text: field - effect transistors exist in two major classifications , the junction fet ( jfet ) and the metal - oxide - semiconductor fet ( mosfet ). a mosfet is a special type of fet that works by electronically varying the width of a channel along which charge carriers ( electrons or holes ) flow . wider channels provide better conductivity . the charge carriers enter the channel at the source , and exit via the drain . the width of the channel is controlled by the voltage on an electrode called the gate , which is located physically between the source and the drain and is insulated from the channel by an extremely thin layer of metal oxide . there are two ways in which a mosfet can function . the first is known as depletion mode . when there is no voltage on the gate , the channel exhibits its maximum conductance . as the voltage on the gate increases ( either positively or negatively , depending on whether the channel is made of p - type or n - type semiconductor material ), the channel conductivity decreases . the second mode of mosfet operation is called enhancement mode . when there is no voltage on the gate , there is in effect no channel , and the device does not conduct . a channel is produced by the application of a voltage to the gate . increasing gate voltage increases conductivity and thus , current flow . the mosfet has certain advantages over the conventional junction fet , or jfet because the gate is insulated electrically from the channel . no current flows between the gate and the channel , regardless of the gate voltage ( as long as it does not become so great that it causes physical breakdown of the metallic oxide layer ). thus , the mosfet has practically infinite impedance . when conventional non - radiation hardened n channels fets are used in applications where radiation is present , the fets become uncontrollable at relatively low radiation levels because the gate threshold voltage of the n channel fet experiences a negative shift , and ultimately falls close to zero . at that point , the n channel fet conducts current with little or no gate voltage applied making it uncontrollable , like a flood gate that cannot be closed . the gate threshold voltage of a conventional , non - radiation hardened p channel fet also shifts negatively with radiation exposure . however , the initial threshold voltage of an ordinary p channel fet is negative to begin with . in the presence of radiation , therefore , the gate threshold voltage does not approach zero and therefore will not become uncontrollable . the gate threshold voltage does change , but from a negative value to a more negative value . conventional p channel fets , therefore , are more robust to total radiation dose effects as compared to conventional n channel fets when the proper gate drive signal is provided . in accordance with an embodiment of the present invention , the gate drive signal should be high enough to saturate the drain to source channel . it should not , however , be so high that the gate to source breakdown voltage rating of the fet is exceeded . preferably , the fet operates close to its maximum gate voltage signal . higher signals can handle higher radiation levels , allowing the fet to function across a larger range of radiation exposure . fig1 is a schematic diagram for a circuit according to a preferred embodiment of the present invention . this particular example operates from a voltage source of 22 vdc to 50 vdc , but nominally 28 vdc . it may , however , be readily scaled for different bus voltages . the circuit consists of an under voltage lockout portion indicated by dashed line 100 , and an inrush limiter portion indicated by dashed line 200 . a common line 300 , a load input line 400 , and a positive input line 500 are all operably connected to the circuit . the common line 300 carries the output of the circuit and the other lines 100 , 200 carry the input to the circuit . for the under voltage lockout portion 100 , under - voltage - lockout - voltage reference - zener diode 21 acts as a voltage reference for the under voltage lockout . the diode 21 and the base emitter junction of the under - voltage - lockout - pnp - bipolar transistor 31 form a voltage reference . bias - current - resistor 41 supplies a minimum , or bias current through the reference - diode 21 to stabilize its zener operation . the diode 22 is operably connected to a filter capacitor 11 . a first - voltage - divider - resistor 44 and second - voltage - divider - resistor 45 form a voltage divider that senses the bus voltage and determines the operating point of the under voltage lockout function . a first - collector - pull - up - resistor 42 and second - collector - pull - up - resistor 43 act as the collector pull up for the transistor 31 . the common connection of resistors 42 and 43 drives the base of the under - voltage - lockout - circuit - npn - bipolar transistor 32 . feeding - current - limiting - resistor 412 limits the current fed to the drive - circuit - stabilizing - zener diode 22 that establishes a stabilized internal voltage with reference to the positive bus voltage . a first - collector - load - resistor 49 is a collector pull - up for the npn transistor 32 . a hysteresis resistor 46 provides a hysteresis action to the operation of the transistor 31 , to prevent intermediate state operation . when the bus voltage exceeds the desired minimum ( approximately 22 vdc in this example ), the under - voltage - lockout - npn - bipolar transistor 32 conducts and applies voltage to one end of a second - collector - load - resistor 410 . this allows the inrush limiter portion 200 to operate . the inrush limiter portion 200 consists of a common - base - amplifier - pnp - bipolar transistor 33 and a gate - drive - voltage - buffer - pnp - bipolar transistor 34 as well as a p - channel fet 35 . the common - base - amplifier - pnp - bipolar transistor 33 is used as a common base amplifier and the gate - drive - voltage - buffer - pnp - bipolar transistor 34 is used to buffer the gate drive voltage to the p channel mosfet 35 . a first - reference - voltage diode 23 and a second - reference - voltage diode 24 establish a reference voltage for the common - base - amplifier - pnp - bipolar transistor 33 . the established reference voltage is slightly more negative than the input bus . diodes 23 and 24 are connected to the return leg of the input bus through bias - current resistor 48 . the purpose of these components is to establish a bias voltage for the base of the common - base - amplifier - pnp - bipolar transistor 33 that should be two diode drops more negative than the positive bus voltage . the common - base - amplifier - pnp - bipolar transistor 33 provides a non - inverting stage of voltage gain . the collector load resistor for the transistor 33 is the second - collector - load - resistor 410 . a reverse - voltage - limiting diode 25 limits the reverse voltage across the base emitter terminal of the common - base - amplifier - pnp - bipolar transistor 33 . a gate - drive - voltage - buffer transistor 34 is a pnp bipolar transistor that acts as an emitter follower , lowering the impedance of the signal on the collector of the common - base - amplifier - pnp - bipolar transistor 33 . a gate - to - source - filter resistor 411 and a current - limiting - resistor 412 act as filter components across the gate - source terminals of the mosfet 35 . a noise - filter - decoupling capacitor 12 is connected across the filter resistor 411 . when bus power is applied , the drain voltage of the mosfet 35 is initially at ground potential . as the mosfet 35 conducts and its drain voltage rises , current flows through a timing capacitor 13 . most of this current flows through a timing resistor 47 , however a small current flows into the emitter of the mosfet 33 . since the emitter voltage of the mosfet 33 should be constant at approximately 1 . 8 vdc below the positive rail , the current through the timing resistor 47 is relatively constant during the turn - on interval . therefore , the current through the timing capacitor 13 should be relatively constant as well . the relatively constant current through the timing capacitor 13 establishes a relatively linear ramp voltage across it . therefore , the output voltage at the drain terminal of the mosfet 35 rises from zero to the positive bus voltage in a linear manner . by controlling the turn on rise time of the mosfet 35 , inrush current flowing into downstream capacitors is determined and controlled . this circuit provides a relatively constant power rise time to the load , essentially independent of the gate to source threshold voltage of the p channel fet 35 . since the gate to source threshold voltage of the fet 35 changes considerably as a result of exposure to ionizing radiation , this circuit produces a relatively uniform power application rise time to the load , despite the parametric changes due to radiation . in the preceding specification , the invention has been described with reference to specific exemplary embodiments thereof . it will however , be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims that follow . the specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense .