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Electron Devices - 1
About the Tutorial
The electronic components exploiting the electronic properties of semiconductor materials,
are termed as semiconductor devices. This tutorial discusses the functional operation of
semiconductor devices, explains the operation of devices in a circuit, etc.
Each topic in this tutorial is explained well using circuit diagrams for better understanding.
After completing this tutorial, readers will be at a moderate level of expertise to explain
the basics related to semiconductor devices.
This tutorial will be useful for all those readers who want to gain knowledge on
semiconductor devices. Upon completion of this tutorial, you will able to explain the
functional operation of semiconductor devices.
We don't assume any prior knowledge of Electronics is necessary to understand this
tutorial. The material is meant for beginners and it should be useful for most readers.
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in this tutorial, please notify us at contact@tutorialspoint.com.
About the Tutorial .................................................................................................................................... i
Audience .................................................................................................................................................. i
Prerequisites ............................................................................................................................................ i
Disclaimer & Copyright............................................................................................................................. i
Table of Contents .................................................................................................................................... ii
1. SEMICONDUCTOR DEVICES ─ INTRODUCTION..................................................................... 1
2. SEMICONDUCTOR DEVICES ─ ATOMIC COMBINATIONS ..................................................... 2
Ionic Bonding .......................................................................................................................................... 3
Covalent Bonding .................................................................................................................................... 4
Metallic Bonding ..................................................................................................................................... 5
3. SEMICONDUCTOR DEVICES ─ CONDUCTION IN SOLID MATERIALS ...................................... 7
4. SEMICONDUCTOR DEVICES – CONDUCTIVITY & MOBILITY .................................................. 9
5. SEMICONDUCTOR DEVICES – TYPES OF SEMICONDUCTOR ............................................... 10
Germanium as a Semiconductor ........................................................................................................... 10
Silicon as a Semiconductor .................................................................................................................... 11
6. SEMICONDUCTOR DEVICES – DOPING IN SEMICONDUCTORS .......................................... 12
Effect of Doping on N-type Material ...................................................................................................... 13
7. SEMICONDUCTOR DEVICES – JUNCTION DIODES .............................................................. 15
8. SEMICONDUCTOR DEVICES ─ DEPLETION ZONE ................................................................ 17
9. SEMICONDUCTOR DEVICES ─ BARRIER POTENTIAL ........................................................... 18
10. SEMICONDUCTOR DEVICES – JUNCTION BIASING ............................................................. 19
Forward Biasing .................................................................................................................................... 19
Reverse Biasing ..................................................................................................................................... 21
11. SEMICONDUCTOR DEVICES ─ LEAKAGE CURRENT ............................................................. 23
12. SEMICONDUCTOR DEVICES – DIODE CHARACTERISTICS .................................................... 25
DIODE IV Characteristics ..................................................................................................................... 25
Forward Characteristic .......................................................................................................................... 26
Reverse Characteristic ........................................................................................................................... 27
Diode Specifications .............................................................................................................................. 27
13. SEMICONDUCTOR DEVICES – LIGHT EMITTING DIODE ...................................................... 29
14. SEMICONDUCTOR DEVICES – ZENER DIODE ...................................................................... 31
15. SEMICONDUCTOR DEVICES – PHOTO DIODE ..................................................................... 34
16. SEMICONDUCTOR DEVICES – PHOTOVOLTAIC CELLS ........................................................ 36
17. SEMICONDUCTOR DEVICES – VARACTOR DIODE ............................................................... 38
18. SEMICONDUCTOR DEVICES – BIPOLAR TRANSISTORS ....................................................... 40
NPN Transitor........................................................................................................................................ 40
PNP Transistor....................................................................................................................................... 41
19. SEMICONDUCTOR DEVICES – CONSTRUCTION OF A TRANSISTOR ..................................... 42
20. SEMICONDUCTOR DEVICES – TRANSISTOR BIASING .......................................................... 43
Working of NPN Transistor .................................................................................................................... 43
Working of PNP Transistor .................................................................................................................... 44
21. SEMICONDUCTOR DEVICES – CONFIGURATION OF TRANSISTORS..................................... 45
22. SEMICONDUCTOR DEVICES ─ FIELD EFFECT TRANSISTORS ................................................ 48
Junction Field Effect Transistor.............................................................................................................. 48
N-Channel JFET ...................................................................................................................................... 48
P-Channel JFETs ..................................................................................................................................... 49
Output Characteristics of JFET ............................................................................................................... 50
Parameters of JFET ................................................................................................................................ 50
23. SEMICONDUCTOR DEVICES ─ JFET BIASING ....................................................................... 52
Self-Bias Method ................................................................................................................................... 52
Voltage Divider Method ........................................................................................................................ 53
24. SEMICONDUCTOR DEVICES ─ MOSFET .............................................................................. 54
D MOSFET ............................................................................................................................................. 54
Depletion Mode .................................................................................................................................... 55
Enhancement Mode .............................................................................................................................. 55
Transfer Characteristics of D – MOSFET ................................................................................................. 56
25. SEMICONDUCTOR DEVICES – OPERATIONAL AMPLIFIERS ................................................. 57
Basic Differential Amplifier ................................................................................................................... 57
26. SEMICONDUCTOR DEVICES – PRACTICAL OP-AMPS .......................................................... 60
Inverting Amplifier ................................................................................................................................ 60
Non-Inverting Amplifier ........................................................................................................................ 60
Inverting Summing Amplifier ................................................................................................................ 61
27. SEMICONDUCTOR DEVICES ─ INTEGRATOR ....................................................................... 62
28. SEMICONDUCTOR DEVICES ─ DIFFERENTIATOR ................................................................ 63
29. SEMICONDUCTOR DEVICES ─ OSCILLATORS ...................................................................... 65
Phase Shift Oscillator ............................................................................................................................ 66
Wien Bridge Oscillator........................................................................................................................... 67
Hartley Oscillator .................................................................................................................................. 68
Piezoelectricity ...................................................................................................................................... 69
Crystal Oscillator ................................................................................................................................... 69
30. SEMICONDUCTOR DEVICES – FEEDBACK & COMPENSATION ............................................ 71
Bias Stabilization Methods .................................................................................................................... 71
Inverse-Voltage Feedback ..................................................................................................................... 72
Inverse-Current Feedback ..................................................................................................................... 73
Bias Compensation ................................................................................................................................ 74
Temperature Compensation Device ...................................................................................................... 75
1. Semiconductor Devices ─ Introduction
It is widely seen that the distance of a nucleus from the electron of a particular atom is
not equal. Normally, electrons rotate in a well-defined orbit. A particular number of
electrons can only hold by outer shell or orbit. The electrical conductivity of an atom is
influenced mainly by the electrons of the outer shell. These electrons have a great deal to
do with the electrical conductivity.
Conductors and Insulators
Electrical conduction is the result of irregular or uncontrolled movement of electrons. These
movements cause certain atoms to be good electrical conductors. A material with such
type of atoms has many free electrons in its outer shell or orbit.
Comparatively, an insulating material has a relatively small number of free electrons.
Consequently, the outer shell electrons of insulators tend to hold their place firmly and
hardly allow any current to flow through it. Therefore, in an insulating material, very little
electrical conductivity takes place.
In between conductors and insulators, there is a third classification of atoms (material)
known as semiconductors. Generally, the conductivity of a semiconductor lies in between
the conductivities of metals and insulators. However, at absolute zero temperature, the
semiconductor also acts like a perfect insulator.
Silicon and germanium are the most familiar semiconductor elements. Copper oxide,
cadmium-sulfide, and gallium arsenide are some other semiconductor compounds that are
frequently used. These kinds of material are generally classified as type IVB elements.
Such atoms have four valence electrons. If they can give up four valence electrons,
stability can be accomplished. It can also be achieved by accepting four electrons.
Stability of an Atom
The concept of stability of an atom is an important factor in the status of semiconductor
materials. The maximum number of electrons in the valence band is 8. When there are
exactly 8 electrons in the valence band, it can be said that the atom is stable. In a stable
atom, the bonding of valence electrons is very rigid. These types of atoms are excellent
insulators. In such atoms, free electrons are not available for electrical conductivity.
Examples of stabilized elements are gases such as Argon, Xenon, Neon, and Krypton. Due
to their property, these gases cannot be mixed with other material and are generally
known as inert gases.
If the number of valence electrons in the outer shell is less than 8, then the atom is said
to be unstable i.e., the atoms having fewer than 8 valence electrons are unstable. They
always try to borrow or donate electrons from the neighboring atoms to become stable.
Atoms in the outer shell with 5, 6, or 7 valence electrons tend to borrow electrons from
other atoms to seek stability, while atoms with one, two, or three valence electrons tend
to release these electrons to other nearby atoms.
2. Semiconductor Devices ─ Atomic Semiconductor
Combinations Devices
Anything that has weight is matter. As per the theory of atom, all matter, whether it is
solid, liquid, or gas is composed of atoms. An atom contains a central part called nucleus,
which holds the neutrons and the protons. Normally, protons are positively charged
particles and neutrons are neutrally charged particles. Electrons which are negatively
charged particles are arranged in orbits around the nucleus in a way similar to the array
of planets around the Sun. The following figure shows the composition of an atom.
