VLSI Interview Questions - 3

1. Explain the voltage transfer characteristics of a CMOS Inverter.




2. What should be done to the size of a nMOS transistor in order to increase its threshold voltage?




3. What are the advantages of CMOS technology?




4. Give the expression for CMOS switching power dissipation.


Pswitching = (1/2)CVdd2/f
Where
Pswitching = Switching power.
C = Load capacitance.
Vdd = Supply voltage.
f = Operating frequency.

5. Why is static power dissipation very low in CMOS technology when compared to others?




6. What is velocity saturation? What are its effects?


In semiconductors, when a strong enough electric field is applied, the carrier velocity in the semiconductor reaches a maximum value. When this happens, the semiconductor is said to be in a state of velocity saturation. As the applied electric field increases from that point, the carrier velocity no longer increases.
In sub-micron technology velocity saturation is an important design characteristic. Velocity saturation greatly affects the voltage transfer characteristics of a circuit. If a semiconductor device enters velocity saturation, an increase in voltage applied to the device will not cause a linear increase in current as would be expected by Ohm's law. Instead, the current may only increase by a small amount, or not at all.

7. Why are pMOS transistor networks generally used to produce high signals, while nMOS networks are used to product low signals?


This is because threshold voltage effect. A nMOS device cannot drive a full 1 or high and pMOS cant drive full '0' or low. The maximum voltage level in nMOS and minimum voltage level in pMOS are limited by threshold voltage. Both nMOS and pMOS do not give rail to rail swing.

8. Expand: DTL, RTL, ECL, TTL, CMOS, BiCMOS.


DTL: Diode-Transistor Logic.
RTL: Resistor-Transistor Logic.
ECL: Emitter Coupled Logic.
TTL: Transistor-Transistor Logic.
CMOS: Complementary Metal Oxide Semiconductor.
BiCMOS: Bipolar Complementary Metal Oxide Semiconductor.

9. On IC schematics, transistors are usually labeled with two, or sometimes one number(s). What do each of those numbers mean?


The two numbers are the width and the length of the channel drawn in the layout. If only one number is present then it is the width of the channel, combined with a default length of the channel.

10. How do you calculate the delay in a CMOS circuit?



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