1. What do you mean by VLSI?

Ans:

Very large-scale integration (VLSI) is the process of integrating or embedding hundreds of thousands of transistors on a single silicon semiconductor microchip.

2.What is MOSFET?

Ans:

A MOSFET is a four-terminal device having source(S), gate (G), drain (D) and body (B) terminals. In general, The body of the MOSFET is in connection with the source terminal thus forming a three-terminal device such as a field-effect transistor. MOSFET is generally considered as a transistor and employed in both the analog and digital circuits.

3.What is MOSFET scaling?

Ans:

MOSFET scaling is the reduction in the parameters (like current, voltage, electric field etc.) due to reduction in length of the transistor with the advancement in technology.

4. What are the different types of MOSFET Scaling?

Ans:

Types of scaling are :
   1) constant field scaling and
   2) constant voltage scaling.

5.Define Depletion Mode and Enhancement Mode.

Ans:

Depletion Mode : When there is no voltage across the gate terminal, the channel shows its maximum conductance. Whereas when the voltage across the gate terminal is either positive or negative, then the channel conductivity decreases.

Enhancement Mode :When there is no voltage across the gate terminal, then the device does not conduct. When there is the maximum voltage across the gate terminal, then the device shows enhanced conductivity.

6. What are short channel effects?

Ans:

The main drives for reducing the size of the transistors, i.e., their lengths, is increasing speed and reducing cost. When you make circuits smaller, their capacitance reduces, thereby increasing operating speed. In the same token, smaller circuits allow more of them in the same wafer, dividing the total cost of a single wafer among more dies.
However, with great reduction come great problems, in this case in the form of unwanted side effects, the so called short-channel effects.

7. Explain the Working Principle of MOSFET

Ans:

The main principle of the MOSFET device is to be able to control the voltage and current flow between the source and drain terminals. It works almost like a switch and the functionality of the device is based on the MOS capacitor. The MOS capacitor is the main part of MOSFET.
The semiconductor surface at the below oxide layer which is located between the source and drain terminal can be inverted from p-type to n-type by the application of either a positive or negative gate voltages respectively. When we apply a repulsive force for the positive gate voltage, then the holes present beneath the oxide layer are pushed downward with the substrate.
The depletion region populated by the bound negative charges which are associated with the acceptor atoms. When electrons are reached, a channel is developed. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied between the drain and source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. Instead of the positive voltage, if we apply a negative voltage, a hole channel will be formed under the oxide layer.

8. List down the advantages and disadvantages of MOSFET Scaling.

Ans:

Advantages of scaling:
   • Increases the switching speed.
   • Reduces chip size.
   • Reduces power dissipation.
Disadvantages of scaling:
   • Effect on SiO2 thickness – on scaling oxide thickness eventually a sage will reach when oxide will lose its dielectric property.
   • Subthreshold current – For smaller device, in addition to VGS voltage, VDS voltage also has greater influence on the carrier flow in the channel. The carrier in the channel can be increased by increasing VDS voltage & also potential barrier can be reduced even VGS < VT. Thus, resulting current through the channel for higher values of VDS is called sub-threshold current.
   • Noise problem – Scaling process inevitably results in noise problem, which degrades the reliability of high density chip.

9. Why do the present VLSI circuits use MOSFETs Instead Of BJTs?

Ans:

In present VLSI circuits, MOSFETs are preferred over BJTs due to the following reasons:
  o MOSFETs are small in size and occupy a minimal silicon area on IC chips as compared to BJTs.
  o MOSFETs are relatively simple in terms of manufacturing as compared to BJTs. o MOSFETs are preferred over BJTs because digital and memory ICs can be implemented with circuits that use only MOSFETs i.e., no resistors, diodes, etc.

10.What is the depletion region in VLSI?

Ans:

When we apply a positive voltage across Gates, it causes the free holes (positive charge) to be repelled from the substrate region under the channel region. When these holes are pushed down the substrate, they create a carrier-depletion region.

11. What are the various regions of operation of MOSFET? How can we use these regions?

Ans:

There are mainly three regions of operation in MOSFET:
  o The cut-off region o The triode region o The saturation region
Here, the cut-off region and the triode region are used to operate as a switch, and the saturation region is used to operate as an amplifier.

12. What are the main steps required to solve setup and hold violations in VLSI?

Ans:

Following is the list of steps that we have to perform to solve the setup and hold violations in VLSI:
  o The optimization and restructuring of the logic between the flops are necessary to make the logic combined and solve this problem.
  o There is a way to modify the flip-flops that offer lesser setup delay and provide faster services to set up a device. Modifying the launch-flop to have a better hold on the clock pin, which provides CK->Q, makes the launch-flop fast and helps fix the setup violations.
  o Modify the network of the clock to reduce the delay or slow down the clock that captures the action of the flip-flop.
  o There is always added delay/buffer available that allows less delay to the function.