Atoms of different elements are found to have different number of protons, neutrons, and
electrons. To distinguish one atom from another or to classify the various atoms, a number
which indicates the number of protons in the nucleus of a given atom, is assigned to the
atoms of each identified element. This number is known as the atomic number of the
element. The atomic numbers for some of the elements which are associated with the
study of semiconductors are given in the following table.
Element Symbol Atomic Number
Silicon Si 14
Germanium Ge 32
Arsenic As 33
Antimony Sb 51
Indium In 49
Gallium Ga 31
Boron B 5
Normally, an atom has an equal number of protons and planetary electrons to maintain
its net charge at zero. Atoms frequently combine to form stabilized molecules or
compounds through their available valence electrons.
The process of combining of free valence electrons is generally called bonding. Following
are the different kinds of bonding that takes place in atom combinations.
 Ionic bonding
 Covalent bonding
 Metallic bonding
Let us now discuss in detail about these atomic bondings.
Ionic Bonding
Each atom is seeking stability when the atoms bond together to form molecules. When the
valence band contains 8 electrons, it is said to be a stabilized condition. When the
valence electrons of one atom combine with those of another atom to become stable, it is
called ionic bonding.
 If an atom has more than 4 valence electrons in the outer shell it is seeking
additional electrons. Such atom is often called an acceptor.
 If any atom holds less than 4 valence electrons in the outer shell, they try to move
out from these electrons. These atoms are known as donors.
In ionic bonding, donor and acceptor atoms frequently combine together and the
combination becomes stabilized. Common salt is a common example of ionic bonding.
The following figures illustrate an example of independent atoms and ionic bonding.
It can be seen in the above figure that the sodium (Na) atom donates its 1 valence electron
to the chloride (Cl) atom which has 7 valence electrons. The chloride atom immediately
becomes overbalanced negatively when it obtains the extra electron and this causes the
atom to become a negative ion. While on the other hand, the sodium atom loses its valence
electron and the sodium atom then becomes a positive ion. As we know unlike charges
attract, the sodium and chloride atoms are bound together by an electrostatic force.
Covalent Bonding
When the valence electrons of neighboring atoms are shared with other atoms, covalent
bonding takes place. In covalent bonding, ions are not formed. This is a unique dissimilarity
in covalent bonding and ionic bonding.
When an atom contains four valence electrons in the outer shell, it can share one electron
with four neighboring atoms. A covalent force is established between the two linking
electrons. These electrons alternately shift orbits between the atoms. This covalent force
bonds the individual atoms together. An illustration of covalent bonding is shown in the
following figures.
In this arrangement, only the nucleus and valence electrons of each atom are shown.
Electron pair are created due to individual atoms are bonded together. In this case, five
atoms are needed to complete the bonding action. The bonding process widens out in all
directions. Each atom is now linked together in a lattice network and a crystal structure is
formed by this lattice network.
Metallic Bonding
The third type of bonding generally occurs in good electrical conductors and it is called as
metallic bonding. In metallic bonding, an electrostatic force exists between the positive
ions and electrons. For example, the valence band of copper has one electron in its outer
shell. This electron has a tendency to roam around the material between different atoms.
When this electron leaves one atom, it instantly enters the orbit of another atom. The
process is repetitive on a nonstop basis. An atom becomes a positive ion when an electron
leaves it. This is a random process. It means that one electron is always linked with an
atom. It does not mean that the electron is associated with one particular orbit. It is always
roaming in different orbits. As a consequence, all atoms are likely to share all the valence
electrons.
Electrons hang around in a cloud that covers the positive ions. This hovering cloud bonds
the electrons randomly to the ions. The following figure shows an example of the metallic
bonding of copper.
3. Semiconductor Devices ─ Conduction in Solid
The number of electrons in the outer ring of an atom is still the reason for the difference
between conductors and insulators. As we know, solid materials are primarily used in
electrical devices to accomplish electron conduction. These materials can be separated
into conductors, semiconductors, and insulators.
However, conductors, semiconductors, and insulators are differentiated by energy-level
diagrams. The amount of energy needed to cause an electron to leave its valence band
and go into conduction will be accounted here. The diagram is a composite of all atoms
within the material. Energy-level diagrams of insulators, semiconductors, and conductors
are shown in the following figure.
Valence Band
The bottom portion is the valence band. It represents the energy levels closest to the
nucleus of the atom and the energy levels in the valance band hold the correct number of
electron necessary to balance the positive charge of the nucleus. Thus, this band is called
the filled band.
In the valence band, electrons are tightly bound to the nucleus. Moving upward in the
energy level, the electrons are more lightly bound in each succeeding level toward the
nucleus. It is not easy to disturb the electrons in the energy levels closer to the nucleus,
as their movement requires larger energies and each electron orbit has a distinct energy
Conduction Band
The top or outermost band in the diagram is called the conduction band. If an electron
has an energy level, which lies within this band, and is comparatively free to move around
in the crystal, then it conducts electric current.
In semiconductor electronics, we are concerned mostly in the valence and conduction
bands. Following are some basic information about it:
 The valence band of each atom shows the energy levels of the valence electrons in
the outer shell.
 A definite amount of energy must be added to the valence electrons to cause them
to go into the conduction band.
Forbidden Gap
The valence and conduction bands are separated by a gap, wherever exists, called
forbidden gap. To cross the forbidden gap a definite amount of energy is needed. If it is
insufficient, electrons are not released for conduction. Electrons will remain in the valence
band till they receive additional energy to cross the forbidden gap.
The conduction status of a particular material can be indicated by the width of the
forbidden gap. In atomic theory, the width of the gap is expressed in electron volts (eV).
An electron volt is defined as the amount of energy gained or lost when an electron is
subjected to a potential difference of 1 V. The atoms of each element have a dissimilar
energy-level value that allows conduction.
Note that the forbidden region of an insulator is relatively wide. To cause an insulator
to go into conduction will require a very large amount of energy. For example, Thyrite.
If insulators are operated at high temperatures, the increased heat energy causes the
electrons of the valence band to move into the conduction band.
As it is clear from the energy band diagram, the forbidden gap of a semiconductor is much
smaller than that of an insulator. For example, silicon needs to gain 0.7 eV of energy to
go into the conduction band. At room temperature, the addition of heat energy may be
sufficient to cause conduction in a semiconductor. This particular characteristic is of great
importance in solid-state electronic devices.
In case of a conductor, the conduction band and the valence band partly overlaps one
another. In a sense, there is no forbidden gap. Therefore, the electrons of valence band
are able to release to become free electrons. Normally at normal room temperature little
electrical conduction takes place within the conductor.
4. Semiconductor Devices – Conductivity & Mobility
As discussed earlier, there may be one or more free electrons per atom which moves all
the way through the interior of the metal under the influence of an applied field.
The following figure shows charge distribution within a metal. It is known as the electron-
gas description of a metal.
The hashed region represents the nucleus with a positive charge. The blue dots represent
the valence electrons in the outer shell of an atom. Basically, these electrons do not belong
to any specific atom and as a result, they have lost their individual identity and roam freely
atom to atom.
When the electrons are in an uninterrupted motion, the direction of transportation is
changed at each collision with the heavy ions. This is based on electron-gas theory of a
metal. The average distance between collisions is called the mean free path. The
electrons, passing through a unit area, in the metal in the opposite direction in a given
time, on a random basis, makes the average current zero.
5. Semiconductor Devices – Types of Semiconductor
When voltage is applied to semiconductor devices, electron current flows toward the
positive side of the source and holes current flows towards the negative side of the source.
Such a situation occurs only in a semiconductor material.
Silicon and Germanium are the most common semiconductor materials. Generally, the
conductivity of a semiconductor lies in between the conductivities of metals and insulators.
Germanium as a Semiconductor
Following are some important points about Germanium:
 There are four electrons in the outermost orbit of germanium. In bonds, atoms are
shown with their outer electrons only.
 The germanium atoms will share valence electrons in a covalent bond. This is shown
in the following figure. Germanium are the ones that are associated with the
covalent bonding. The crystalline form of germanium is called the crystal lattices.
This type of structure has the atoms arranged in the way as shown in the following
figure.
 In such an arrangement, the electrons are in a very stable state and thus are less
appropriate to be associated with conductors. In the pure form, germanium is an
insulating material and is called as an intrinsic semiconductor.
The following figure shows the atomic structures of Silicon and Germanium.
Atomic Structure of Silicon & Germanium
Silicon as a Semiconductor
Semiconductor devices also use silicon in the manufacturing of various electronic
components. The atomic structure of silicon and germanium is shown in the above figure.
The crystal lattice structure of silicon is similar to that of Germanium.
Following are some of the important points about Silicon:
 It has four electrons in its outermost shell like germanium.
 In pure form, it is of no use as a semiconductor device.
 A desired amount of conductivity can be obtained by adding up of impurities.
 Adding up of impurity must be done carefully and in a controlled environment.
 Depending on the type of impurity added, it will create either an excess or a deficit
of electrons.
The following figure shows the intrinsic crystal of Silicon.
6. Semiconductor Devices – Doping in Semiconductor
SemiconductorsDevices
Pure Silicon or Germanium are rarely used as semiconductors. Practically usable
semiconductors must have controlled quantity of impurities added to them. Addition of
impurity will change the conductor ability and it acts as a semiconductor. The process of
adding an impurity to an intrinsic or pure material is called doping and the impurity is
called a dopant. After doping, an intrinsic material becomes an extrinsic material.
