# VLSI Questions and Answers – Limitations of Scaling -1

This set of VLSI Multiple Choice Questions & Answers (MCQs) focuses on “Limitations of Scaling -1”.

1. Built-in junction potential Vb depends on
a) Vdd
b) Vgs
c) substrate doping level
d) oxide thickness

Explanation: Built-in junction potential Vb depends on the substrate doping level and this will be acceptable so long as Vb is small compared with Vdd.

2. As the channel length is reduced in a MOS transistor, depletion region width must be
a) increased
b) decreased
c) must not vary
d) exponentially decreased

Explanation: As the channel length is reduced in a MOS transistor, depletion width must also be scaled down to prevent the source and drain depletion regions from meeting.

3. Vdd is scaled by
a) α
b) β
c) 1/α
d) 1/β

Explanation: Supply voltage Vdd is scaled by 1/β. All voltages are scaled by 1/β.

4. If doping level of substrate Nb increases then depletion width
a) increases
b) decreases
c) does not change
d) increases and then decreases

Explanation: If the substrate doping length Nb increases then depletion width decreases because depletion width is inversely proportional to Nb.

5. Maximum electric field can be given as
a) V/d
b) d/V
c) 2V/d
d) d/2V

Explanation: Maximum electric field can be given by 2V/d and this is induced in one-sided step junction.
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6. The size of a transistor is usually defined in terms of its
a) channel length
b) feature size
c) width
d) thickness ‘d’

Explanation: The size of a transistor is usually defined in terms of its channel length L because feature size only gives area capacitance etc.

7. What is the minimum value of L to maintain transistor action?
a) d
b) d/2
c) 2d
d) d2

Explanation: The channel length L should be atleast 2d to maintain the transistor action and to prevent punch-through.

8. L depends on
a) substrate concentration
b) Vgs
c) Vt
d) Vds

Explanation: Channel length L depends on the supply voltage Vdd and substrate concentration Nb.

9. Drift velocity can be given as
a) E/µ
b) µ/E
c) µ * E
d) E

Explanation: Carrier drift velocity can be given as the product of µ and E and the maximum carrier drift velocity is approximately equal to Vsat regardless of the supply voltage.

10. The transit time can be given as
a) 2d
b) 2d/µE
c) µE/d
d) µE/2d

Explanation: The transit time can be given by L/Vdrift which is equivalent to 2d/µE as L = 2d and Vdrift is µE.

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