MOS Transistor Theory Questions and Answers

Q: Identify which one of the following materials cannot be used for the gate electrode in a MOSFET ?

The material that cannot be used as the gate electrode is hafnium dioxide.The gate electrode must be a conductor. Aluminium was the original gate metal, tungsten and other refractory metals are used in metal gate stacks, and gold, though rarely used in silicon processing because it contaminates silicon, is a conductor that can serve as a gate in experimental and compound-semiconductor devices.Hafnium dioxide (HfO2) is an insulator. It is used in modern high-k metal gate transistors as the gate dielectric, the layer under the electrode, precisely because it does not conduct, so it cannot be the gate electrode itself.

Q: In nMOS device, gate material could be :

The gate material could be polysilicon.Heavily doped polycrystalline silicon replaced aluminium as the standard MOS gate material in the 1970s. It withstands the high-temperature steps after gate formation, which allows the self-aligned gate process in which the gate itself masks the source and drain implants, and its doping can be chosen to set the threshold voltage. Advanced processes have since moved to metal gates with high-k dielectrics.Single-crystal silicon is the substrate, not the gate. Boron and phosphorus are dopants used to make p-type and n-type regions; they are not gate electrode materials themselves.

Q: In a MOSFET devices, the N-channel type is better than the P-channel type in the following respect :

The N-channel MOSFET is better because it is faster.In an N-channel device the current is carried by electrons, whose mobility in silicon (about 1350 cm2/V s) is roughly two to three times the hole mobility (about 480 cm2/V s) that governs a P-channel device. For the same size, an NMOS transistor therefore has higher transconductance and lower on-resistance, so it charges load capacitance faster and switches faster.Noise immunity is a property of the logic family's voltage levels, not of the channel type. TTL compatibility and drive capability depend on the circuit design and supply voltages, so they are not the defining advantage of N-channel over P-channel devices.

Q: n-channel MOSFET is preferred than p-channel MOSFET because :

The n-channel device is preferred because the electron mobility in Si is larger than the mobility of holes.In silicon, electrons have a mobility of about 1350 cm\(^{2}\)/V·s and holes about 480 cm\(^{2}\)/V·s. Since drain current is proportional to carrier mobility, an n-channel MOSFET of the same size carries two to three times the current, switches faster and has a lower on-resistance than a p-channel device.Input impedance is not the reason: in both types the gate is insulated by oxide, so both have extremely high input impedance. The option saying electron mobility is smaller than hole mobility states the fact the wrong way round.

Q: The scaling factor of current density in constant voltage model is :

In the constant voltage model, current density scales by \(\alpha^{2}\).In the scaling scheme used in VLSI design texts, lateral dimensions shrink by \(1/\alpha\) and supply voltage and oxide thickness by \(1/\beta\). The saturation current scales as \(1/\beta\) (gate capacitance per unit area rises by \(\beta\), voltage squared falls by \(1/\beta^{2}\)), and the area it flows through shrinks by \(1/\alpha^{2}\), so current density scales as \(\alpha^{2}/\beta\).Constant voltage scaling keeps the voltage fixed, so \(\beta = 1\) and the factor is \(\alpha^{2}\). \(\alpha^{2}/\beta\) is the general expression, not its value in this model. \(1/\alpha^{2}\) is how gate area scales, and 1 would mean no change, which is wrong when current is pushed through a smaller area.

An enhancement type, basically termed as normally OFF N-MOSFET works only with :

A.Large positive gate voltage
B.Large negative gate voltage
C.Large positive drain voltage
D.Large negative drain voltage

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

A normally OFF n-channel enhancement MOSFET works only with a large positive gate voltage.

An enhancement MOSFET has no channel at zero gate voltage, so no drain current flows: it is normally off. Applying a gate-source voltage more positive than the threshold voltage attracts electrons to the surface of the p-type substrate and induces an n-type inversion channel between source and drain, and the device conducts.

A negative gate voltage only pushes electrons away and deepens the cut-off. The drain voltage, positive or negative, cannot create the channel; it only drives current once the gate has formed one.

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Identify which one of the following materials cannot be used for the gate electrode in a MOSFET ?

A.Aluminium
B.Tungsten
C.Gold
D.Hafnium dioxide

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: D

The material that cannot be used as the gate electrode is hafnium dioxide.

The gate electrode must be a conductor. Aluminium was the original gate metal, tungsten and other refractory metals are used in metal gate stacks, and gold, though rarely used in silicon processing because it contaminates silicon, is a conductor that can serve as a gate in experimental and compound-semiconductor devices.

Hafnium dioxide (HfO₂) is an insulator. It is used in modern high-k metal gate transistors as the gate dielectric, the layer under the electrode, precisely because it does not conduct, so it cannot be the gate electrode itself.

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In nMOS device, gate material could be :

A.Silicon
B.Polysilicon
C.Boron
D.Phosphorus

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

The gate material could be polysilicon.