Practically only after doping these materials become usable.
When an impurity is added to silicon or germanium without modifying the crystal structure,
an N-type material is produced. In some atoms, electrons have five electrons in their
valence band such as arsenic (As) and antimony (Sb). Doping of silicon with either impurity
must not change the crystal structure or the bonding process. The extra electron of
impurity atom does not take part in a covalent bonding. These electrons are loosely held
together by their originator atoms. The following figure shows alteration of silicon crystal
with the addition of an impurity atom.
Effect of Doping on N-type Material
The effect of doping on an N-type material is as follows:
 On addition of Arsenic to pure Silicon, the crystal becomes an N-type material.
 Arsenic atom has additional electrons or negative charges that do not take part in
the process of covalent bonding.
 These impurities give up or donate, one electron to the crystal and they are referred
to as donor impurities.
 An N-type material has extra or free electrons than an intrinsic material.
 An N-type material is not negatively charged. Actually all of its atoms are all
electrically neutral.
 These extra electrons do not take part in the covalent bonding process. They are
free to move about through the crystal structure.
 An N-type extrinsic silicon crystal will go into conduction with only 0.005eV of
energy applied.
 Only 0.7eV is required to move electrons of intrinsic crystal from the valence band
into the conduction band.
Normally, electrons are considered to be the majority current carriers in this type of crystal
and holes are the minority current carriers. The quantity of donor material added to Silicon
finds out the number of majority current carriers in its structure.
The number of electrons in an N-type silicon is many times greater than the electron-hole
pairs of intrinsic silicon. At room temperature, there is a firm difference in the electrical
conductivity of this material. There are abundant current carriers to take part in the current
flow. The flow of current is achieved mostly by electrons in this type of material. Therefore,
an extrinsic material becomes a good electrical conductor.
The effect of doping on a P-type material is as follows:
 When Indium (In) or Gallium (Ga) is added to pure silicon, a P-type material is
formed.
 This type of dopant material has three valence electrons. They are eagerly looking
for a fourth electron.
 In P type material, each hole can be filled with an electron. To fill this hole area,
very less energy is required by electrons from the neighboring covalent bonded
 Silicon is typically doped with doping material in the range of 1 to 106. This means
that P material will have much more holes than the electron-hole pairs of pure
silicon.
 At room temperature, there is a very determined characteristic difference in the
electrical conductivity of this material.
The following figure shows how the crystal structure of Silicon is altered when doped with
an acceptor element — in this case, Indium. A piece of P material is not positively charged.
Its atoms are primarily all electrically neutral.
There are, however, holes in the covalent structure of many atom groups. When an
electron moves in and fills a hole, the hole becomes void. A new hole is created in the
bonded group where the electron left. Hole movement in effect is the result of electron
movement. A P-type material will go into conduction with only 0.05 eV of energy applied.
The above figure shows how a P-type crystal will respond when connected to a voltage
source. Note that there are larger numbers of holes than electrons. With voltage applied,
the electrons are attracted to the positive battery terminal.
Holes move, in a sense, toward the negative battery terminal. An electron is picked up at
this point. The electron immediately fills a hole. The hole then becomes void. At the same
time, an electron is pulled from the material by the positive battery terminal. Holes
therefore move toward the negative terminal due to electrons shifting between different
bonded groups. With energy applied, hole flow is continuous.
7. Semiconductor Devices – Junction Diodes
A crystal structure made of P and N materials is generally known as junction diode. It is
generally regarded as a two-terminal device. As shown in the following diagram one
terminal is attached to P-type material and the other to N-type material.
The common bond point where these materials are connected is called a junction. A
junction diode allows current carriers to flow in one direction and obstruct the flow of
current in the reverse direction.
The following figure shows the crystal structure of a junction diode. Take a look at the
location of the P type and N type materials with respect to the junction. The structure of
crystal is continuous from one end to the other. The junction acts only as a separating
point that represents the end of one material and the beginning of the other. Such
structure allows electrons to move thoroughly in the entire structure.
The following diagram shows two portions of semiconductor substance before they are
shaped into a P-N junction. As specified, each part of material has majority and minority
current carriers.
The quantity of carrier symbols shown in each material indicates the minority or majority
function. As we know electrons are the majority carriers in the N type material and holes
are the minority carriers. In P type material, holes are the majority carriers and electrons
are in the minority.
8. Semiconductor Devices ─ Depletion Zone
Initially, when a junction diode is formed, there is a unique interaction between current
carriers. In N type material, the electrons move readily across the junction to fill holes in
the P material. This act is commonly called diffusion. Diffusion is the result of high
accumulation of carriers in one material and a lower gathering in the other.
Generally, the current carriers which are near to the junction only takes part in the process
of diffusion. Electrons departing the N material cause positive ions to be generated in their
place. While entering the P material to fill holes, negative ions are created by these
electrons. As a result, each side of the junction contains a large number of positive and
negative ions.
The area where these holes and electrons become depleted is generally known by the term
depletion region. It is an area where there is lack of majority current carriers. Normally, a
depletion region is developed when P-N junction is formed. The following figure shows the
depletion region of a junction diode.
9. Semiconductor Devices ─ Barrier Potential
N-type and P-type material are considered as electrically neutral before they are joined
together at a common junction. However, after joining diffusion takes place
instantaneously, as electrons cross the junction to fill holes causing negative ions to
emerge in the P material, this action causes the nearby area of the junction to take on a
negative charge. Electrons departing the N material causes it to generate positive ions.
All this process, in turn, causes the N side of the junction to take on a net positive charge.
This particular charge creation tends to force the remaining electrons and holes away from
the junction. This action makes it somewhat hard for other charge carriers to diffuse across
the junction. As a result, the charge is built up or barrier potential emerges across the
junction.
As shown in the following figure. the resultant barrier potential has a small battery
connected across the P-N junction. In the given figure observe the polarity of this potential
barrier with respect to P and N material. This voltage or potential will exist when the crystal
is not connected to an external source of energy.
The barrier potential of germanium is approximately 0.3 V, and of silicon is 0.7 V. These
values cannot be measured directly and appears across the space charge region of the
junction. In order to produce current conduction, the barrier potential of a P-N junction
must be overcome by an external voltage source.
10. Semiconductor Devices – Junction Biasing
The term bias refers to the application of DC voltage to set up certain operating conditions.
Or when an external source of energy is applied to a P-N junction it is called a bias voltage
or simply biasing. This method either increases or decreases the barrier potential of the
junction. As a result, the reduction of the barrier potential causes current carriers to return
to the depletion region. Following two bias conditions are applied w.r.t. PN junctions.
 Forward Biasing – An external voltage is added of the same polarity to the barrier
potential, which causes an increase in the width of the depletion region.
 Reverse Biasing - A PN junction is biased in such a way that the application of
external voltage action prevents current carriers from entering the depletion
Forward Biasing
The following figure shows a forward biased PN junction diode with external voltage
applied. You can see that the positive terminal of the battery is connected to the P material
and the negative terminal of the battery is connected to the N material.
Following are the observations:
 This bias voltage repels the majority current carriers of each P and N type material.
As a result, large number of holes and electrons start appearing at the junction.
 At the N-side of the junction, electrons move in to neutralize the positive ions in
the depletion region.
 On the P-side material, electrons are dragged from negative ions, which cause them
to become neutral again. This means that forward biasing collapses the depletion
region and hence the barrier potential too. It means that when P-N junction is
forward biased, it will allow a continuous current flow.
The following figure shows the flow of current carriers of a forward-biased diode. A
constant supply of electrons is available due to an external voltage source connected to
the diode. The flow and direction of the current is shown by large arrows outside the diode
in the diagram. Note that the electron flow and the current flow refers to the same thing.
 Suppose electrons flow through a wire from the negative battery terminal to the N
material. Upon entering this material, they flow immediately to the junction.
 Similarly, on the other side an equal number of electrons are pulled from P side
and are returned to the positive battery terminal. This action creates new holes and
causes them to move toward the junction.
 When these holes and electrons reach the junction they join together and
effectively disappear. As a result, new holes and electrons emerge at the outer
ends of the diode. These majority carriers are created on a continuous basis. This
action continues as long as the external voltage source is applied.
 When diode is forward biased it can be noticed that electrons flow through the
entire structure of diode. This is common in N type material, whereas in the P
material holes are the moving current carriers. Notice that the hole movement in
one direction must begin by electron movement in the opposite direction.
Therefore, the total current flow is the addition of holes and electrons flow through
a diode.
Reverse Biasing
The following figure shows reverse biased PN junction diode with external voltage applied.
You can see that the positive terminal of the battery is connected to the N material and
the negative terminal of the battery is connected to the P material. Note that in such an
arrangement, battery polarity is to oppose the material polarity of the diode so that
dissimilar charges attract. Hence, majority charge carriers of each material are dragged
away from the junction. Reverse biasing causes the diode to be nonconductive.
The following figure shows the arrangement of the majority current carriers in a reverse
biased diode.
 Due to circuit action electrons of the N material are pulled toward the positive
battery terminal.
 Each electron that moves or departs the diode causes a positive ion to emerge in
its place. As a result, this causes an equivalent increase in the width of the depletion
region on the N side of the junction.
