# Electronic Devices and Circuits Questions and Answers – MOSFETs Device Strucuture and Physical Operation

This set of Electronic Devices and Circuits Questions and Answers for Campus interviews focuses on “MOSFETs Device Strucuture and Physical Operation”.

1. For NMOS transistor which of the following is not true?
a) The substrate is of p-type semiconductor
b) Inversion layer or induced channel is of n type
c) Threshold voltage is negative
d) None of the mentioned

Explanation: The threshold voltage is positive for NMOS.

2. Process transconductance parameter is directly proportional to
a) Electron mobility only
b) (Electron mobility)-1 only
c) Oxide capacitance only
d) Product of oxide capacitance and electron mobility

Explanation: It is the product of the electronic mobility with the oxide capacitance (F/m2).

3. The SI Units of the Process transconductance Parameter (k’) is
a) V2/A
b) A/V2
c) V/A
d) A/V

Explanation: k’ = μn Cox where μn is electronic mobility (m2/Vs) and Cox is oxide capacitance is (F/m2).

4. Aspect ratio of the MOSFET has the units of
a) No units
b) m
c) m2
d) m-1

Explanation: It is the ratio of the induced channel width (w) to the induced channel length (l).

5. The MOSFET transconductance parameter is the product of
a) Process transconductance and inverse of aspect ratio
b) Inverse of Process transconductance and aspect ratio
c) Inverse of Process transconductance and inverse of aspect ratio
d) Process transconductance and aspect ratio

Explanation: This statement only satisfies the mathematical expression.
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6. With the potential difference between the source and the drain kept small (VDS is small), the MOSFET behaves as a resistance whose value varies __________ with the overdrive voltage
a) Linearly
b) Inversely
c) Exponentially
d) Logarithmically

Explanation: For small VDS, resistance r is given by
R = 1 / ((μn Cox)(w/l)(VOV)).

7. For a p channel MOSFET which of the following is not true?
a) The source and drain are a p type semiconductor
b) The induced channel is p type region which is induced by applying a positive potential to the gate
c) The substrate is a n type semiconductor
d) None of the mentioned

Explanation: The induced channel is p type region which is induced by applying a negative potential to the gate.

8. When the voltage across the drain and the source (VDS) is increased from a small amount (assuming that the gate voltage, VG with respect to the source is higher than the threshold voltage, Vt), then the width of the induced channel in NMOS (assume that VDS is always small when compared to the Vov)
a) Will remain as was before
b) Will become non uniform and will take a tapered shape with deepest width at the drain
c) Will become non uniform and will take a tapered shape with deepest width at the source
d) Will remain uniform but the width of the channel will increase

Explanation: The voltage across the source will be VOV and the voltage will decrease linearly to VOV – VDS as we reach the drain end. The width of the induced channel is proportional to the voltage.

9. The saturation current of the MOSFET is the value of the current when
a) The voltage between the drain and drain becomes equal to the overdrive voltage
b) The voltage between the drain and drain becomes equal to the threshold voltage
c) The voltage between the drain and drain becomes equal to the voltage applied to the gate
d) The voltage between the drain and drain becomes equal to difference the overdrive voltage and the threshold voltage

Explanation: By definition of the MOSFET saturation current.

10. At channel pinch off
a) The width of the induced channel becomes non linear
b) The width of the induced channel becomes very large (resulting in very large resistance and very low, practically zero, current)
c) width becomes 1/e times the maximum possible width
d) The width of the induced channel becomes zero and the current saturates

Explanation: It is a characteristics of a channel pinch off.

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