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GATE VLSI Questions and answers

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1) Why Silicon is preferred over Germanium in semiconductor devices?

– The major raw material for Si wafer fabrication is sand which is widely present in the nature.
– SiO2 which is a very good insulator can be easily processed. This layer of oxide is used for the gate oxide in MOSFET
-Temperature stability of silicon is good, it can withstand in temperature range typically 140C to 180C whereas Germanium is much temperature sensitive only up to 70C.

2) What is the difference between dry oxidation and wet oxidation and which is purest among these two?

    Dry Oxidation

Si + O2 -> SiO2

This growth is relatively slow.But this process yields high quality oxide layers.

    Wet Oxidation

Si +2H20 –> SiO2 + 2H2 ( Fast growth, less pure)

3) Effect of Body effect on Threshold Voltage if vsB > 0

Ans : We note that the voltage vSB (voltage source-to-body) is not
necessarily equal to zero (i.e., vSB > 0), which means that this voltage vSB will attract some electrons from the substrate (In Case of P Substrate), thus more voltage is needed to create the channel(SO THRESHOLD VOLTAGE INCREASES).

4) In MOSFET, the polarity of the inversion layer is same as that of (GATE 1989)

a) Charge on the gate electrode
b) Minority carriers in the drain
c) Majority carriers in the substrate
d) Majority carriers in the source

Ans : d

5) In an n-MOSFET, the susbstrate is

a) Heavily doped p type
b) lightly doped p type
c) heavily doped n type
d) lightly doped n type

Ans : a

( Why P+ is used ahead of P-, post your observations in comment section)


6) Vt of n-MOSFET is 0.5V. It is biased with V_{GS}= 3V, V_{DS}=1V, then the MOSFET is in

a) Cut-off
b) Saturation
c) Linear
d) Inverse Saturatiom

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