 The P side of the diode has a similar effect alike the N side. In this action, a number
of electrons leave the negative battery terminal and enter the P type material.
 These electrons then straight away move in and fill a number of holes. Each
occupied hole then becomes a negative ion. These ions in turn are then repelled by
the negative battery terminal and driven toward the junction. Due to this, there is
an increase in the width of the depletion region on the P side of the junction.
The overall width of the depletion region directly depends on an external voltage source
of a reverse-biased diode. In this case, the diode cannot efficiently support the current
flow through the wide depletion region. As a result, the potential charge starts developing
across the junction and increases until the barrier potential equals the external bias
voltage. After this, the diode behaves as a nonconductor.
11. Semiconductor Devices ─ Leakage Current
An important conduction limitation of PN junction diode is leakage current. When a diode
is reverse biased, the width of the depletion region increases. Generally, this condition is
required to restrict the current carrier accumulation near the junction. Majority current
carriers are primarily negated in the depletion region and hence the depletion region acts
as an insulator. Normally, current carriers do not pass through an insulator.
It is seen that in a reverse-biased diode, some current flows through the depletion region.
This current is called leakage current. Leakage current is dependent on minority current
carriers. As we know that the minority carriers are electrons in the P type material and
holes in the N type material.
The following figure shows how current carriers react when a diode is reverse biased.
 Minority carriers of each material are pushed through the depletion zone to the
junction. This action causes a very small leakage current to occur. Generally,
leakage current is so small that it can be considered as negligible.
 Here, in case of leakage current, temperature plays an important role. The minority
current carriers are mostly temperature dependent.
 At room temperatures of 25°C or 78°F, there is negligible amount of minority
carriers present in a reverse bias diode.
 When the surrounding temperature rises, it causes significant increase in minority
carrier creation and as a result it causes a corresponding increase in leakage
current.
In all reverse-biased diodes, occurrence of leakage current is normal to some extent. In
Germanium and Silicon diodes, leakage current is only of few microamperes and
nanoamperes, respectively. Germanium is much more susceptible to temperature than
silicon. For this reason, mostly Silicon is used in modern semiconductor devices.
12. Semiconductor Devices – Diode Characteristics
There are diverse current scales for forward bias and reverse bias operations. The forward
portion of the curve indicates that the diode conducts simply when the P-region is made
positive and the N-region negative.
The diode conducts almost no current in the high resistance direction, i.e. when the P-
region is made negative and the N-region is made positive. Now the holes and electrons
are drained away from the junction, causing the barrier potential to increase. This
condition is indicated by the reverse current portion of the curve.
The dotted section of the curve indicates the ideal curve, which would result if it were
not for avalanche breakdown. The following figure shows the static characteristic of a
junction diode.
DIODE IV Characteristics
The forward and reverse current voltage (IV) characteristics of a diode are generally
compared on a single characteristic curve. The figure depicted under the section Forward
Characteristic shows that Forward Voltage and Reverse Voltage are usually plotted on the
horizontal line of the graph.
Forward and reverse current values are shown on the vertical axis of the graph. Forward
Voltage represented to the right and Reverse Voltage to the left. The point of beginning or
zero value is at the center of the graph. Forward Current lengthens above the horizontal
axis with Reverse Current extending downward.
The combined Forward Voltage and Forward Current values are located in the upper right
part of the graph and Reverse Voltage and Reverse Current in the lower left corner.
Different scales are normally used to display forward and reverse values.
Forward Characteristic
When a diode is forward biased it conducts current (IF) in forward direction. The value of
IF is directly dependent on the amount of forward voltage. The relationship of forward
voltage and forward current is called the ampere-volt, or IV characteristic of a diode. A
typical diode forward IV characteristic is shown in the following figure.
 Forward Voltage is measured across the diode and Forward Current is a measure
of current through the diode.
 When the forward voltage across the diode equals 0V, forward current (IF) equals
0 mA.
 When the value starts from the starting point (0) of the graph, if VF is progressively
increased in 0.1-V steps, IF begins to rise.
 When the value of VF is large enough to overcome the barrier potential of the P-N
junction, a considerable increase in IF occurs. The point at which this occurs is
often called the knee voltage VK. For germanium diodes, VK is approximately 0.3
V, and 0.7 V for silicon.
 If the value of IF increases much beyond VK, the forward current becomes quite
large.
This operation causes excessive heat to develop across the junction and can destroy a
diode. To avoid this situation, a protective resistor is connected in series with the diode.
This resistor limits the forward current to its maximum rated value. Normally, a current-
limiting resistor is used when diodes are operated in the forward direction.
Reverse Characteristic
When a diode is reverse biased, it conducts Reverse current that is usually quite small. A
typical diode reverse IV characteristic is shown in the above figure.
The vertical reverse current line in this graph has current values expressed in
microamperes. The amount of minority current carriers that take part in conduction of
reverse current is quite small. In general, this means that reverse current remains
constant over a large part of reverse voltage. When the reverse voltage of a diode is
increased from the start, there is a very slight change in the reverse current. At the
breakdown voltage (VBR) point, current increases very rapidly. The voltage across the
diode remains reasonably constant at this time.
This constant-voltage characteristic leads to a number of applications of diode under
reverse bias condition. The processes which are responsible for current conduction in a
reverse-biased diode are called as Avalanche breakdown and Zener breakdown.
Diode Specifications
Like any other selection, selection of a diode for a specific application must be considered.
Manufacturer generally provides this type of information. Specifications like maximum
voltage and current ratings, usual operating conditions, mechanical facts, lead
identification, mounting procedures, etc.
Following are some of the important specifications:
 Maximum forward current (IFM): The absolute maximum repetitive forward
current that can pass through a diode.
 Maximum reverse voltage (VRM): The absolute maximum or peak reverse bias
voltage that can be applied to a diode.
 Reverse breakdown voltage (VBR): The minimum steady-state reverse voltage
at which breakdown will occur.
 Maximum forward surge current (IFM-surge): The maximum current that can
be tolerated for a short interval of time. This current value is much greater than
IFM.
 Maximum reverse current (IR): The absolute maximum reverse current that
can be tolerated at device operating temperature.
 Forward voltage (VF): Maximum forward voltage drop for a given forward
current at device operating temperature.
 Power dissipation (PD): The maximum power that the device can safely absorb
on a continuous basis in free air at 25° C.
 Reverse recovery time (Trr): The maximum time that it takes the device to
switch from on to off stat.
 Breakdown Voltage: It is the minimum reverse bias voltage at which PN junction
breaks down with sudden rise in reverse current.
 Knee Voltage: It is the forward voltage at which the current through the junction
starts to increase rapidly.
 Peak Inverse Voltage: It is the maximum reverse voltage that can be applied to
the PN junction, without damaging it.
 Maximum Forward Rating: It is the highest instantaneous forward current that
a PN junction can pass, without damaging it.
 Maximum Power Rating: It is the maximum power that can be dissipated from
the junction, without damaging the junction.
13. Semiconductor Devices – Light Emitting Diode
Light Emitting Diodes are directly or indirectly influencing our day-to-day activities. From
the message display to LED TVs, everywhere these LEDs exist. It is basically a P-N junction
diode that emits light when a forward current is allowed to pass through it. The following
figure shows the logic symbol of an LED.
Logic Symbol of LED
How Does a PN Junction Diode Emit Light?
LEDs are not made from Silicon or Germanium and elements like Gallium Arsenide (GaAs)
and Gallium Phosphide (GaP). These materials are deliberately used as they emit light.
Hence, when an LED is forward-biased, as usual electrons cross the junction and unite
with holes.
This action causes electrons of N-type region to fall out of conduction and return to the
valence band. In doing so, the energy possessed by each free electron is then released. A
part of released energy emerges as heat and the rest of it is given as visible light energy.
If LEDs are made from Silicon and Germanium, then during recombination of electrons, all
the energy is dissipated in the form of heat only. On the other hand, materials such as
Gallium Arsenide (GaAs) and Gallium Phosphide (GaP) possess enough photons that are
sufficient to produce visible light.
 If LEDs are made from gallium arsenide, they produce red light.
 If LEDs are made from Gallium Phosphide, then such LEDs emit green light.
Now consider two LEDs connected back to back across an external voltage supply source,
such that anode of one LED is connected to the cathode of another LED or vice versa.
When an external voltage is applied to this circuit, one LED will operate at a time and due
to this circuit action, it emits a different light when one LED is forward biased and the
other is reverse biased or vice versa.
Advantages of LEDs
LEDs offer the following advantages:
 Quite small in size.
 Very fast switching.
 Can be operated with very low voltage.
 A very long life expectancy.
 Construction procedure permits manufacturing in different shapes and patterns.
Applications of LEDs
LEDs are mostly used in numeric displays indicating the numbers 0 through 9. They are
also used in seven-segment display found in digital meters, clocks, calculators, etc.
14. Semiconductor Devices – Zener Diode
It is a specific type of semiconductor diode, which is made to operate in the reverse
breakdown region. The following figure depicts the crystal structure and the symbol of a
Zener diode. It is mostly similar to that of a conventional diode. However, small
modification is done to distinguish it from a symbol of a regular diode. The bent line
indicates letter 'Z' of the Zener.
