FET & MOSFET MCQs

Practice FET & MOSFET MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Electronic Devices & CircuitsFET & MOSFET

For an n-channel JFET operating normally, the gate-source junction is generally?

AForward biased heavily
BReverse biased
CShort-circuited
DDriven into avalanche continuously

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Correct Answer: B. Reverse biased

Explanation: The JFET gate-channel PN junction is normally reverse biased, which keeps gate current very small and controls the channel depletion width.

Reference: Standard electronics: JFET operation

Question 2

Electronic Devices & CircuitsFET & MOSFET

An enhancement-mode n-channel MOSFET at VGS = 0 ideally has?

AA fully enhanced conducting channel
BNo induced conducting channel between drain and source
CMaximum drain current
DA forward-biased gate junction

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Correct Answer: B. No induced conducting channel between drain and source

Explanation: An enhancement-mode nMOS requires a positive gate-to-source voltage above threshold to induce a conducting channel; ideally it is off at VGS = 0.

Reference: Standard electronics: enhancement MOSFET

Question 3

Electronic Devices & CircuitsFET & MOSFET

A major characteristic of an ideal FET input is?

AVery low input impedance
BVery high input impedance
CZero output resistance in all configurations
DLarge gate current under normal operation

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Correct Answer: B. Very high input impedance

Explanation: A FET is voltage controlled and normally draws very little gate current, giving it high input impedance.

Reference: Standard electronics: FET characteristics

Question 4

Electronic Devices & CircuitsFET & MOSFET

For an n-channel JFET, Shockley’s transfer equation is commonly written as ID = IDSS(1 – VGS/VP)^2 in the appropriate operating region. If VGS = 0, ID is?

A0
BIDSS
CVP
D2IDSS

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Correct Answer: B. IDSS

Explanation: Substituting VGS = 0 gives ID = IDSS(1 - 0)^2 = IDSS.

Reference: Standard electronics: JFET transfer characteristic

Question 5

Electronic Devices & CircuitsFET & MOSFET

An n-channel enhancement MOSFET begins forming a strong inversion channel when VGS exceeds approximately its?

ADrain breakdown voltage
BThreshold voltage
CGate reverse current
DPinch-off current

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Correct Answer: B. Threshold voltage

Explanation: The threshold voltage VTH is the gate-source voltage at which a conducting inversion channel begins to form according to the device model.

Reference: Standard electronics: MOSFET threshold

Question 6

Electronic Devices & CircuitsFET & MOSFET

In an ideal MOSFET, the DC gate current is approximately?

AEqual to drain current
BEqual to source current
CZero
DInfinite

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Correct Answer: C. Zero

Explanation: The MOS gate is insulated from the channel by an oxide layer, so ideal steady-state gate current is zero, giving very high input resistance.

Reference: Standard electronics: MOSFET gate

Question 7

Electronic Devices & CircuitsFET & MOSFET

For a JFET, transconductance gm represents the small-signal ratio of change in?

ADrain current to gate-source voltage
BGate current to drain-source voltage
CDrain voltage to source current
DGate voltage to gate current only

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Correct Answer: A. Drain current to gate-source voltage

Explanation: JFET transconductance is gm = dID/dVGS at the operating point, describing how effectively gate voltage controls drain current.

Reference: Standard electronics: FET transconductance

Question 8

Electronic Devices & CircuitsFET & MOSFET

An n-channel JFET has IDSS = 8 mA and VP = -4 V. Using Shockley’s equation, the drain current at VGS = -2 V is?

A1 mA
B2 mA
C4 mA
D8 mA

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Correct Answer: B. 2 mA

Explanation: ID = IDSS(1 - VGS/VP)^2 = 8(1 - (-2/-4))^2 = 8(0.5)^2 = 2 mA.

Reference: Standard electronics: JFET Shockley-equation calculation

Question 9

Electronic Devices & CircuitsFET & MOSFET

In the ideal square-law saturation model of an enhancement nMOSFET, drain current is primarily proportional to?

AVGS only
B(VGS - VTH)^2
C1/VGS
DVDS^2 regardless of VGS

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Correct Answer: B. (VGS - VTH)^2

Explanation: In the long-channel ideal saturation model, ID is proportional to the square of the overdrive voltage VGS - VTH, neglecting channel-length modulation.

Reference: Standard electronics: MOSFET square-law model

Question 10

Electronic Devices & CircuitsFET & MOSFET

For an n-channel enhancement MOSFET to be in the ideal saturation region, a common long-channel condition is?

