Important Short Questions and Answers: VLSI Design - CMOS Technology

1. Define Threshold voltage

The threshold voltage VT for a MOS transistor can be defined as the voltage between the gate and the source terminals below which the drain to source current effectively drops to zero.

2. Define body effect or substrate bias effect.

The threshold voltage VT is not a constant with respect to the voltage difference between the substrate and the source of the MOS transistor. This effect is called the body effect or substrate bias effect.

3. Give the different modes of operation of MOS transistor

Cut off mode Linear mode Saturation mode

4.   What are the different regions of operation of a MOS transistor?

a. Cut off region

Here the current flow is essentially zero (accumulation mode)

b. Linear region

It is also called weak inversion region where the drain current is dependent on the gate and the drain voltage w. r. to the substrate.

c. Saturation region

Channel is strongly inverted and the drain current flow is ideally independent of the drain-source voltage (strong-inversion region).

5. Give the expressions for drain current for different modes of operation of

MOS transistor.

a. Cut off region ID =0

b. Linear region

ID = kn [(VGS – VT) VDS – VDS2/2]

c. Saturation region

ID = (kn /2) (VGS – VT)²

6. Plot the current-voltage characteristics of a nMOS transistor.

7. Define accumulation mode.

The initial distribution of mobile positive holes in a p type silicon substrate of a mos transistor for a voltage much less than the threshold voltage

8. What are the secondary effects of MOS transistor?

a.        Threshold voltage variations

b.        Source to drain resistance

c.         Variation in I-V characteristics

d.        Subthreshold conduction

e.         CMOS latchup

9. What is CMOS latchup? How it can be prevented?

The MOS technology contains a number of intrinsic bipolar transistors.These are especially troublesome in CMOS processes, where the combination of wells and subtrates results in the formation of p-n-p-n structures. Triggering these thyristor like devices leads to a shorting of VDD & VSS lines, usually resulting in a destruction of the chip.

10.The remedies for the latch-up problem include:

(i)       an increase in substrate doping levels with a consequent drop in the value of Rpsubs.

(ii)    reducing Rnwell by control of fabrication parameters and ensuring a low contact resistance to VDD.

(iii)  by introducing guard rings.

11.   What are the different fabrication processes available to CMOS technology?

a.        p-well process

b.        n-well process

c.         Twin-tub process

d.        Silicon On Insulator (SOI) / Silicon On Sapphire (SOS) process

12.   What is intrinsic and extrinsic semiconductor?

The pure silicon is known as Intrinsic Semiconductor. When impurity is added with pure silicon, it is electrical properties are varied. This is known as Extrinsic semiconductor.

13. What are the steps involved in manufacturing of IC?

 i. wafer preparation

ii.        Epitaxial growth

iii.     Oxidation

iv.     photo lithography

v.        Diffusion and Ion Implantation

vi.     Isolation

vii.   Metallization

14. What is meant by ‘ epitaxy ’ ?

Epitaxy means arranging atoms in single crystal fashion upon a single crystal substrate.

15.   what are the process involved in photo lithography?

i.          making process

ii.        photo etching process

these are important process involved in photolithography.

16. what is the purpose of masking in fabrication of IC?

Masking is used to identify the place in which Ion Implantion should not be occurred.

17. what are the materials used for masking?

Photo resist, Sio₂, SiN, poly Silicon.

18.what are the types of etching ?

Wet etching and dry etching are the types of photo etching.

19. what is diffusion process ? what are doping impurities?

Diffusion is a process in which impurities are diffused in to the silicon chip at 1000⁰C temperature. B₂O₃ and P₂O₅ are used as impurities.

20. what is isolation?

It is a process used to provide electrical isolation between different components and interconnections.

21. what are the various CMOS technologies?

Various CMOS technologies are

i.          n-well process or n-tub process

ii.        p-well process or p-tub process

iii.     twin-tub process

iv.     Silicon on Insulator (SOI) process.

22. what is channel stop implantation?

In n-well fabrication, n-well is protected with resist material. Because, it should not be affected by Boron implantation. The boron is implanexcept n-well. It is done using photoresist mask. This type of implantation is known as channel implantation.

23. what is LOCS?

LOCOS means Local Oxidation Of Silicon. This is one type of oxide construction.

24. what is SWAMI?

SWAMI means Side Wall Masked Isolation. It is used to reduce bird’s beak effect.

25. what is LDD?

LDD means Light Doped Drain Structures. It is used for implantation of n- in n-well process.

26. what is twin-tub process? Why it is called so?

Twin-tub process is one of the CMOS technology. There are two wells are available in this process. The other name of well is tub. So, because of these two tubs, this process is known as twin-tub process.