Mastering CMOS: Challenging VLSI Interview Questions for Freshers Quiz

1. Understanding Latch-Up

   In CMOS circuits, what is 'latch-up' and what is one practical method to prevent it? Consider a scenario where a sudden high current flows between the power rails after triggering a parasitic component.

   1. A. Latch-up is a temporary logic error; it is prevented by increasing the clock speed.
   2. B. Latch-up is accidental creation of a low resistance path caused by parasitic thyristors; it is prevented using guard rings and sufficient substrate contact.
   3. C. Latch-up is an excessive power-up delay; it is prevented by reducing load capacitance.
   4. D. Latch-up is threshold voltage instability; it is prevented by scaling transistor widths.
   5. E. Latch-up is frequency instability; it is prevented using decoupling inductors.

2. Gate Selection in CMOS Logic

   Why is the NAND gate typically preferred over a NOR gate for CMOS fabrication when considering factors like carrier mobility and speed? For instance, compare switching speeds when driving identical loads.

   1. A. NAND has higher gate leakage current than NOR, making it preferable.
   2. B. NOR has more symmetric rise and fall delays than NAND, so NOR is preferred.
   3. C. NAND is faster due to higher electron mobility and lower gate-leakage overall.
   4. D. NOR uses fewer transistors for equivalent logic, increasing speed.
   5. E. NAND's threshold voltage is always lower, increasing robustness.

3. CMOS Noise Margins

   How is the 'noise margin' of a CMOS inverter determined, and why is it critical for reliable digital operation? Imagine an environment where random voltage spikes occur on signal lines.

   1. A. By calculating the maximum allowable input noise voltage so the output is not affected; it ensures tolerance to spurious signals.
   2. B. By minimizing the rise time of the output; it prevents glitches.
   3. C. By measuring the delay between input and output; it avoids race conditions.
   4. D. By using only n-type transistors; it prevents cross-talk.
   5. E. By increasing frequency above threshold; it rejects noise automatically.

4. Transistor Sizing for Performance

   When designing a CMOS inverter chain to drive a large capacitive load, why is it preferable to gradually increase the size of each successive inverter rather than connecting a small inverter directly to a large one?

   1. A. Large inverters are always slower regardless of previous stage size.
   2. B. Gradually increasing inverter size allows optimal driving of the load by limiting each stage's delay and reducing overall propagation time.
   3. C. Smaller inverters consume less power when connected to a large inverter.
   4. D. Suddenly increasing size reduces noise margin drastically.
   5. E. All-stage inverters must be identical in size for symmetric delays.

5. Techniques to Minimize CMOS Power Dissipation

   What is a valid set of techniques to minimize dynamic power consumption in a CMOS digital circuit according to the relationship Power = CV²f? Assume you are designing a low-power system.

   1. A. Increase supply voltage, maximize load capacitance, and double frequency.
   2. B. Lower supply voltage, reduce load capacitance, and decrease operating frequency.
   3. C. Use only PMOS transistors, and remove all output resistances.
   4. D. Increase the threshold voltage, and alternate signal lines.
   5. E. Raise ambient temperature, and use thicker gate oxides.