1. Field of the Invention
The present invention relates to a Gunn diode using an InP semiconductor to be used as a microwave source or a millimeter wave source.
2. Description of the Background Art
It is well known that a drift velocity of conduction electrons in an InP semiconductor has a property called negative differential resistance, i.e., it decreases for an electric field strength greater than a certain level. This is graphically shown in FIG. 1, where a vertical axis represents a drift velocity V of the conduction electron and a horizontal axis represents an electric field strength E. As shown in FIG. 1, in a region A1 the drift velocity V increases as the electric field strength E increases, so that the differential resistance which is inversely proportional to dV/dE has a positive value in this region A1. On the other hand, in a region A2 the drift velocity V decreases as the electric field strength E increases until it reaches to a saturation velocity Vs, so that the differential resistance has a negative value in this region A2, and the drift velocity is said to have a property called the negative differential resistance.
This negative differential resistance is caused by the fact characteristic to this type of a semiconductor that the effective mass of the conduction electron at a high energy level is heavier than an effective mass of the conduction electron at a low energy level.
A Gunn diode which utilizes this property of the negative differential resistance is widely used as a solid state oscillator element for generating microwaves and millimeter waves.
An example of a conventional Gunn diode is shown in FIG. 2, where the Gunn diode comprises an n-type active layer 3 sandwiched by two n.sup.+ -type layers 4 and 5, where metallic electrodes 6 and 7 are connected to the n.sup.+ -type layers 4 and 5, respectively. A distribution of a donor impurity concentration Nd in this Gunn diode of FIG. 2 is shown in FIG. 3. Here, the contacts between the n.sup.+ -type layers 4 and 5 and the metallic electrodes 6 and 7 can easily be made to be ohmic contacts, because the donor concentration of the n.sup.+ -type layers 4 and 5 is sufficiently high as shown in FIG. 3.
In this configuration, by setting a thickness and the donor impurity concentration of the active layer 3 appropriately, and applying an appropriate DC voltage between the two electrodes 6 and 7, a structure called a Gunn domain can be formed in an negative electrode side (cathode side) of the active layer 3, due to the negative differential resistance property of the semiconductor conduction electrons. In the Gunn domain, a region in which the conduction electron concentration is greater than surrounding (accumulation layer) or a portion at which a region with greater conduction electron concentration and a region with less conduction electron concentration are facing each other (dipole layer) is generated, and such a portion moves toward the positive electrode side (anode side) of the active layer 3. Such a Gunn domain disappears when it reaches to the n.sup.+ -type layer of the anode side, and then another Gunn domain is generated at the cathode side.
As this disappearance of an old Gunn domain and an appearance of a new Gunn domain is repeated, it appears that an AC current is superposed onto a DC current between the electrodes, where the frequency of this AC current is the characteristic frequency of the Gunn diode which is mainly determined from a time for the Gunn domain to move through the active layer 3 from the cathode side to the anode side. On the other hand, the velocity of the Gunn domain in the active layer 3 is approximately equal to the saturation velocity Vs of the conduction electrons, so that the characteristic frequency f of the Gunn diode is roughly given as f=Vs/L where L is a thickness of the active layer 3. Conversely, an appropriate thickness L of the active layer 3 can be given as a function of the desired characteristic frequency f in a form of L=Vs/f. Thus, it can be said that the thickness of the active layer is roughly inversely proportional to the characteristic frequency f.
Such a Gunn diode is widely used as an oscillator for high frequency waves. For example, the high frequency electromagnetic waves such as the microwaves can be obtained from the Gunn diode by appropriately arranging the Gunn diode inside a resonant cavity. Such an oscillator using the Gunn diode is called a Gunn oscillator, for which the most important quality index is a conversion efficiency indicating how much AC output can be obtained from a given DC power input, and the improvement of this conversion efficiency in the Gunn oscillator has been a major problem associated with a conventional Gunn oscillator.
Now, in such a Gunn diode having n.sup.+ nn.sup.+ structure, it is known that the Gunn domain is generated not exactly at a contact plane between the active layer and the cathode side n.sup.+ -type layer, but at a position inside the active layer which is a certain distance inward from the contact plane, as shown in FIG. 4. This FIG. 4 shows a result of a computer simulation of a Gunn domain movement, which clearly indicates an existence of a region A nearby the cathode side n.sup.+ -type layer contact plane in which no Gunn domain is present.
This phenomenon can be explained as follows. Namely, the conduction electrons entering into the active layer have not gained sufficient amount of energy from the electric field so that their energy is not sufficiently high for realizing the negative differential resistance property which is indispensable for the formation of the Gunn domain. On the other hand, as the conduction electrons passes through the active layer for a while, these conduction electrons are raised to the high energy states and start to have the negative differential resistance property, so that the formation of the Gunn domain become possible.
The region A between the cathode side n.sup.+ -type layer contact plane and the actual Gunn domain generation position where no Gunn domain is called a dead zone. The existence of this dead zone severely damages the characteristics of the Gunn diode, so that it is highly desirable to make this dead zone as small as possible, or to remove this dead zone completely. In particular, in a case of using the Gunn diode for an application using a high frequency over 40 GHz, the appropriate thickness of the active layer becomes as thin as less than 2 .mu.m as explained above, so that the thickness of the dead zone becomes unignorable compared with the thickness of the active layer, and the existence of the dead zone affects the conversion efficiency of the Gunn diode considerably.
As a method of removing the dead zone, there is known a configuration in which the cathode side n.sup.+ -type layer is removed and the metallic electrode is connected directly to the active layer on the cathode side, as shown in FIG. 5. In this configuration of FIG. 5, where the anode side metallic electrode 9 remains to contact with the anode side n.sup.+ -type layer 5, while the cathode side metallic electrode 8 is directly contacting the n-type active layer 3. In such a configuration, the contact between the active layer 3 and the cathode side metallic electrode 8 is not an ohmic contact but a Schottky contact, so that the dead zone can be removed completely by appropriately setting the characteristic of this Schottky contact, and as a result the conversion efficiency can be improved.
However, in the Gunn diode of this configuration, the appropriate setting of the Schottky contact is extremely difficult because a minute change in the Schottky contact can affect the Gunn diode characteristic considerably. In addition, compared with the ohmic contact, the Schottky contact is far more difficult technically to manufacture at a high reproducibility. Furthermore, the characteristic of the Schottky contact is quite sensitive to the environmental temperature, so that the reliability of this type of Gunn diode has been rather low.