The most significant difference in Zener diodes and regular PN junction diodes is in the
mode which they are used in circuits. These diodes are normally operated only in the
reverse bias direction, which implies that the anode must be connected to the negative
side of the voltage source and the cathode to the positive.
If a regular diode is used in the same way as Zener diode, it will be destroyed due to
excessive current. This property makes the Zener diode less significant.
The following illustration shows a regulator with a Zener diode.
The Zener diode is connected in reverse bias direction across unregulated DC supply
source. It is heavily doped so that the reverse breakdown voltage is reduced. This results
in a very thin depletion layer. Due to this, the Zener diode has sharp reverse breakdown
voltage Vz.
As per the circuit action, breakdown occurs sharply with a sudden increase in current as
shown in the following figure.
Voltage Vz remains constant with an increase in current. Due to this property, Zener diode
is widely used in voltage regulation. It provides almost constant output voltage irrespective
of the change in current through the Zener. Thus, the load voltage remains at a constant
We can see that at a particular reverse voltage known as knee voltage, current increases
sharply with constant voltage. Due to this property, Zener diodes are widely used in
voltage stabilization.
15. Semiconductor Devices – Photo Diode
A photodiode is a P-N junction diode that will conduct current when exposed to light. This
diode is actually designed to operate in the reverse bias mode. It means that larger the
intensity of falling light, the greater will be the reverse bias current.
The following figure shows a schematic symbol and constructional detail of a photo diode.
Working of a Photo Diode
It is a reverse-biased diode. Reverse current increases as the intensity of incident light
increases. This means that reverse current is directly proportional to the intensity of falling
It consists of a PN junction mounted on a P-type substrate and sealed in a metallic case.
The junction point is made of transparent lens and it is the window where the light is
supposed to fall.
As we know, when PN junction diode is reverse biased, a very small amount of reverse
current flows. The reverse current is generated thermally by electron-hole pairs in the
depletion region of the diode.
When light falls on PN junction, it is absorbed by the junction. This will generate more
electron-hole pairs. Or we can say, characteristically, the amount of reverse current
increases.
In other words, as the intensity of falling light increases, resistance of the PN junction
diode decreases.
 This action makes the diode more conductive.
 These diodes have very fast response time
 These are used in high computing devices.
 It is also used in alarm circuits, counter circuits, etc.
16. Semiconductor Devices – Photovoltaic Cells
A basic photovoltaic cell consists of a n-type and a p-type semiconductor forming a p-n
junction. The upper area is extended and transparent, generally exposed to the sun. These
diodes or cells are exceptional that generate a voltage when exposed to light. The cells
convert light energy directly into electrical energy.
The following figure shows the symbol of photovoltaic cell.
Working of a Photovoltaic Cell
The construction of a photovoltaic cell is similar to that of a PN junction diode. There is no
current flow through the device when no light is applied. In this state, the cell will not be
able to generate current.
It is essential to bias the cell properly which requires a fair amount of light. As soon as
light is applied, a remarkable state of PN junction diode can be observed. As a result, the
electrons acquire sufficient energy and break away from the parent atoms. These newly
generated electron-hole pairs in the depletion region crosses the junction.
In this action, the electrons move into the N type material because of its normal positive
ion concentration. Likewise holes sweep into the P type material because of its negative-
ion content. This causes the N type material to instantly take on a negative charge and
the P material to take on a positive charge. The P-N junction then delivers a small voltage
as a response.
Characteristics of a Photovoltaic Cell
The following figure on the left, shows one of the characteristics, a graph between reverse
current (IR) and illumination (E) of a photo diode. IR is measured on the vertical axis and
illumination is measured on the horizontal axis. The graph is a straight line passing through
the zero position.
i.e., IR = mE
m = graph straight line slope
The parameter m is the sensitivity of the diode.
The figure on the right, shows another characteristic of the photo diode, a graph between
reverse current (IR) and reverse voltage of a photo diode. It is clear from the graph that
for a given reverse voltage, reverse current increases as the illumination increases on the
PN junction.
These cells generally supply electrical power to a load device when light is applied. If a
larger voltage is required, array of these cells are used to provide the same. For this
reason, photovoltaic cells are used in applications where high levels of light energy are
17. Semiconductor Devices – Varactor Diode
This is a special P-N junction diode with an inconsistent concentration of impurities in its
P-N materials. In a normal PN junction diode, doping impurities are usually dispersed
equally throughout the material. Varactor diode doped with a very small quantity of
impurities near the junction and impurity concentration increases moving away from the
In conventional junction diode, the depletion region is an area which separates the P and
N material. The depletion region is developed in the beginning when the junction is initially
formed. There are no current carriers in this region and thus the depletion region acts as
a dielectric medium or insulator.
The P-type material with holes as majority carriers and N type material with electrons as
majority carriers now act as charged plates. Thus the diode can be considered as a
capacitor with N- and P-type opposite charged plates and the depletion region acts as
dielectric. As we know, P and N materials, being semiconductors, are separated by a a
depletion region insulator.
Diodes which are designed to respond to the capacitance effect under reverse bias are
called varactors, varicap diodes, or voltage-variable capacitors.
The following figure shows the symbol of Varactor diode.
Varactor diodes are normally operated in the reverse bias condition. When the reverse
bias increases, the width of the depletion region also increases resulting in less
capacitance. This means when reverse bias decreases, a corresponding increase in
capacitance can be seen. Thus, diode capacitance varies inversely proportional to the bias
voltage. Usually this is not linear. It is operated between zero and the reverse breakdown
voltage.
The capacitance of Varactor diode is expressed as:
 CT = Total capacitance of the junction
 E = Permittivity of the semiconductor material
 A = Cross-sectional area of the junction
 Wd = Width of the depletion layer
These diodes are variable used in microwave applications. Varactor diodes are also used
in resonant circuits where some level of voltage tuning or frequency control is required.
This diode is also employed in Automatic Frequency Control (AFC) in FM radio and
television receivers.
18. Semiconductor Devices – BipolarSemiconductor
Transistors Devices
Bipolar transistors are mainly formed of two layers of semiconductor material of the
opposite type, connected back to back. The type of impurity added to silicon or germanium
decides the polarity when it is formed.
NPN Transitor
An NPN transistor is composed of two N type material separated by a thin layer of P type
semiconductor material. The crystal structure and schematic symbol of the NPN transistor
are shown in the above figure.
There are three leads taken out from each type of material recognized as the emitter,
base, and collector. In the symbol, when the arrowhead of the emitter is directed
outwards from the base, it indicates that the device is of the NPN type.
PNP Transistor
A PNP transistor is composed of two P type material separated by a thin layer of N type
semiconductor material. The crystal structure and schematic symbol of a PNP transistor is
shown below.
In the symbol, when the arrowhead of the emitter is directed inwards towards the base,
it indicates that the device is of the PNP type.
19. Semiconductor Devices – Construction of a
Following are some manufacturing techniques used in the construction of a transistor:
Diffusion Type
In this method, the wafer of semiconductor is subjected to some gaseous diffusion of both
N type and P type impurities to form emitter and collector junctions. First, base-collector
junction is determined and photo-etched just prior to base diffusion. Later, the emitter is
diffused on the base. Transistors manufactured by this technique have better noise figure
and improvement in current gain is also seen.
Grown Type
It is formed by drawing a single crystal from melted silicon or germanium. The required
concentration of impurity is added during crystal drawing operation.
Epitaxial Type
A very high purity and thin single-crystal layer of silicon or germanium is grown on a
heavily doped substrate of the same type. This improved version of the crystal forms the
collector on which the emitter and base junctions are formed.
Alloy Type
In this method, the base section is made of a thin slice of N type material. At the opposite
sides of the slice, two small dots of Indium are attached and the complete formation is
kept to a high temperature for a shorter time. The temperature would be above melting
temperature of Indium and below Germanium. This technique is also known as fused
Electrochemically Etched Type
In this method, on the opposite sides of a semiconductor wafer, depression is etched in
order to reduce the width of the base region. Then a suitable metal is electroplated into
the depressions area to form emitter and collector junctions.
20. Semiconductor Devices – Transistor Biasing
Transistors have three sections namely – the emitter, the base, and the collector.
 The base is much thinner than the emitter, and the collector is comparatively wider
than both.
 The emitter is heavily doped so that it can inject large number of charge carriers
for current conduction.
 The base passes most of the charge carriers to the collector as it is comparatively
lightly doped than emitter and the collector.
For a proper functioning of the transistor, the emitter-base region must be forward-biased
and collector-base region must be reverse-biased.
In semiconductor circuits, the source voltage is called as the bias voltage. In order to
function, bipolar transistors must have both junctions biased. This condition causes a
current to flow through the circuit. The depletion region of the device is reduced and
majority current carriers are injected towards the junction. One of the junctions of a
transistor must be forward biased and other must be reverse biased when it operates.
Working of NPN Transistor
As shown in the above figure, the emitter to base junction is forward biased and the
collector to base junction is reverse biased. Forward bias on the emitter to base junction
causes the electrons to flow from N type emitter towards the bias. This condition
formulates the emitter current (IE).
While crossing the P-type material, electrons tend to combine with holes, generally very
few, and constitute the base current (IB). Rest of the electrons cross the thin depletion
region and reach the collector region. This current constitutes collector current (IC).
