MOSFET (Metal Oxide Semiconductor FET) MCQs

Correct Answer: (A) Voltage-controlled device

Explanation:
MOSFETs are voltage-controlled devices where the gate voltage (V_GS) controls the channel conductivity between drain and source. Unlike BJTs which are current-controlled, MOSFETs require negligible gate current to operate, making them highly efficient for many applications.

Correct Answer: (C) Gate, Drain, Source

Explanation:
The three main terminals of a MOSFET are:
1. Gate: Controls the channel formation (insulated by oxide layer)
2. Drain: Where current exits the channel
3. Source: Where current enters the channel
Many MOSFETs also have a substrate/body terminal (option D), but this isn't always externally accessible.

Correct Answer: (A) Enhancement mode & Depletion mode

Explanation:
MOSFETs are classified by their default state:
1. Enhancement mode: Normally OFF (no channel at V_GS=0)
2. Depletion mode: Normally ON (channel exists at V_GS=0)
Both types can be either N-channel or P-channel (option C refers to channel type, not MOSFET type).

Correct Answer: (A) Cutoff, Triode, and Saturation regions

Explanation:
MOSFETs have three operating regions:
1. Cutoff (V_GS < V_TH): No channel, no conduction
2. Triode/Linear (V_GS > V_TH, V_DS small): Acts like variable resistor
3. Saturation (V_GS > V_TH, V_DS > V_GS-V_TH): Constant current source (for amplification)

Correct Answer: (B) Electrons

Explanation:
In N-channel MOSFETs:
- Majority carriers are electrons in the induced channel
- Current flows from drain to source via electron movement
P-channel MOSFETs use holes as majority carriers. The "N-channel" designation refers to the type of charge carriers in the channel.

Correct Answer: (A) The minimum gate voltage required to turn the MOSFET ON

Explanation:
Threshold voltage (V_TH) is:
- Minimum V_GS needed to form conducting channel (enhancement MOSFET)
- Typically 0.5-3V for enhancement MOSFETs
- Depends on oxide thickness, doping, and material properties
- Negative for depletion-mode N-MOSFETs (must be made more negative to turn OFF)

Correct Answer: (D) Lower gain

Explanation:
MOSFET advantages over BJTs:
• Higher input impedance (almost infinite)
• Faster switching (no minority carrier storage)
• Lower power dissipation (especially in digital circuits)
• Higher gain is possible with modern MOSFETs (transconductance can be very high)

Correct Answer: (D) All of the above

Explanation:
MOSFET applications include:
• Digital circuits (CMOS logic gates, memory cells)
• Microprocessors (billions of MOSFETs in modern CPUs)
• Power amplifiers (RF and audio)
• Power electronics (switching regulators, motor control)
• Analog switches and multiplexers

Correct Answer: (B) Susceptibility to static charge damage

Explanation:
MOSFET disadvantages:
• Gate oxide is vulnerable to electrostatic discharge (ESD)
• Requires careful handling (anti-static precautions)
• Can be damaged by overvoltage (gate breakdown typically at 15-20V)
Modern MOSFETs include protection diodes, but ESD remains a concern.

Correct Answer: (B) High-power applications

Explanation:
Power MOSFETs are designed for:
• High current handling (up to hundreds of amps)
• High voltage ratings (up to 1000V)
• Low on-resistance (R_DS(on))
Applications include power supplies, motor control, and audio amplifiers. They switch faster than power BJTs.

Correct Answer: (B) Enhancement-mode MOSFET

Explanation:
Enhancement-mode MOSFETs are preferred because:
• Naturally OFF at V_GS=0 (no power consumption in static state)
• Clear distinction between ON/OFF states
• Enable CMOS technology (complementary pairs of N and P-channel MOSFETs)
• Form the basis of modern digital ICs (microprocessors, memory chips)