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 VP 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 VGS(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 vDS with gate voltage

VGS with keeping ID constant.

Thus, µ ≡ ∆ vDS / ∆ vGS

15. What

1. Its

(o

2. It

3. It

4. It

5. It

ch

16. List

• In

ou

• Cu

• It

ru

• It

17. Wha

The m

comparis

18. Dra

n-chann

19. Defin

at are the spe

Its operation d

(one type of c

It is simpler to

It exhibits a hi

It is less noisy

It exhibits zer

chopper.

ist the advanta

Input impedan

output circuit.

Current carrie

It has a negat

runaway.

It has a smalle

hat are the m

he main disadv

arison with tha

raw the symb

annel FET

efine drain res

special feature

n depends upo

f carrier) devic

r to fabricate an

a high input res

isy than a bipo

zero offset volt

ntages of FET

dance is very h

uit.

riers are not cr

gative temperat

aller size, longe

main drawba

dvantage of th

that which can

mbol of JFET.

P-C

resistance and

ures of FET?

pon the flow o

vice.

e and occupies

resistance, typ

ipolar transisto

oltage at zero d

ET.

y high. This all

t crossing the ju

erature Co-effic

nger life high e

backs of FET

the FET is its

an be obtained

T.

Channel FET

and Transcond

of majority ca

ies less space in

typically many

stor.

ro drain current

allows high de

e junctions hen

fficient of resis

h efficiency.

ET?

its relatively sm

ed with a conv

ET

onductance.

y carriers only.

e in integrated

ny Mega ohms

ent, and hence

degree of isola

ence noise is

sistance. This

small gain ban

nventional tran

ly. It is therefo

ed form.

ms.

ce makes an ex

solation betwee

highly reduc

is avoids the th

band width pro

transistor.

efore a unipola

n excellent sign

een the input

uced.

e thermal

product in

olar

ignal

ut &

20. Wri

21. Wha

Because t

voltage or

with the l

22. Wha

Parameter

Terminals

Type of d

Input Res

To Contro

Voltage g

Output cu

24. Wha

1. ac drain

2. transco

3. Amplif

rite Shockley

hat are the p

se the glass like

e or even static

e leads shorted

hat are the di

eter

als

f device

esistance

ntrol the signal

e gain

t current

hat are the pa

rain resistance(

sconductance (

plification facto

ley’s equation

e precautions t

like silicon oxi

tic electricity.

ted together.

differences be

BJT

Emitter,

Current

Low com

nal BJT use

High

parameters of

ce(rd)

e (gm)

ctor (µ)

ion.

ns to be taken

oxide layer belo

To avoid da

s between BJT

ter, Base and C

ent controlled B

compared to F

uses junctions

s of JFET?

en when hand

elow the gate i

damage to the

JT and JFET?

d Collector

d Bi-polar dev

o FET

ns

ndling MOSF

te is do thin, it

he device, MO

T?

FET

Source

evice Voltag

High

A JFE

lower

SFET?

, it can be pierc

OSFETs are u

rce, Gate and D

ltage controlled

FET uses a cha

er voltage gain

ierced by too m

re usually shipp

nd Drain

lled Uni-polar

channel

ain

o much

ipped

lar device

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 = gm* rd

µ = 96 and rd = 32X10^3

gm = 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 p-channel FET

In an N-channel JFET the current carriers are

electrons

the current carriers are holes in a P-channel

JFET.

Mobility of electrons is large in N-channel JFET mobility of holes is poor in P-channel JFET

input noise is less in N-channel JFET high

transconductance is larger less

36. Draw the transfer characteristics for JFET and N-Channel D-MOSFET.

The transfer characteristics is the plot of the drain current (ID) Vs Gate source voltage VGS.

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

Resistance

108 ohm 10

10 to 10

15 ohm

Gate leakage

current

10-9

A 10-12

A

Drain

resistance

0.1 to 1Mohm 1 to 50Kohm

Out put

characteristic

Much flatter because of high drain

resistance

Not much flatter

Handling

precautions

Not much care is needed Careful handling is a must

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

E- only (Enhancement mode only) Two modes of operation

Depletion as well as enhancement

mode.

Constructional

difference

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

popular because easier and cheaper to produce

Not very popular

Mobility of

carriers

Electron mobility is faster The hole mobility is nearly 2.5

times lower than the electron

mobility

Drain

resistance

and Packing

density

Less area to integrate and high packing

densities

p-channel MOSFET occupies a

larger area than an n-channel

MOSFET having the same ID

rating and less packing density

Turn on Due to the +ve charged contaminants, the

n-channel MOSFET may turn ON

prematurely.

Not affected by premature turn

on.