In other words, the emitter current actually flows through the collector circuit. Therefore,
it can be considered that the emitter current is the summation of the base and the collector
current. It can be expressed as,
IE = IB + IC
Working of PNP Transistor
As shown in the following figure, the emitter to base junction is forward biased and the
causes the holes to flow from P type emitter towards the bias. This condition formulates
the emitter current (IE).
While crossing the N-type material, the electrons tend to combine with electrons, generally
very few, and constitute the base current (IB). Rest of the holes cross the thin depletion
region and reach the collector region. This current constitutes the collector current (IC).
21. Semiconductor Devices – Configuration of
When a transistor is connected in a circuit, four terminals or leads or legs are required,
two both for input and output. As we know that transistors have only 3 terminals, this
situation can be overcome by making one of the terminal common for both input and
output section. Accordingly, a transistor can be connected in three configurations as
 Common Base Configuration
 Common Emitter Configuration
 Common Collector Configuration
Following are some important points to note about transistor operation.
 A transistor can be operated in three regions namely active, saturation, and cut-
off region.
 A transistor when used in the active region, the base-emitter junction is forward
biased and the collector-base junction is reverse biased.
 A transistor when used in the saturation region, the base-emitter junction is
forward biased and the collector-base junction is also forward biased.
 A transistor when used in the cut-off region, both the base-emitter junction and
collector-base junction are reverse biased.
Comparison of Transistor Configuration
The following table shows the comparison of transistor configuration.
Characteristics Common Emitter Common Base
Current Gain High No Considerable
Applications Audio frequency High frequency
Input Resistance Low Low Very high
Output Resistance High Very high Low
Voltage Gain Approx. 500 Approx. 150 Less than 1
Advantages and Disadvantages of Transistors
The following table lists the advantages and disadvantages of transistors.
Low source voltage Temperature dependency
High voltage gain Lower power dissipation
Smaller in size Low input impedance
Current Amplification Factor (α)
The ratio of change in the collector current to the change in the emitter current at constant
collector to base voltage Vcb is known as current amplification factor 'α'. It can be
expressed as
∆ Ic
α= at constant VCB
∆ IB
It is clear that current amplification factor is less than unity and it is inversely proportional
to the base current considered that the base is lightly doped and thin.
Base Current Amplification Factor (β)
It is the ratio of change in collector current to the change in base current. A small variation
in base current results in a very large change in collector current. Therefore, the transistor
is able to attain current gain. It can be expressed as
β=
Transistor as an Amplifier
The following figure shows that a load resistor (RL) is in series with the collector supply
voltage (Vcc). A small voltage change ∆Vi between the emitter and the base causes a
relatively large emitter-current change ∆IE.
We define by the symbol 'a' - the fraction of this current change - which is collected and
passes through RL. The change in output voltage across the load resistor ∆Vo = a'RL ∆IE
may be many times the change in input voltage ∆VI. Under these circumstances, the
voltage amplification A==VO/∆VI will be greater than unity and the transistor acts as an
22. Semiconductor Devices ─ Field Effect Transistors
A Field Effect Transistor (FET) is a three-terminal semiconductor device. Its operation is
based on a controlled input voltage. By appearance JFET and bipolar transistors are very
similar. However, BJT is a current controlled device and JFET is controlled by input voltage.
Most commonly two types of FETs are available.
 Junction Field Effect Transistor (JFET)
 Metal Oxide Semiconductor FET (IGFET)
Junction Field Effect Transistor
The functioning of Junction Field Effect Transistor depends upon the flow of majority
carriers (electrons or holes) only. Basically, JFETs consist of an N type or P type silicon
bar containing PN junctions at the sides. Following are some important points to remember
about FET:
 Gate: By using diffusion or alloying technique, both sides of N type bar are heavily
doped to create PN junction. These doped regions are called gate (G).
 Source: It is the entry point for majority carriers through which they enter into the
semiconductor bar.
 Drain: It is the exit point for majority carriers through which they leave the
 Channel: It is the area of N type material through which majority carriers pass
from the source to drain.
There are two types of JFETs commonly used in the field semiconductor devices:
N-Channel JFET and P-Channel JFET.
N-Channel JFET
It has a thin layer of N type material formed on P type substrate. Following figure shows
the crystal structure and schematic symbol of an N-channel JFET. Then the gate is formed
on top of the N channel with P type material. At the end of the channel and the gate, lead
wires are attached and the substrate has no connection.
When a DC voltage source is connected to the source and the drain leads of a JFET,
maximum current will flow through the channel. The same amount of current will flow
from the source and the drain terminals. The amount of channel current flow will be
determined by the value of VDD and the internal resistance of the channel.
A typical value of source-drain resistance of a JFET is quite a few hundred ohms. It is clear
that even when the gate is open full current conduction will take place in the channel.
Essentially, the amount of bias voltage applied at ID, controls the flow of current carriers
passing through the channel of a JFET. With a small change in gate voltage, JFET can be
controlled anywhere between full conduction and cutoff state.
P-Channel JFETs
It has a thin layer of P type material formed on N type substrate. The following figure
shows the crystal structure and schematic symbol of an N-channel JFET. The gate is formed
on top of the P channel with N type material. At the end of the channel and the gate, lead
wires are attached. Rest of the construction details are similar to that of N- channel JFET.
Normally for general operation, the gate terminal is made positive with respect to the
source terminal. The size of the P-N junction depletion layer depends upon fluctuations in
the values of reverse biased gate voltage. With a small change in gate voltage, JFET can
be controlled anywhere between full conduction and cutoff state.
Output Characteristics of JFET
The output characteristics of JFET are drawn between drain current (I D) and drain source
voltage (VDS) at constant gate source voltage (VGS) as shown in the following figure.
Initially, the drain current (ID) rises rapidly with drain source voltage (VDS) however
suddenly becomes constant at a voltage known as pinch-off voltage (VP). Above pinch-off
voltage, the channel width becomes so narrow that it allows very small drain current to
pass through it. Therefore, drain current (ID) remains constant above pinch-off voltage.
Parameters of JFET
The main parameters of JFET are:
 AC drain resistance (Rd)
 Transconductance
 Amplification factor
AC drain resistance (Rd): It is the ratio of change in the drain source voltage ( VDS) to
the change in drain current ( ID) at constant gate-source voltage. It can be expressed
Rd = (∆ VDS) / (∆ ID) at constant VGS
Transconductance (gfs ): It is the ratio of change in drain current ( ∆ ID) to the change
in gate source voltage (∆ VGS) at constant drain-source voltage. It can be expressed as,
gfs = (∆ID)/ (∆VGS) at constant VDS
Amplification Factor (u): It is the ratio of change in drain-source voltage (∆VDS) to the
change in gate source voltage (∆VGS) constant drain current (∆ID). It can be expressed as,
u = (∆ VDS) / (∆VGS) at constant ID
23. Semiconductor Devices ─ JFET Biasing
There are two methods in use for biasing the JFET: Self-Bias Method and Potential Divider
Method. In this chapter, we will discuss these two methods in detail.
Self-Bias Method
The following figure shows the self-bias method of n-channel JFET. The drain current flows
through Rs and produces the required bias voltage. Therefore, Rs is the bias resistor.
Therefore, voltage across bias resistor,
Vs = IDRs
As we know, gate current is negligibly small, the gate terminal is at DC ground, VG=o,
VGS = VG – VS = 0-IDRS
Or VGS = -IDRS
VGS keeps gate negative w.r.t. to the source.
Voltage Divider Method
The following figure shows voltage divider method of biasing the JFETs. Here, resistor R1
and R2 form a voltage divider circuit across drain supply voltage (VDD), and it is more or
less identical to the one used in transistor biasing.
The voltage across R2 provides necessary bias:
=V2 + VGS + ID + RS
or VGS = V2 – IDRS
The circuit is so designed that VGS is always negative. The operating point can be found
using the following formula:
ID = V2-VGS
and VDS = VDD – ID (RD + RS)
24. Semiconductor Devices ─ MOSFET
Metal-oxide semiconductor field-effect transistors, also known as MOSFETs, have
greater importance and are a new addition to the FET family.
It has a lightly doped P type substrate into which two highly doped N type zones are
diffused. A unique feature of this device is its gate construction. Here, the gate is
completely insulated from the channel. When voltage is applied to the gate, it will develop
an electrostatic charge.
At this point of time, no current is allowed to flow in the gate region of the device. Also,
the gate is an area of the device, which is coated with metal. Generally, silicon dioxide is
used as an insulating material between the gate and the channel. Due to this reason, it is
also known as insulated gate FET. There are two MOSFETS widely used i) Depletion
MOSFET ii) Enhancement MOSFET.
D MOSFET
The following figures show n-channel D-MOSFET and the symbol. The gate forms a
capacitor with gate as one plate, and the other plate is the channel with SiO2 layer as
dielectric. When the gate voltage varies, the electric field of the capacitor changes, which
in turn varies the resistance of the n-channel.
In this case, we can either apply positive or negative voltage to the gate. When MOSFET
is operated with negative gate voltage, it is called depletion mode and when operated with
positive gate voltage it is called as enhancement mode of operation of MOSFET.
Depletion Mode
The following figure shows an n-channel D-MOSFET under depletion mode of operation.