AVGS < VTH
BVGS > VTH and VDS >= VGS - VTH
CVDS = 0 for all VGS
DVGS = 0 and VDS < 0 only

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Correct Answer: B. VGS > VTH and VDS >= VGS - VTH

Explanation: For the standard long-channel model, the device is on when VGS > VTH and enters saturation when VDS is at least the overdrive voltage VGS - VTH.

Reference: Standard electronics: MOSFET operating regions

Question 11

Electronic Devices & CircuitsFET & MOSFET

A common-source FET amplifier normally produces approximately what phase relationship between input and output voltage?

A0 degrees
B90 degrees
C180 degrees
D270 degrees only

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Correct Answer: C. 180 degrees

Explanation: Like the BJT common-emitter stage, the common-source FET voltage amplifier is inverting and produces approximately 180 degrees phase reversal.

Reference: Standard FET amplifiers: common-source stage

Question 12

Electronic Devices & CircuitsFET & MOSFET

A source follower is analogous to which BJT configuration?

ACommon emitter
BCommon base
CEmitter follower
DDifferential pair only

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Correct Answer: C. Emitter follower

Explanation: The common-drain FET stage, called a source follower, has behavior analogous to the BJT common-collector or emitter-follower stage.

Reference: Standard FET amplifiers: source follower

Question 13

Electronic Devices & CircuitsFET & MOSFET

An n-channel enhancement MOSFET has VTH = 2 V. In the ideal long-channel model, if VGS = 5 V, the overdrive voltage VOV is?

A2 V
B3 V
C5 V
D7 V

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Correct Answer: B. 3 V

Explanation: MOSFET overdrive voltage is VOV = VGS - VTH = 5 - 2 = 3 V.

Reference: Standard MOSFET analysis: overdrive voltage

Question 14

Electronic Devices & CircuitsFET & MOSFET

For the MOSFET in the ideal long-channel model with VGS = 5 V and VTH = 2 V, saturation requires VDS to be at least approximately?

A1 V
B2 V
C3 V
D5 V

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Correct Answer: C. 3 V

Explanation: The ideal saturation boundary is VDS >= VGS - VTH. Here the overdrive voltage is 3 V, so VDS must be at least about 3 V.

Reference: Standard MOSFET analysis: saturation condition

Question 15

Electronic Devices & CircuitsFET & MOSFET

In a MOSFET, channel-length modulation in saturation causes the drain current to?

AIncrease slightly as VDS increases
BRemain exactly independent of VDS in every real device
CFall exactly to zero
DReverse direction automatically

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Correct Answer: A. Increase slightly as VDS increases

Explanation: Channel-length modulation shortens the effective channel as VDS rises, giving the saturation-region output characteristic a finite positive slope.

Reference: Standard electronics: MOSFET channel-length modulation

Question 16

Electronic Devices & CircuitsFET & MOSFET

The body effect in an NMOS transistor generally causes threshold voltage magnitude to do what when source-to-body reverse bias increases?

AIncrease
BDecrease to zero
CRemain exactly unchanged
DBecome an AC frequency

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Correct Answer: A. Increase

Explanation: Increasing source-body reverse bias increases the threshold voltage required to form the inversion channel; this is the MOSFET body effect.

Reference: Standard electronics: MOSFET body effect

Question 17

Electronic Devices & CircuitsFET & MOSFET

In an n-channel depletion MOSFET, a conducting channel can exist at VGS=0. Applying a sufficiently negative VGS generally?

AReduces drain current by depleting the channel
BAlways increases drain current
CHas no effect on the channel
DForward biases an insulated gate junction

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Correct Answer: A. Reduces drain current by depleting the channel

Explanation: A negative gate voltage repels electrons from the n-channel, depleting it and reducing drain current.

Reference: Standard electronics: depletion MOSFET

Question 18

Electronic Devices & CircuitsFET & MOSFET

For a long-channel MOSFET in saturation, the small-signal transconductance gm indicates how strongly drain current changes with?

AGate-source voltage at the bias point
BDrain resistance only
CBody temperature only
DGate current

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Correct Answer: A. Gate-source voltage at the bias point

Explanation: MOSFET transconductance is the incremental derivative gm=∂ID/∂VGS at the operating point.

Reference: Standard electronics: MOSFET transconductance

Question 19

Electronic Devices & CircuitsFET & MOSFET

For an ideal square-law NMOS in saturation with ID=2 mA and overdrive voltage VOV=0.2 V, gm=2ID/VOV is approximately?

A2 mS
B10 mS
C20 mS
D200 mS

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Correct Answer: C. 20 mS

Explanation: gm=2×2 mA/0.2 V=20 mS.

Reference: Standard electronics: MOSFET transconductance calculation