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# B.Tech S6 Model Question Paper VLSI Electronics & Communication Engineering

VI Semester B.Tech Degree Examination - 2012
Model Question Paper
VLSI DESIGN

Time:3 hrs                                                                                                   max.marks : 100

PART-A

Answer all questions. Each question carries 2 marks

1. Differentiate between positive and negative photoresists.
2. State Fick’s law of diffusion.
3. A proximity printer operates with a 20μm mask to wafer gap and a wavelength of 250nm.
Find the minimum linewidth that can be obtained.
4. Define threshold voltage. Write the expression for threshold voltage under body bias
condition.
5. Explain channel length modulation. How is it incorporated in the current relationship of
MOS transistor?
6. Explain the junction capacitances associated with a MOSFET.
7. Illustrate a case of progressive transistor sizing to reduce propagation delay in CMOS
design.
8. Pseudo NMOS is a ratioed logic. Justify.
9. How the test patterns are minimized by modeling the circuit with independent logic
paths.
10. Consider a DRAM cell with Cs=50fF and Cbit=8Cs. Assuming a maximum voltage of
Vs=Vmax=2.5V on the storage capacitor, what is the final voltage during a logic 1 read
operation. (2x10 = 20 marks)

PART B

Answer one full question from each module

MODULE I

11. a) Portray the different steps involved in an n well process for CMOS. (12)
b) With neat sketches, explain Czochralsky method of wafer processing. List the

OR

12. a) Derive Deal Grove model of oxidation. (10)
b) With the aid of neat diagram explain ion implantation process (10)
MODULE II

13. a)Derive the current relationship in a MOS transistor in deep triode and saturation
regions. (12)
b) Define flatband voltage. Obtain an expression for threshold voltage of a MOS
transistor in terms of flat band voltage and body effect parameter. (8)

OR

14. Elucidate the short channel effects in MOS with neat diagrams. (20)
MODULE III

15. Derive expressions for noise margin (NML and NMH) and switching threshold of a CMOS
inverter. (20)

OR

16. a) Design a 14 bit square root carry select adder. Calculate the worst case delay. (10)
b) Explain the function of a 4 bit multiplier with block diagram. Write down the
expression for maximum delay. (10)

MODULE IV

17. a) Draw the CMOS implementation of a NOR ROM cell to store 4 words of 4bits which
are as follows 1011, 1110, 1101 and 1001. (10)
b) Design a regular PLA structure using MOS to implement the function
Y1= X4, Y2=X1X3+X2 X4 and Y3=X1X2+X2X3 (10)
OR
18. a) Explain the working of a 1 transistor DRAM cell with the help of timing diagrams.(10)
b) Explain the methods of design for testability