VLSI Questions and Answers – Latch-up in CMOS

This set of VLSI Multiple Choice Questions & Answers (MCQs) focuses on “Latch-up in CMOS”.

1. In latch-up condition, parasitic component gives rise to __________ conducting path.
a) low resistance
b) high resistance
c) low capacitance
d) high capacitance
View Answer

Answer: a
Explanation: In latch-up condition, the parasitic component gives rise to low resistance conducting path between Vdd and Vss with disastrous results. Careful control during fabrication is necessary to avoid this problem.

2. Latch-up can be induced by __________
a) incident radiation
b) reflected radiation
c) etching
d) diffracted radiation
View Answer

Answer: a
Explanation: Latch-up can be induced by glitches on the supply rail or by incident radiation.

3. How many transistors might bring up latch up effect in p-well structure?
a) two
b) three
c) one
d) four
View Answer

Answer: a
Explanation: Two transistors and two resistances might bring up the latch-up effect in p-well structure. These are associated with p-well and with regions of the substrate.
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4. Substrate doping level should be decreased to avoid the latch-up effect.
a) true
b) false
View Answer

Answer: b
Explanation: An increase in substrate doping level with a consequent drop in the value of Rs can be used as a remedy for latch-up problem.

5. What can be introduced to reduce the latch-up effect?
a) latch-up rings
b) guard rings
c) latch guard rings
d) substrate rings
View Answer

Answer: b
Explanation: The introduction of guard rings can reduce the effect of latch-up problem. Guard rings are diffusions which decouple the parasitic bipolar transistors.

6. Which process produces a circuit which is less prone to latch-up effect?
a) CMOS
b) nMOS
c) pMOS
d) BiCMOS
View Answer

Answer: d
Explanation: BiCMOS process produces circuits that are less likely to suffer from latch-up problems where as CMOS circuits are very highly prone to latch-up problems.

7. Which one of the following is the main factor for reducing the latch-up effect?
a) reduced p-well resistance
b) reduced n-well resistance
c) increased n-well resistance
d) increased p-well resistance
View Answer

Answer: b
Explanation: One of the main factors in reducing the latch-up effect is reduced n-well resistance Rw. Reduction in Rw means that a larger lateral current is necessary to invite latch-up and higher value of holding current is also required.
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8. The parasitic PNP transistor has the effect of _______ carrier lifetime.
a) increasing
b) decreasing
c) exponentially decreasing
d) exponentially increasing
View Answer

Answer: b
Explanation: The parasitic PNP transistor has the effect of reducing carrier lifetime in the n-base region.

9. The reduction in carrier lifetime brings about __________
a) reduction in alpha
b) reduction in beta
c) reduction in current
d) reduction in voltage
View Answer

Answer: b
Explanation: The parasitic PNP transistor has the effect of reducing carrier lifetime in the n-base region which results in radiation in beta.
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10. To reduce latch-up effect substrate resistance should be high.
a) true
b) false
View Answer

Answer: b
Explanation: To reduce the latch-up effect, substrate resistance Rs should be low. Reduction of Rs and Rw means that larger lateral current is necessary to invite latch-up.

11. Latch-up is the generation of __________
a) low impedance path
b) high impedance path
c) low resistance path
d) high resistance path
View Answer

Answer: a
Explanation: Latch-up is the generation of low-impedance path in CMOS chips between the power supply and ground rails.

12. Latch-up is brought about by BJTs __________
a) with positive feedback
b) with negative feedback
c) with no feedback
d) without BJT
View Answer

Answer: a
Explanation: Latch-up occurs due to BJTs for silicon-controlled rectifiers with positive feedback and virtually short circuit the power and ground rail.

13. Sudden transient in power can cause latch-up.
a) true
b) false
View Answer

Answer: a
Explanation: Sudden transient in power and ground buses are also among the reason which causes latch-up effect.

14. BJT gain should be ______ to avoid latch-up effect.
a) increased
b) decreased
c) should be maintained constant
d) changed randomly
View Answer

Answer: b
Explanation: BJT gain should be reduced by lowering the minority carrier lifetime through doping of the substrate to lower the latch-up effect.

Sanfoundry Global Education & Learning Series – VLSI.

To practice all areas of VLSI, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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