Capgemini VLSI Interview Questions

  Capgemini VLSI Interview Questions

1) Why do the present VLSI circuits use MOSFETs Instead Of BJTs?

In present VLSI circuits, MOSFETs are preferred over BJTs due to the following reasons:

1. MOSFETs are small in size and occupy a minimal silicon area on IC chips as compared to BJTs.

2. MOSFETs are relatively simple in terms of manufacturing as compared to BJTs.

3. 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.

2) What are the different gates where Boolean logic is used?

There are mainly three types of gates used in Boolean logic:

1. NOT Gate: The NOT gate is a logic gate that is used to implement logical negation. It has one input and one output. For example, if A = 0, then the Value of B =1 and vice versa.

2. AND Gate: The AND gate is a logic gate that implements logical conjunction. It has one output due to the combination of two outputs. For example, if A and B = 1, then the value of output should be 1. On the other hand, if any input value is 0, then the output will be 0.

3. OR Gate: The OR gate is a logic gate that is used to implement logical disjunction. If the value of one or both the inputs to the gate is 1, the output will be 1. If both the inputs are 0, the output will be 0. For example, if A's value is 1 or B is 0, then the value of output will be 1.

3) What do you understand by the threshold voltage?

The threshold voltage is commonly abbreviated as V??. It can be defined as a voltage between Gate and Source, i.e., VGS. A sufficient number of mobile electrons accumulate in the channel region and create a conducting channel. It is the minimum gate-to-source voltage required to create a conducting path between the source and the drain terminals. It is an essential scaling factor to maintain power efficiency.

4) How can binary numbers give a signal or be converted into a digital signal?

Binary number contains two numbers, either 0 or 1. In simple words, we can say that the number 1 represents the ON state, and number 0 represents the OFF state. Using these binary numbers, we can combine billions of machines into one machine or circuit and operate those machines by performing arithmetic calculations and sorting operations.

5) What do you understand by Channel-length Modulation?

When we increase the VDS beyond the saturation point, it affects the MOSFET characteristics, and the channel pinch-off point starts moving away from the Drain towards the Source. Due to this process, the effective channel length decreases. This phenomenon is called Channel Length Modulation.

6) What is the depletion region in VLSI?

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.

7) What is the reason behind the number of gate inputs to CMOS gates usually limited to four?

The number of gate inputs to CMOS gates usually limited to four because a higher number of stacks make the gate slower. In NOR and NAND gates, the number of gates present in the stack is usually like the number of inputs plus one. That's why the input gates are restricted to four.

8) What are the different types of skews used in VLSI?

In the clock, a skew is used to reduce the delay or better understand the process. There are mainly three different types of skews used in VLSI.

1. Local skew: The local skew is generally used to include the difference between the launching flip-flop and the destination flip-flop. This differentiation helps to define a time path between the two.

2. Global skew: The global skew defines the difference between the earliest components reaching the flip flop within the same clock domain. It needs to be mentioned in this skew. The delays are not measured while the clock is uniform for both.

3. Useful skew: The useful skew is used to define the delay in capturing flip flop paths, which later helps set up an environment with precise requirements for the launch and capture of the timing path. It needs to be mentioned for design purposes to met the hold requirements.

9) What do you understand by an SCR?

SCR is a short form that stands for Silicon Controlled Rectifier. It is a four-layered, 3-terminal solid-state device used to control the flow of current. It is a type of rectifier that is controlled by a logical gate signal.

10) What is the function of tie-high and tie-low cells?

The tie-high and tie-low cells are used to connect the transistors of the gate by using either the power or the ground. The gates are connected using the power or ground to turn off and on them because the power bounces from the ground. The cells stop the bouncing and ease the current from one cell to another. These cells require Vdd that connects to the tie-high cell as a power supply is high, and tie-low gets connected to Vss. After the connection establishment, the transistors function correctly without any ground bounce occurring in any cell.

11) What is the primary function of metastability in VSDL?

Metastability is a phenomenon of unstable equilibrium in digital electronics. In metastability, the sequential element is not able to resolve the state of the input signal. That's why the output goes into an unresolved state for an unbounded interval of time. Metastability is used in designing a system that violates the setup or holds time requirements. The setup time requirement needs the data to be stable before the clock edge, and the hold time requires the data to be stable after the clock edge has passed. There are potential violations that can lead to setup and hold violations as well.

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