# VLSI Questions and Answers – Device Modelling and Performance Estimation – 2

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This set of VLSI Assessment Questions and Answers focuses on “Device Modelling and Performance Estimation – 2”.

1. Depletion mode MESFET operates as
a) reverse biased
b) forward biased
c) both reverse and forward biased
d) none of the mentioned

Explanation: Depletion mode MESFET operates as reverse biased (Vt is lesser than Vgs) and enhancement mode MESFET operates as forward biased.

2. Pinch-off voltage is equal to
a) built-in potential
b) applied voltage
c) sum of built-in potential and applied voltage
d) difference of built-in potential and applied voltage

Explanation: Pinch-off voltage is the total voltage, both built-in potential and applied voltage necessary to completely deplete the channel of mobile charge carriers.

3. Pinch-off voltage is a function of
a) channel depth
b) channel thickness
c) channel length
d) channel density

Explanation: Pinch-off voltage is a function of both channel thickness ‘a’ and concentration density Nd and it is always positive.

4. The threshold voltage is sensitive to
a) channel length
b) channel depth
c) doping density
d) doping of the channel layer

Explanation: The threshold voltage Vt is very sensitive to both the channel thickness ‘a’ and the doping of the channel layer.

5. The dynamic switching energy must exceed the capacitive load.
a) true
b) false

Explanation: In logic structure, the dynamic switching energy must exceed the energy stored in the capacitive load.

6. To keep dynamic switching energy small
a) logic voltage swing must be large
b) logic current swing must be large
c) logic voltage swing must be small
d) logic current swing must be small

Explanation: To keep dynamic switching energy small, the logic voltage swing must be kept small. This requires proper control over threshold voltages.

7. Standard deviation of threshold voltage should be ______ of logic voltage swing.
a) less than 5%
b) more than 5%
c) less than 10%
d) more than 10%

Explanation: To achieve proper control over the threshold voltage, the standard deviation of threshold voltage should be maintained less than 5% of the logic voltage swing.

8. In D-MESFET, voltage swing is less than 1V.
a) true
b) false

Explanation: In D-MESFET, the logic voltage swing can be larger than 1V which means tolerance to higher threshold voltage variation can be accommodated.

9. Threshold voltage is ________ on implant depth.
a) proportionally dependent
b) inversely proportionally dependent
c) exponentially dependent
d) logarithmically dependent

Explanation: Threshold voltage is exponentially dependent on implant depth and there is the need for proper control of channel thickness.

10. The drain current is independent of
a) Vgs
b) Vds
c) Vt
d) Vs

Explanation: The drain current saturates at the same drain to source voltage Vds and is independent of gate to source voltage Vgs.

11. Impurity concentration should be
a) greater than 20%
b) lesser than 20%
c) greater than 10%
d) lesser than 10%

Explanation: The channel thickness should be controlled withint 20 armstrong and change is impurity concentration should be less than 20%.

12. Threshold voltage is independent of pinch-off voltage.
a) true
b) false

Explanation: Threshold voltage is dependent upon pinch-off voltage Vpo and barrier potential and this is given as the difference between the two.

13. Pinch-off voltage is ______ to channel concentration density.
a) directly related
b) inversely related
c) exponentially related
d) is not related

Explanation: Pinch-off voltage is directly related to the effective channel concentration density Nd. 