Heavily doped polycrystalline silicon replaced aluminium as the standard MOS gate material in the 1970s. It withstands the high-temperature steps after gate formation, which allows the self-aligned gate process in which the gate itself masks the source and drain implants, and its doping can be chosen to set the threshold voltage. Advanced processes have since moved to metal gates with high-k dielectrics.

Single-crystal silicon is the substrate, not the gate. Boron and phosphorus are dopants used to make p-type and n-type regions; they are not gate electrode materials themselves.

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In a MOSFET devices, the N-channel type is better than the P-channel type in the following respect :

A.It has better noise immunity
B.It is faster
C.It is TTL compatible
D.It has drive capability

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The N-channel MOSFET is better because it is faster.

In an N-channel device the current is carried by electrons, whose mobility in silicon (about 1350 cm²/V s) is roughly two to three times the hole mobility (about 480 cm²/V s) that governs a P-channel device. For the same size, an NMOS transistor therefore has higher transconductance and lower on-resistance, so it charges load capacitance faster and switches faster.

Noise immunity is a property of the logic family's voltage levels, not of the channel type. TTL compatibility and drive capability depend on the circuit design and supply voltages, so they are not the defining advantage of N-channel over P-channel devices.

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n-channel MOSFET is preferred than p-channel MOSFET because :

A.input impedance of n-channel is higher than the input impedance of p-channel MOSFET
B.input impedance of p-channel is higher than the input impedance of n-channel MOSFET
C.the electron mobility in Si is smaller than the mobility of holes
D.the electron mobility in Si is larger than the mobility of holes

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

The n-channel device is preferred because the electron mobility in Si is larger than the mobility of holes.

In silicon, electrons have a mobility of about 1350 cm\(^{2}\)/V·s and holes about 480 cm\(^{2}\)/V·s. Since drain current is proportional to carrier mobility, an n-channel MOSFET of the same size carries two to three times the current, switches faster and has a lower on-resistance than a p-channel device.

Input impedance is not the reason: in both types the gate is insulated by oxide, so both have extremely high input impedance. The option saying electron mobility is smaller than hole mobility states the fact the wrong way round.

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The scaling factor of current density in constant voltage model is :

A.\(1/\alpha^{2}\)
B.1
C.\(\alpha^{2}\)
D.\(\alpha^{2}/\beta\)

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

In the constant voltage model, current density scales by \(\alpha^{2}\).

In the scaling scheme used in VLSI design texts, lateral dimensions shrink by \(1/\alpha\) and supply voltage and oxide thickness by \(1/\beta\). The saturation current scales as \(1/\beta\) (gate capacitance per unit area rises by \(\beta\), voltage squared falls by \(1/\beta^{2}\)), and the area it flows through shrinks by \(1/\alpha^{2}\), so current density scales as \(\alpha^{2}/\beta\).

Constant voltage scaling keeps the voltage fixed, so \(\beta = 1\) and the factor is \(\alpha^{2}\). \(\alpha^{2}/\beta\) is the general expression, not its value in this model. \(1/\alpha^{2}\) is how gate area scales, and 1 would mean no change, which is wrong when current is pushed through a smaller area.

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The threshold voltage of a MOSFET can be lowered by

1. using a thinner gate oxide
2. reducing the carrier concentration in the substrate
3. increasing the carrier concentration in the substrate Of these statements :

A.3 alone is correct.
B.1 and 2 are correct
C.1 and 3 are correct
D.2 alone is correct

Exam Year(s)

UGC NET - 2014,

Answer: B

Statements 1 and 2 are correct. The threshold voltage of a MOSFET is

\[ V_{T} = V_{FB} + 2\phi_{F} + \frac{Q_{d}}{C_{ox}} \]

where \(Q_{d} = \sqrt{2\epsilon q N_{A}(2\phi_{F})}\) is the depletion charge.

A thinner gate oxide raises \(C_{ox}\) and lowers the last term, so statement 1 is right. Reducing the substrate carrier concentration lowers the depletion charge and also lowers \(2\phi_{F}\), so \(V_{T}\) falls and statement 2 is right. Increasing the concentration does the opposite and raises \(V_{T}\), so statement 3 is wrong. That rules out the options that contain 3.

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The threshold voltage of an n-channel MOSFET can be increased by

A.increasing the channel doping concentration
B.reducing the channel doping concentration
C.reducing the gate oxide thickness
D.reducing the channel length

Exam Year(s)

UGC NET - 2014,

Answer: A

The threshold voltage of an n-channel MOSFET is increased by increasing the channel doping concentration. A heavier p-type doping needs a larger gate voltage to invert the surface, so the threshold voltage rises.

Reducing the doping lowers the threshold. A thinner gate oxide raises the oxide capacitance and so also lowers the threshold. Shortening the channel does not raise it either; in short-channel devices the threshold tends to fall.

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