Its operation is as follows:
 Most electrons are available on the gate as the gate is negative and it repels the
electrons of n channel.
 This action leaves positive ions in the part of the channel. In other words, some of
the free electrons of the n channel are depleted. As a result, less number of
electrons are available for current conduction through the n channel.
 The greater the negative voltage at the gate, less is the current from the source to
the drain. Thus, we can change the resistance of the n channel and the current
from the source to the drain by varying the negative voltage on the gate.
Enhancement Mode
The following figure shows n channel D MOSFET under enhancement mode of operation.
Here, the gate acts as a capacitor. However, in this case the gate is positive. It provokes
the electrons in the n channel and the number of electrons increases in the n channel.
A positive gate voltage enhances or increases conductivity of the channel. The larger the
positive voltage on the gate, greater the conduction from the source to the drain.
Thus, we can change the resistance of the n channel and the current from the source to
the drain by varying the positive voltage on the gate.
Transfer Characteristics of D – MOSFET
The following figure shows transfer characteristics of D-MOSFET.
When VGS goes negative, ID falls below the value of IDSS, till it reaches zero and VGS = VGS
(off) (Depletion mode). When VGS is zero, ID = IDSS because the gate and the source
terminals are shorted. ID increases above the value of IDSS, when VGS is positive and the
MOSFET is in enhancement mode.
25. Semiconductor Devices – Operational Amplifiers
An operational amplifier, or op-amp, is a very high gain differential amplifier with high
input impedance and low output impedance. Operational amplifiers are typically used to
provide voltage amplitude changes, oscillators, filter circuits, etc. An op-amp may contain
a number of differential amplifier stages to achieve a very high voltage gain.
This is a high gain differential amplifier using direct coupling between the output and the
input. This is suitable for DC as well as AC operations. Operational amplifiers perform
numerous electronic functions such as instrumentation devices, signal generators, active
filters, etc. besides various mathematical operations. This versatile device is also used in
many non–linear applications, such as voltage comparators, Analog–to–digital converters
and Digital–to–Analog converters, Logarithmic amplifiers, non–linear function generators,
Basic Differential Amplifier
The following illustration shows a basic differential amplifier:
In the above figure -
 VDI = differential input
 VDI = V1 –V2
 VDO = differential output
 VDO = VC1 –VC2
This amplifier amplifies the difference between the two input signals, V1 and V2.
Differential voltage gain,
Ad = VDO/VDI
Ad = (VC1 –VC2) / VDI
As shown in the following figure, basic operational amplifier consists of three stages:
Input Stage
This is the first stage and has the following characteristics.
 High CMR (Common Mode Rejection)
 High input impedance
 Wide band width
 Low (DC) input offset
These are some significant characteristics for the performance of the operational amplifier.
This stage consists of a differential amplifier stage and a transistor is biased so that it acts
as a constant current source. The constant current source greatly increases the CMR of
the differential amplifier.
Following are the two inputs to the differential amplifier:
 V1 = Non inverting input
 V2 = Inverting input
This is the second stage and designed to get better voltage and current gains. The current
gain is required to supply sufficient current to drive the output stage, where most of the
operational amplifier power is generated. This stage consists of one or more differential
amplifiers followed by an emitter follower and a DC level shifting stage. Level shifting
circuit enables an amplifier to have two differential inputs with a single output.
Vout = +ve when V1 > V2
Vout = –ve when V2 < V1
Vout = 0 when V1 = V2
Output Stage
This is the last stage of the op-amp and is designed to have low output impedance. This
provides the needed current to drive the load. More or less current will be drawn from the
output stage as and when the load varies. Therefore, it is essential that the previous stage
operates without being influenced by the output load. This requirement is met by designing
this stage so as to have high input impedance and high current gain, however with low
output impedance.
The operational amplifier has two inputs: Non-inverting input and Inverting input.
The above figure shows inverting type of operational amplifier. A signal which is applied
at the inverting input terminal is amplified however the output signal is out of phase with
the input signal by 180 degrees. A signal applied at the non–inverting input terminal is
amplified and the output signal is in phase with the input signal.
The op-amp can be connected in large number of circuits to provide various operating
characteristics.
26. Semiconductor Devices – Practical Op-Amps
Inverting Amplifier
The following figure shows an inverting amplifier. The input signal is amplified and
inverted. This is the most widely used constant-gain amplifier circuit.
VO = –Rf. Vin/R1
Voltage gain A = (–Rf / R1)
Non-Inverting Amplifier
The following figure shows an op-amp circuit that works as a non-inverting amplifier or
constant-gain multiplier and it has better frequency stability.
The input signal is amplified but it is not inverted.
Output VO = [(R1+Rf)/R1] V1
Voltage gain A = (R1+ Rf)/R1
Inverting Summing Amplifier
The following figure shows an inverting summing amplifier. It is the most used circuit of
the op-amp. The circuit shows a three-input summing amplifier, which provides a means
of algebraically summing three voltages, each multiplied by a constant-gain factor. The
output voltage is expressed as,
VO = [(–R4/R1)V1] [(–R4/R2)V2] [(–R4/R3)V3]
VO = –R4 (V1/R1 + V2/R2 + V3/R3)
If, R1 = R2 = R3 = R4 = R & R5 = R/3.
VO = – (V1 + V2 + V3)
27. Semiconductor Devices ─ Integrator
The following figure shows that the feedback component used is a capacitor and the
resulting connection is called as an integrator.
The virtual-ground equivalent shows that an expression for the voltage between the input
and the output can be derived in terms of the current (I), from the input to the output.
Recall that virtual ground means we can consider the voltage at the junction of R and X C
to be ground (since Vi ≈ 0 V) however no current goes into the ground at that point. The
capacitive impedance can be expressed as
Where s =jw as in the Laplace notation. Solving equation for Vo/Vi yields the following
V1 −V0 − 0 V0
I= = = =
R1 Xc sC V1
V0 −1
V1 sCR1
It can be written in the time domain as
28. Semiconductor Devices ─ Differentiator
A differentiator circuit is shown in the following figure.
The differentiator provides a useful operation, the resulting relation for the circuit being
Following are some important parameters of Operational amplifier:
Open Loop Voltage Gain (AVOL)
The open loop voltage gain of an operational amplifier is its differential gain under
conditions where no negative feedback is used. AVOL ranges from 74 db to 100 db.
AVOL = [VO/(V1–V2)]
Output Offset Voltage (VOO)
The output offset voltage of an operational amplifier is its output voltage when its
differential input voltage is zero.
Common Mode Rejection (CMR)
If both the inputs are at the same potential, causing the differential input zero, and if the
output is zero, the operational amplifier is said to have a good common mode rejection.
Common Mode Gain (AC)
Common mode gain of an operational amplifier is the ratio of the common mode output
voltage to the common mode input voltage.
Differential Gain (AD)
The differential gain of an operational amplifier is the ratio of the output to the differential
Ad = [VO/(V1–V2)]
Common Mode Rejection Ratio (CMRR)
CMRR of an operational amplifier is defined as the ratio of the closed loop differential gain
to the common mode gain.
CMRR = Ad/AC
Slew Rate (SR)
Slew rate is the rate of output voltage change caused by a step input voltage. An ideal
slew rate is infinite, which means that the operational amplifier output should change
instantly in response to an input step voltage.
We have already discussed some applications of op-amp such as differentiator, integrator,
summing amplifier, etc. Some other common applications of operational amplifiers are:
 Logarithmic amplifier
 Gyrator (Inductance simulator)
 DC & AC voltage follower
 Analog to Digital converter
 Digital to Analog converter
 Power supplies for over voltage protection
 Polarity indicator
 Voltage follower
 Active filters
29. Semiconductor Devices ─ Oscillators
An oscillator is an electronic circuit that generates sinusoidal oscillations known as
sinusoidal oscillator. It converts input energy from a DC source into AC output energy
of periodic waveform, at a specific frequency and is known amplitude. The characteristic
feature of the oscillator is that it maintains its AC output.
The following figure shows an amplifier with feedback signal even in the absence of an
externally applied input signal. A sinusoidal oscillator is essentially a form of feedback
amplifier, where special requirements are placed on the voltage gain Av and the feedback
networks β.
Consider the feedback amplifier of the above figure, where the feedback voltage Vf = βVO
supplies the entire input voltage
𝑉𝑖 = 𝑉𝑓 = 𝛽𝑉𝑂 = 𝐴𝑣 𝛽𝑉𝑖 (1)
𝑉𝑖 = 𝐴𝑣 𝛽𝑉𝑖 or (1 − 𝐴𝑣 𝛽)𝑉𝑖 = 0 (2)
If an output voltage is to be produced, the input voltage cannot be zero. Hence, for 𝑉𝑖 to
exist, Equation (2) requires that
(1 − 𝐴𝑣 𝛽) = 0 or 𝐴𝑣 𝛽 = 1 (3)
Equation (3) is known as "Barkhausen criterion", which states two basic requirements
for oscillation:
 The voltage gain around the amplifier and feedback loop, called the loop gain, must
be unity, or 𝐴𝑣 𝛽 = 1.
 The phase shift between 𝑉𝑖 and 𝑉𝑓 , called the loop phase shift, must be zero.
If these two conditions are satisfied, the feedback amplifier of the above figure will
generate a sinusoidal output waveform consistently.
