This set of VLSI Multiple Choice Questions & Answers (MCQs) focuses on “Rules for Proper Design”.
1. The Zp.u./Zp.d. ratio for nMOS inverter is
Explanation: For nMOS inverters the Zp.u./Zp.d. ratio is 4:1 when driven from another inverter and 8:1 when driven through one or more pass transistors.
2. The impedance ratio for pseudo-nMOS is
Explanation: For pseudo-nMOS, the Zp.u./Zp.d. ratio is 3:1 and for CMOS 1:1 ratio is required for minimum area.
3. What is the value for peripheral capacitance for 5 micron technology?
a) 4 x 10(-4) pf/µm2
b) 5 x 10(-4) pf/µm2
c) 8 x 10(-4) pf/µm2
d) 12 x 10(-4) pf/µm2
Explanation: Peripheral capacitance is the side wall capacitance. Peripheral capacitance of 5 micron technology is 8 x 10(-4) pf/µm2.
4. 1 square Cg is ___________ of MOS transistor.
a) gate to source capacitance
b) gate to drain capacitance
c) source to drain capacitance
d) gate to channel capacitance
Explanation: 1 square Cg is defined as the gate to channel capacitance of a MOS transistor having standard feature size (W=L).
5. What is the delay value Ʈ for 1.2 micron technology?
a) 0.1 nsec
b) 0.12 nsec
c) 0.046 nsec
d) 0.064 nsec
Explanation: The delay Ʈ is the time constant and for 1.2 micron technology its value is 0.046 nsec.
6. Which is used to increase Ʈ?
a) parasitic capacitance
b) peripheral capacitance
c) area capacitance
d) load capacitance
Explanation: Circuit wiring and parasitic capacitance must be allowed to increase the value of Ʈ by the factor of 2 or 3.
7. The inverter pair delay is given by
b) (1+ Zp.u./Zp.d.)Ʈ
c) (1+ Zp.u./Zp.d.)Ʈ
d) (1+ Ʈ)Zp.u./Zp.d.
Explanation: The inverter delay is given by (1+ Zp.u./Zp.d.)Ʈ. The inverter pair delay for CMOS is 7Ʈ.
8. The number of stages N is given by
Explanation: To calculate the value for N, where N inverters are cascaded, each one of which is larger than the preceding stage by a width factor f the formula used is ln(y)/ln(f).
9. If f assumes the value e then delay is
c) does not change
Explanation: Total delay is minimized if f assumes the value of e which is the base of the natural logarithm. This applies to both nMOS and CMOS.
10. Propogation delay is given by
Explanation: Propogation delay through cascaded pass transistors or transmission gate can be given as n2rcƮ.
11. Using _____ long wires are possible.
Explanation: Using silicide, reasonable long wires are possible. It is a modest RC product. Silicides are used in place of polysilicon in some nMOS processes.
12. One pass transistor can be driven through output of another.
Explanation: No pass transistor gate must be driven through the output of one or more pass transistors since logic 1 levels are degraded by the threshold voltage.
13. Pass transistors are allowed to be constructed under
a) diffusion layer
b) polysilicon layer
c) metal layer
d) silicon layer
Explanation: Pass transistors are allowed to be constructed under metal layers to save space and is more convenient.
14. Maximum allowable current density in aluminium is
a) 0.1 mA/µm2
b) 0.5 mA/µm2
c) 2 mA/µm2
d) 1 mA/µm2
Explanation: The maximum allowable current density in aluminium wire is 1 mA/µm2. Otherwise metal migration may occur.
Sanfoundry Global Education & Learning Series – VLSI.
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