Unit III FET and its Applications 2 Marks Questions and Answers 1. Why do you call FET as field effect transistor? The name field effect is derived from the fact that the current is controlled by an electric field set up in the device by an external voltage, applied across gate and source terminals, which reverse bias the junctions. 2. What is a FET? A field effect (FET) is a three terminal semiconductor device in which current conduction takes place by only one type of carriers (either holes or electron) and is controlled by an electric field. 3. Why FET is called an unipolar device? The operation of FET depends upon the flow of majority carriers only (either holes or electrons) so the FET is said to be an unipolar device. 4. Define pinch off voltage? It is the voltage at which the channel is pinched off, i.e. all the free charge from the channel get removed. At Pinch-off voltage V P the drain current becomes constant. 5. Define drain resistance? Drain resistance (rd) is defined as the ratio of small change in drain to source voltage ( Vds) to the corresponding change in drain current ( Id) at constant gate to source voltage (Vgs). rd = Vds / Id at constant gate to source voltage (Vgs) 6. Write down the relationship between various FET parameters? Amplification factor = drain resistance * Transconductance µ = rd * gm 7. Mention the applications of FET. FET is used as a buffer in measuring instruments, receivers since it has high i/p impedance and low o/p impedance. FETS are used in RF amplifiers in FM tuners and communication equipment for the low noise level. Since the device is voltage controlled, it is used as a voltage variable resistor in op-amps and tone controls. Used as phase shift oscillator because frequency drifts is low. 8. Why the input impedance of FET is more than that of a BJT? The input impedance of FET is more than that of a BJT because the input circuit of FET is reverse biased whereas the input circuit of BJT is forward biased.
9. What is meant by gate source threshold voltage of a FET? The voltage at which the channel is completely cur off and the drain current becomes zero is called as gate source threshold voltage. Also called as V GS(off). 10. Why N channel FET s are preferred over P channel FET s? In N channel FET the charge carriers are the electrons which have a mobility of about 1300 cm2/ VS, whereas in P channel FET s the charge carriers are the holes which have a mobility of about 500 cm2 /VS. the current in a semiconductor is directly proportional to mobility. Therefore the current in N channel FET is more than that of P channel FET. 11. How FET devices are classified? The Field Effect Transistor (FET) can be broadly classified into following categories: 12. What are the operating regions of a JFET? 1. Ohmic region 2. Pinch-off region 3. Breakdown region 13. Give the drain current equation of JFET. the relationship is quadratic: where VP is the pinch-off voltage and IDSS is the saturation drain current for VGS = 0 (i.e. gate shorted to source). 14. Define the amplification factor in the JFET Amplification factor (µ) is the negative of rate of change of drain voltage v DS with gate voltage V GS with keeping I D constant. Thus, µ v DS / v GS
15. What are the special features of FET? 1. Its operation depends upon the flow of majority carriers only. It is therefore a unipolar (one type of carrier) device. 2. It is simpler to fabricate and occupies less space in integrated form. 3. It exhibits a high input resistance, typically many Mega ohms. 4. It is less noisy than a bipolar transistor. 5. It exhibits zero offset voltage at zero drain current, and hencee makes an excellent signal chopper. 16. List the advantages of FET. Input impedance is very high. This allows high degree of isolation between the input & output circuit. Current carriers are not crossing the junctions hence noise is highly reduced. It has a negative temperature Co-efficient of resistance. This avoids the thermal runaway. It has a smaller size, longer life high efficiency. 17. What are the main drawbacks of FET? The main disadvantage of the FET is its relatively small gain band width product in comparison with that which can be obtained with a conventional transistor. 18. Draw the symbol of JFET. n-channel FET P-Channel FET 19. Define drain resistance and Transconductance.
20. Write Shockley s equation. 21. What are the precautions to be taken when handling MOSFET? Because the glass like silicon oxide layer below the gate is do thin, it can be pierced by too much voltage or even static electricity. To avoid damage to the device, MOSFETs are usually shipped with the leads shorted together. 22. What are the differences between BJT and JFET? Parameter Terminals Type of device Input Resistance To Control the signal Voltage gain Output current BJT Emitter, Base and Collector Current controlled Bi-polar device Low compared to FET BJT uses junctions High FET Source, Gate and Drain Voltage controlled Uni-polar device High A JFET uses a channel lower voltage gain 24. What are the parameters of JFET? 1. ac drain resistance(r d ) 2. transconductance (g m ) 3. Amplification factor (µ)
25. Draw the symbol for i) P-channel JFET, ii) N-channel JFET iii) P-channel depletion MOSFET iv) N-channel depletion MOSFET (v) P-channel enhancement MOSFET vi) N-channel enhancement MOSFET 26. What is Darlington connection? What are the benefits? If two transistors are connected as shown above then it is Darlington connection. Benefits (i) High Input Impedance (ii) High current gain (iii) Less space to integrate in ICs 27. Draw small signal model of Common source amplifier. 28. Why the input impedance in FET is very high in comparison with BJT? JFET have very high input impedance because of the reverse biased Gate-Source pn-junction.
