MOSFET Important MCQs — SSC JE Electrical

Practice set: 20 carefully selected multiple-choice questions with short, clear explanations. Good for quick revision and blog publishing.

MOSFET Power Electronics SSC JE AE Electrical

Tip: Use the Expand arrow on each question to view the correct answer and explanation.

Q1. What does MOSFET stand for?

1. Metal Oxide Semiconductor Field Effect Transistor
2. Metal Oxide Silicon Field Effect Transistor
3. Metal Oxide Silicon Familiar Electrical Transistor
4. Metal Oxide Semiconductor Frequency Effect Transistor

Answer & Explanation

Correct: A. Metal Oxide Semiconductor Field Effect Transistor. This is the full form used universally.

Q2. Which terminal sequence is correct for an n-channel enhancement MOSFET when looking at the front package terminal order (left to right) in many to-220 packages?

1. Gate - Drain - Source
2. Gate - Source - Drain
3. Drain - Gate - Source
4. Source - Gate - Drain

Answer & Explanation

Correct: A. Gate - Drain - Source is a common physical pin order for many TO-220 MOSFETs (but always check datasheet — packages vary).

Q3. An n-channel enhancement MOSFET is normally:

1. Normally ON at VGS = 0
2. Normally OFF at VGS = 0
3. Conducts at negative VGS
4. Has no threshold voltage

Answer & Explanation

Correct: B. Enhancement-type MOSFETs are normally OFF at V_GS=0. A positive V_GS greater than threshold (V_th) is required to form a conducting channel in n-channel devices.

Q4. The primary advantage of MOSFETs over bipolar junction transistors (BJTs) is:

1. Higher current gain
2. Voltage-driven gate with very high input impedance
3. Better linearity at low power only
4. No need for heat-sinking

Answer & Explanation

Correct: B. MOSFETs are voltage-driven devices with very high input impedance (negligible gate current), making drive circuits simpler and reducing drive power compared to current-driven BJTs.

Q5. In MOSFET nomenclature: V_th refers to:

1. Gate leakage voltage
2. Drain threshold voltage
3. Gate threshold voltage (minimum V_GS to form channel)
4. Maximum allowed gate voltage

Answer & Explanation

Correct: C. V_th is the threshold voltage — the minimum gate-to-source voltage needed to create a conductive channel between drain and source.

Q6. Which of the following is true about MOSFET body diode?

1. Only present in P-channel MOSFET
2. Intrinsic diode from drain to source (direction depends on channel type)
3. Can be eliminated by gate drive
4. Prevents MOSFET from blocking voltage

Answer & Explanation

Correct: B. A MOSFET has an intrinsic body (or substrate) diode between body and drain/source; for an n-channel MOSFET the diode is from body (p) to drain/source and typically conducts when V_DS is negative beyond diode drop.

Q7. Which region of operation is used when MOSFET is used as a switch (fully ON)?

1. Cut-off region
2. Triode (ohmic) region
3. Saturation (active) region
4. Avalanche region

Answer & Explanation

Correct: B. For switching (fully ON) MOSFETs operate in the triode (ohmic) region where they behave like a low-value resistor (R_DS(on)).

Q8. R_DS(on) of a MOSFET refers to:

1. Drain–source resistance in OFF state
2. Drain–source resistance when device is fully ON
3. Gate–source leakage resistance
4. Internal body resistance

Answer & Explanation

Correct: B. R_DS(on) is the drain–source on-state resistance when the MOSFET is fully enhanced (ON). Lower R_DS(on) reduces conduction losses.

Q9. Which parameter largely determines switching losses in a MOSFET during turn-on and turn-off?

1. Gate capacitance and drive speed
2. R_DS(on) only
3. Body diode forward voltage only
4. Package color

Answer & Explanation

Correct: A. Gate charge/capacitance and drive speed determine how fast V_GS changes; slower transitions cause the MOSFET to spend more time in the linear region, increasing switching losses.

Q10. Which of the following is a common method to reduce MOSFET switching losses?

1. Use extremely slow gate drive
2. Use snubbers or soft-switching techniques
3. Increase R_DS(on)
4. Reverse the gate and drain pins

Answer & Explanation

Correct: B. Using snubbers, RC/RCD networks, or soft-switching (like zero-voltage switching) reduces voltage/current overlap and switching losses.

Q11. Which MOSFET parameter directly influences conduction loss at a given current?

