# VLSI Questions and Answers – Sheet Resistance of MOS Transistors and Inverters

This set of VLSI Multiple Choice Questions & Answers (MCQs) focuses on “Sheet Resistance of MOS Transistors and Inverters”.

1. The resistance of uniform slab of the conducting material is?
a) Linear(proportional) with length
b) Inversely proportional to thickness
c) Inversely proportional to width
d) All of the mentioned

Explanation: The resistance of a uniform slab of conducting material can be expressed as
R = (ρ.l)/(t.w).

2. The sheet resistance of the conducting material is?
a) RS = resistivity/length
b) RS = resistivity/width
c) RS = resistivity/thickness
d) None of the mentioned

Explanation: The sheet resistance of the conducting material is given by RS = resistivity/thickness.

3. In CMOS manufacturing process Sheet resistance is used instead of resistivity because _______________
a) Resistivity is same for all doped regions
b) Resistivity and thickness are characteristics which cannot be controlled by the circuit designer, and it is expressed as the single sheet resistance parameter
c) Sheet resistance is dimensionless quantity
d) Sheet resistance is equal to resistivity

Explanation: It is convenient to use Sheet resistance instead of resistivity because Resistivity and thickness are characteristics which cannot be controlled by the circuit designer, and it is expressed as the single sheet resistance parameter.

4. Compute the sheet resistance of a 0.17 µm thick Cu wire if resistivity of Cu wire is 1.7 µΩ-cm.
a) 0.01 Ω/square
b) 0.001 Ω/square
c) 10.0 Ω/square
d) 0.10 Ω/square

Explanation: Sheet resistance of copper is = (1.7*10-8)/(0.17*10-6)=0.10 Ω/square.

5. For semiconductors doped through diffusion or through surface peaked ion implantation we derive the sheet resistance as ___________
a) Average resistivity of semiconductor/thickness
b) Resistivity/thickness
c) Conductivity/thickness
d) None of the mentioned

Explanation: For semiconductors doped through diffusion or through surface peaked ion implantation we derive the sheet resistance using the average resistivity of the material.
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6. Sheet resistance of a semiconductor is ___________
a) Inherent property of the material
b) Function of thickness of the material
c) Also called as Specific Resistance
d) All of the mentioned

Explanation: Resistivity is the inherent property of any conducting material. It is also called Specific Resistance. Sheet resistance is function of thickness as resistivity for a material is fixed.

7. Sheet resistance of semiconductor is directly measured using ___________
a) Ohmmeter
b) Four point probe measurement
c) Non-contact eddy current based testing device
d) Any of the mentioned

Explanation: Sheet resistance of semiconductor is directly measured using Four point probe measurement.

8. The resistance of the semiconductor material is 800Ω. The sheet resistance if the dimensions of the material is 0.125µm wide and 1 mm long is?
a) 10 Ω/square
b) 0.01 Ω/square
c) 0.10 Ω/square
d) 1 Ω/square

Explanation: We know that R=Rs(L/W).
Therefore Rs=R x W/L
Substituting the values of R, W and L, Rs is found to be 0.10 Ω/square.

9. The typical values of sheet resistance for the n-well semiconductor is ____________
a) 1-5 KΩ/square
b) 10-50 KΩ/square
c) 1-5 Ω/square
d) 100-500 Ω/square

Explanation: The n-well semiconductors have high sheet resistance in the range of 1-5 KΩ/square.

10. The typical values of sheet resistance for polysilicon semiconductor is?
a) 15-30 Ω/square
b) 150-300 Ω/square
c) 1.5-3 KΩ/square
d) 0.15-0.3 Ω/square

Explanation: The typical values of polysilicon semiconductor is 15-30 Ω/square.

11. For λ based design, what is the standard unit of capacitance, (λ=5µm)?
a) 0.01pF
b) 0.0032pF
c) 0.0023pF
d) All of the mentioned

Explanation: 5 µm x 5 µm x 4 pF x 10-4/µm2 = 0.01pF.

12. If standard area is 2λ x 2λ, then the standard capacitance of a gate of length 30λ and width 6λ is?
a) 180oCg
b) 45oCg
c) 90oCg
d) 4oCg

Explanation: 30λ x 6λ/2λ x 2λ = 45 oCg.

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