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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
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3. It
4. It
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22. Wha
Parameter
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24. Wha
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hat are the p
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BJT
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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.