1. Gate threshold V_th
2. R_DS(on)
3. Gate capacitance
4. Max drain current rating

Answer & Explanation

Correct: B. Conduction loss P = I² × R_DS(on). A lower R_DS(on) reduces conduction losses for the same current.

Q12. Which type of MOSFET is most commonly used for high-side switching in low-voltage applications?

1. N-channel enhancement MOSFET with bootstrap gate driver
2. P-channel depletion MOSFET with no driver
3. JFET
4. BJT

Answer & Explanation

Correct: A. N-channel MOSFETs are preferred for low R_DS(on). For high-side switching their gate is often driven by a bootstrap or dedicated high-side driver to provide V_GS above supply.

Q13. Which MOSFET parameter indicates how much charge is needed to switch the device?

1. Gate threshold voltage
2. Total gate charge (Q_g)
3. R_DS(on)
4. Drain capacitance only

Answer & Explanation

Correct: B. Total gate charge Q_g (often specified at a particular V_GS) shows the amount of charge required to change the gate voltage; it's used to size gate drivers and estimate switching energy.

Q14. For protection against voltage spikes generated by inductive loads, a MOSFET circuit commonly uses:

1. Series resistor only
2. Freewheeling diode or snubber network
3. Short across the supply
4. No protection needed

Answer & Explanation

Correct: B. Freewheeling diodes, RCD snubbers, or TVS diodes clamp inductive voltage spikes to protect the MOSFET from overvoltage during switching of inductive loads.

Q15. Which MOSFET parameter should be checked to ensure the device will withstand transient high drain-source voltages?

1. Maximum V_GS
2. Drain-Source breakdown voltage (V_DSrating)
3. R_DS(on)
4. Gate charge

Answer & Explanation

Correct: B. The V_DS rating (often V_BR) is the maximum drain-source voltage the MOSFET can sustain; choose a margin above expected operating and transient voltages.

Q16. Which statement about P-channel MOSFETs is correct?

1. P-channel MOSFETs have electrons as majority carriers.
2. P-channel MOSFETs conduct when gate is made sufficiently positive relative to source.
3. P-channel MOSFETs are symmetric in operation with N-channel but have lower mobility and usually higher R_DS(on) for same area.
4. P-channel devices are always preferred over N-channel for low R_DS(on).

Answer & Explanation

Correct: C. P-channel MOSFETs use holes as majority carriers and typically have lower mobility resulting in higher R_DS(on) compared to an equivalent-sized N-channel device.

Q17. Avalanche rating in MOSFET datasheets refers to:

1. Ability to dissipate heat via avalanche breakdown repeatedly
2. Energy the MOSFET can absorb during single, non-repetitive avalanche event
3. Maximum continuous current rating only
4. Gate leakage during breakdown

Answer & Explanation

Correct: B. Avalanche energy rating (E_AS or similar) indicates the energy the device can safely absorb during a single avalanche event; repetitive avalanches require stricter limits.

Q18. When selecting a MOSFET for a PWM motor drive at high frequency, the most important trade-off is between:

1. Low R_DS(on) (conduction loss) and low gate charge (switching loss)
2. Package weight and color
3. Body diode only
4. Gate threshold only

Answer & Explanation

Correct: A. At high switching frequencies both conduction losses (dependent on R_DS(on)) and switching losses (related to gate charge and capacitances) matter; choose MOSFET balanced for application.

Q19. A logic-level MOSFET refers to a device that:

1. Needs very high V_GS like 20–30V to turn ON
2. Is optimized to turn ON strongly at low gate voltages (e.g., 4.5V or 5V)
3. Has no gate threshold
4. Is used only in analog circuits

Answer & Explanation

Correct: B. Logic-level MOSFETs are designed to have low R_DS(on) at logic gate drive levels (e.g., 4.5V, 5V, or even 3.3V) so they can be driven directly by microcontrollers.

Q20. In MOSFET driving, a gate resistor (R_G) is often used to:

1. Increase gate charge permanently
2. Control switching speed and damp oscillations between driver and MOSFET gate capacitance
3. Decrease R_DS(on)
4. Eliminate body diode

Answer & Explanation

Correct: B. A series gate resistor limits peak drive current, controls dv/dt and switching speed, and damps parasitic oscillations (Miller effect & gate-driver loop).

Want more sets (PYQs, numerical practice, or topic-wise MCQs like power MOSFETs, RF MOSFETs)? Reply and I will extend this set.

© Electrical JE Education — MOSFET MCQs • Prepared for SSC JE & AE aspirants