Let us now discuss in detail about some typical oscillator circuits.
Phase Shift Oscillator
An oscillator circuit that follows the fundamental progress of a feedback circuit is the
phase-shift oscillator. A phase-shift oscillator is shown in the following figure. The
requirements for oscillation are that the loop gain (βA) should be greater than unity and
the phase shift between input and output should be 360o.
Feedback is provided from the output of the RC network back to the amplifier input. The
op-amp amplifier stage provides an initial 180-degree shift and the RC network introduces
an additional amount of phase shift. At a specific frequency, the phase shift introduced by
the network is exactly 180 degrees, so the loop will be 360 degrees and the feedback
voltage is in phase input voltage.
The minimum number of RC stages in the feedback network is three, as each section
provides 60 degrees of phase shift. The RC oscillator is ideally suited to the range of audio
frequencies, from a few cycles to approximately 100 KHz. At the higher frequencies, the
network impedance becomes so low that it may seriously load the amplifier, thereby
reducing its voltage gain below the required minimum value, and the oscillations will cease.
At low frequencies, the loading effect is not usually a problem and the required large
resistance and capacitance values are readily available. Using the basic network analysis,
frequency oscillation can be expressed as
Wien Bridge Oscillator
A practical oscillator circuit uses an op-amp and RC bridge circuit, with the oscillator
frequency set by the R and C components. The following figure shows a basic version of a
Wien bridge oscillator circuit.
Note the basic bridge connection. Resistors R1 and R2 and capacitors C1 and C2 form the
frequency-adjustment elements, while resistors R3 and R4 form part of the feedback path.
In this application, the input voltage (Vi) to the bridge is the amplifier output voltage, and
the output voltage (Vo) of the bridge is feedback to the amplifier input. Neglecting the
loading effects of the op-amp input and output impedances, the analysis of the bridge
circuit results in
If R1 = R2 = R and C1 = C2 = C, the resulting oscillator frequency is
Hartley Oscillator
The following figure shows the Hartley oscillator. It is one of the most common RF circuits.
It is normally used as the local oscillator in a communication broadcast receiver. The
bipolar junction transistor in the common emitter connection is the voltage amplifier and
is biased by a universal bias circuit consisting of R1, R2, RE. Emitter bypass capacitor (CE)
increases the voltage gain of this single transistor stage.
The Radio Frequency Choke (RFC) in the collector circuit acts as an open circuit at the RF
frequency and prevents RF energy from entering the power supply. The tank circuit
consists of L1, L2, and C. The frequency of oscillations is determined by the value of L1,
L2, and C and is determined by the oscillates at the resonant frequency of the LC tank
circuit. This resonant frequency is expressed as
The output signal can be taken from the collector by capacitive coupling, provided that the
load is large and the frequency of oscillation is not affected.
Piezoelectricity
Piezoelectric properties are exhibited by a number of natural crystal substances, of which
the most important are quartz, Rochelle salt, and tourmaline. When a sinusoidal voltage
is applied across these materials, they vibrate at the applied voltage frequency.
On the other hand, when these materials are compressed and placed under mechanical
strain to vibrate, they produce an equivalent sinusoidal voltage. Therefore, these materials
are called as piezoelectric crystal. Quartz is the most popular piezoelectric crystal.
Crystal Oscillator
The circuit diagram of the crystal oscillator is shown in the following figure.
The crystal here acts as a tuned circuit. The equivalent circuit of a crystal is given below.
A crystal oscillator has two resonant frequencies: Series Resonant Frequency and Parallel
Resonant Frequency.
Series Resonant Frequency:
Parallel Resonant Frequency:
The two resonant frequencies are almost same, since C/Cm is very small. In the above
figure, the crystal is connected to operate in parallel resonant mode.
The resistors R1, R2, RE, and transistor together form an amplifier circuit. Resistors R1
and R2 provide a voltage stabilized DC bias. The capacitor (CE) provides AC bypass of the
emitter resistor (RE) and the RFC provides high impedance to the frequency generated by
the oscillator, so that they do not enter the power lines.
The crystal is in parallel with capacitor C1 and C2 and permits maximum voltage feedback
from the collector to the emitter, when its impedance is maximum. At other frequencies,
the crystal impedance is low and so the resultant feedback is too small to sustain
oscillations. The oscillator frequency is stabilized at the parallel resonant frequency of the
crystal.
30. Semiconductor Devices – Feedback & Compensation
The basic purpose of the bias network is to establish collector–base–emitter voltage and
current relationships at the operating point of the circuit (the operating point is also known
as the quiescent point, Q–point, no–signal point, idle point, or static point). Since,
transistors rarely operate at this Q-point, the basic bias networks are generally used as a
reference or starting point for design.
The actual circuit configuration and especially, the bias network values are selected on the
basis of dynamic circuit conditions (desired output voltage swing, expected input signal
level, etc.) Once the desired operating point is established, the next function of the bias
network is to stabilize the amplifier circuit at this point. The basic bias network must
maintain the desired current relationships in the presence of temperature and power
supply changes, and possible transistor replacement.
In some cases, frequency changes and changes caused by the component again must also
be offset by the bias network. This process is generally referred to as bias stabilization.
Proper bias stabilization will maintain the amplifier circuit at the desired operating point
(within practical limits), and will prevent thermal runaway.
Stability Factor 'S'
It is defined as the rate of change of collector current w.r.t. reverse saturation current,
keeping β and VBE constant. It is expressed as
𝑑𝐼𝑐
𝑆=
𝑑𝐼𝑜
Bias Stabilization Methods
The method of making the operating point independent of temperature changes or
variations in transistors' parameters is known as stabilization. There are several schemes
for providing bias stabilization of solid–state amplifiers. All these schemes engage a form
of negative feedback. That is any stage in transistor currents produce a corresponding
voltage or current change that tends to counterbalance the initial change.
There are two fundamental methods for producing negative feedback, inverse–voltage
feedback and inverse–current feedback.
Inverse-Voltage Feedback
The following figure shows the basic inverse–voltage bias network. The emitter–base
junction is forward biased by the voltage at the junction of R 1 and R2. The base–collector
junction is reverse biased by the differential between voltages at the collector and the
Normally, the collector of a resistance coupled amplifier is at a voltage about one half that
of the supply Resistor (R3), connected between the collector and base. Since the collector
voltage is positive, a portion of this voltage is feedback to the base to support the forward
bias.
The normal (or Q point) forward bias on the emitter–base junction is the result of all the
voltages between the emitter and the base. As the collector current increases, a larger
voltage drop is produced across RL. As a result, the voltage on the collector decreases,
reducing the voltage feedback to the base through R3. This reduces the emitter–base
forward bias, reducing the emitter current and lowering the collector current to its normal
value. As there is an initial decrease in the collector current, an opposite action takes
place, and the collector current is raised to its normal (Q point) value.
Any form of negative or inverse feedback in an amplifier has a tendency to oppose all
changes even those produced by the signal being amplified. This inverse or negative
feedback tends to reduce and stabilize gain, as well as undesired change. This principle of
stabilizing gain by means of feedback is used in more or less all types of amplifiers.
Inverse-Current Feedback
The following figure shows a distinctive inverse–current (emitter– feedback) bias network
using an NPN transistor. Current feedback is more commonly used than voltage feedback
in solid–state amplifiers. This is because transistors are mainly current–operated devices,
rather than voltage–operated devices.
The use of an emitter–feedback resistance in any bias circuit can be summed up as follows:
Base current depends upon the differential in voltage between the base and the emitter.
If the differential voltage is lowered, less base current will flow.
The opposite is true when the differential is increased. All current flowing through the
collector. The voltage drops across the emitter resistor and is therefore not fully
dependent. As the collector current increases, the emitter current and the voltage drop
across the emitter resistor, will also increase. This negative feedback tends to decrease
the differential between the base and the emitter, thus lowering the base current. In turn,
the lower base current tends to decrease the collector current, and counterbalance the
initial collector–current increases.
Bias Compensation
In solid state amplifiers, when the loss in signal gain is intolerable in a particular
application, compensating techniques are often used to reduce the drift of operating point.
In order to provide maximum bias and thermal stabilization, both compensation and
stabilization methods can be employed together.
The following figure shows diode compensation technique that utilized both the diode
compensation and self-bias stabilization. If both diode and transistor are of the same type,
then they have the same temperature coefficient across the circuit. Here, diode is forward
biased. KVL for the given circuit can be expressed as:
It is clear from the above equation that VBE follows VO w.r.t. temperature and Ic will have
no effect to variations in VBE. This is an effective method to take care of the operating point
of the transistor due to variation in VBE.
Temperature Compensation Device
We can also use some temperature sensitive device to compensate for variations of
transistor internal characteristics. Thermistor has a negative temperature coefficient,
which means with the rise in temperature, its resistance decreases exponentially. The
following figure shows a circuit that uses thermistor (RT) to reduce the increase in collector
current due to change in VBE, ICO, or β with temperature.
When temperature increases, RT decreases and the current fed through RT into RE
increases. The action voltage drop across RE is in the opposite direction to reverse bias the
transistor. RT acts so as to tend to compensate the increase in IC, which increases due to
the rise in temperature.
P–N Junction
Doping (Semiconductor)
Chemical Bond
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