29. Why is FET preferred as a Buffer Amplifier? FET is used as a buffer in measuring instruments, receivers since it has high input impedance and low output impedance. 30. In a n-channel JFET, IDSS = 20 m A and VP = -6 V. Calculate the drain current when VGS = -3 V. =20x10-3 x (1-(3/6)) 2 = 5mA 31. Determine the transconductance of a JFET if its amplification factor is 96 and drain resistance is 32 KΩ. µ = g m * r d µ = 96 and r d = 32X10^3 g m = 96/32X10^3 = 2.66mS 32. What is meant by cascade connection? In a multistage amplifier circuit, the output of one stage is connected to the input of the next stage. Mostly similar type of stages are cascaded. 33. What is meant by cascode connection? The cascode is a two-stage amplifier composed of a common source amplifier followed by common gate amplifier. The cascode improves input-output isolation (or reverse transmission) as there is no direct coupling from the output to input. This eliminates the Miller effect and thus contributes to a much higher bandwidth. Compared to a single amplifier stage, it also provides higher input impedance, high output impedance, higher gain or higher bandwidth. 34. Mention the operating modes of MOSFET. 1. Enhancement mode. 2. Depletion mode. 35. Comparison of n-channel FET and p-channel FET n-channel FET In an N-channel JFET the current carriers are electrons Mobility of electrons is large in N-channel JFET input noise is less in N-channel JFET transconductance is larger p-channel FET the current carriers are holes in a P-channel JFET. mobility of holes is poor in P-channel JFET high less
36. Draw the transfer characteristics for JFET and N-Channel D-MOSFET. The transfer characteristics is the plot of the drain current (I D ) V s Gate source voltage V GS. 38. Draw the high frequency model of JFET 39. Comparison of the three FET configurations (CS,CD and CG) Parameter CG CD CS Voltage gain High Low Medium Current gain Low High Medium Power gain Low Medium High Input / output phase relationship 0 0 180 Input resistance Low High Medium Output resistance High Low Medium Application Current Buffer Voltage Buffer Inverts Input
40. What are the differences between JFET and MOSFET? Parameter JFET MOSFET Basic difference in operation The transverse electric field applied across the reverse biased pn-junction controls the conductivity of the channel The transverse electric field applied across the insulating layer deposited on the semiconductor material controls the conductivity of the channel Input 10 8 ohm 10 10 to 10 15 ohm Resistance Gate leakage 10-9 A 10-12 A current Drain 0.1 to 1Mohm 1 to 50Kohm resistance Out put Much flatter because of high drain Not much flatter characteristic resistance Handling Not much care is needed Careful handling is a must precautions Fabrication Not easy to fabricate Easy to fabricate Operation Operated only in depletion mode DMOSFET can be operated in enhancement mode and depletion mode Advantages and uses High input impedance and low output impedance, low noise levels Zero offset voltage Symmetrical (drain and source can be interchange) Very useful in analog signal switching and digital VLSI 41. Comparison of E only MOSFET and DMOSFET Parameter E only MOSFET DMOSFET Modes of operation Constructional difference E- only (Enhancement mode only) Two modes of operation Depletion as well as enhancement mode. There is no channel in between Source and drain A channel is diffused between the source and drain based on the type of FET for (n-channel MOSFET a n-type channel is diffused between source and drain.
Symbol Non continuous channel is indicated by Break in channel line The channel line is continuous. 42. Comparison of n-channel MOSFET and p-channel MOSFET Parameter n-channel MOSFET p-channel MOSFET popularity p-channel enhancement MOSFET is very Not very popular popular because easier and cheaper to produce Mobility of carriers Electron mobility is faster The hole mobility is nearly 2.5 times lower than the electron Drain resistance and Packing density Turn on Less area to integrate and high packing densities Due to the +ve charged contaminants, the n-channel MOSFET may turn ON prematurely. mobility p-channel MOSFET occupies a larger area than an n-channel MOSFET having the same I D rating and less packing density Not affected by premature turn on.