Very Large-Scale Integration (VLSI) is the process of integrating or embedding hundreds of thousands of transistors on a single silicon semiconductor microchip. VLSI is widely used technologies for microchip processors, integrated circuits (IC), and component design. Most companies and businesses have moved to VLSI. 

On that note, getting called in for an interview is only the first step toward your ideal VSLI job. You must prepare thoroughly for the interview, which usually determines whether or not you will be hired. So, what is the best way to prepare? This article covers 30+ common VLSI interview questions you should be prepared to answer.

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Top 30+ Most Asked VLSI Interview Questions

The list below contains the most frequently asked VLSI Interview questions and their best possible answers.

1. What do you understand by Boolean logic?

Boolean logic is a type of algebra in which the results are either TRUE or FALSE (known as truth values or truth variables). Instead of arithmetic operators such as addition, subtraction, and multiplication, Boolean logic employs three fundamental logical operators: AND, OR, and NOT.

2. What is the usage of Boolean logic?

Boolean logic examines a reported relationship between things to determine whether or not the relationship is true. Consider the following equation:

2 + 2 = 4

A Boolean expression is the combination of the two parts 2 + 2 and 4 and the relationship (= equals) in this example.

Remember that Boolean logic works only when an expression can be TRUE or FALSE.

For example, 3 + 8 is not a Boolean expression because it is not being compared or related to anything else. However, the expression 3 + 8 = 10 is a Boolean expression because we can now evaluate each side and determine whether the reported relationship between them is TRUE or FALSE (in this case, FALSE).

3. How does a Boolean logic control the logical gates?

The true state in Boolean algebra is denoted by the number one, also known as logic one or logic high. On the other hand, the false state is represented by the number zero, also known as logic zero or logic low. In digital electronics, the presence of a voltage potential denotes a logic high.

4. Why do the present VLSI circuits use MOSFETs instead of BJTs?

The advantage of using MOSFETs over BJTs is that power dissipation and leakage currents are minimized in the circuits, so they have been used instead.

BJTs

MOSFETs

Larger

Can be made very small as they occupy a tiny silicon area on IC chip

More complex to manufacture

Relatively simple in terms of manufacturing

Poler device

Unipoler device

Current controller

Voltage controller

3 terminal devices which are emitter, collector, and base

MOSFET is a four terminal device, mainly source, gate, drain, and substrate (also known as the body)

A BJT is limited to something like 0.3v for the lowest voltage drop on the current path

MOSFETs don't need current on their control pin but require more voltage

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

MOSFETs operate in three regions: the cut-off region, the triode region, and the saturation region. The cut-off region and the triode region serve as switches. The saturation region serves as an amplifier.

6. What are the different gates where Boolean logic is used?

Different gates in Boolean Logic:

  • NOT Gate
  • AND Gate
  • OR Gate
  • NAND Gate
  • NOR Gate
  • XOR Gate
  • XNOR Gate

7. What do you understand by the threshold voltage?

The threshold voltage is the voltage above which a specific phenomenon occurs depending on the device. A MOSFET's threshold voltage is the gate voltage's value when a conductive band forms between the transistor's source and drain.

8. How can binary numbers give a signal or be converted into a digital signal?

The process of converting binary data, a sequence of bits, to a digital signal is known as line coding. Line encoding can be polar, unipolar, or bipolar.

9. What does "the channel is pinched off" mean?

Pinch-off occurs when a voltage is applied to the drain (VD) until it reaches the drain voltage saturation (VDS). This causes the depletion region at the drain to increase due to a decrease in the electrical field at the drain edge, resulting in only a small electron, if any, being induced at the drain edge.

10. What are the key differences between the TTL chips and CMOS chips?

The differences are:

TTL Chips

CMOS Chips

TTL chips are used in transistor logic. Each logic gate is built with two Bi-polar Junction Transistors.

CMOS is an abbreviation for Complementary Metal Oxide Semiconductor. It is also an integrated chip but designed with field effect transistors. 

TTL chips can contain many components, such as resistors.

CMOS has a higher density of logic gates. A single logic gate in a CMOS chip can be made up of as few as two FETs.

The TTLS chip uses a lot more power, especially at rest. A single gate in a TTL chip consumes approximately mW of power.

CMOS chips use less energy. A single CMOS chip consumes approximately 10nW of power.

TTL chips are suitable for use in computers.

In mobile phones, CMOS chips are used.

11. What is the most significant advantage of the CMOS chips over the TTL chips?

The main advantage of the CMOS chip over the TTL chip is that it has a higher density of logic gates within the same material, which improves its efficiency. Another significant advantage is that CMOS chips consume less power than TTL chips even when they are idle.

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12. What do you understand by Channel-length Modulation?

Channel length modulation (CLM) is a field effect transistor effect that causes the length of the inverted channel region to shorten as the drain bias increases for significant drain biases. CLM causes an increase in current with drain bias and a decrease in output resistance. In MOSFET scaling, it is one of several short-channel effects.

13. What do you understand by a sequential circuit?

A sequential circuit is a logic circuit with inputs (X), logic gates (Computational circuit), and outputs (Y) (Z). A sequential circuit generates an output based on the current and previous input variables, whereas a combinatorial circuit generates an output based solely on the input variable.

14. What is the depletion region in VLSI?

Electrons diffuse across the junction to combine with holes, forming a "depletion region." A depletion region is near the p-n junction, where the flow of charge carriers (free electrons and holes) is reduced over time until no charge carriers are left—semiconductors with P-N junctions.

15. What is Verilog? How is it different from normal programming languages?

Verilog is a programming language that aids in designing and verifying digital circuits. IEEE 1364-2005 is the most recent stable version.

The primary distinction between Verilog and C is that Verilog is a Hardware Description Language, whereas C is a high-level, general-purpose programming language. 

16. What are the various factors that can affect the threshold voltage?

Several factors, including the influence of the threshold voltage are as follows:

1. Doping of the Substrate

With substrate doping, the threshold voltage rises. During channel formation, the p-type substrate will have to invert to n-type near the gate. As the doping level rises, a stronger bias is required to move the majority of carriers away from the channel region. You can avoid this by adjusting the doping near the gate area.

2. Doping at the Gate

The conductivity of the polysilicon gate is increased by poly doping. Poly depletion aids in channel formation, lowering the threshold voltage as doping increases.

3. Length of the Channel

The threshold voltage decreases as the channel length decreases.

4. Oxide Gate

Thicker gate oxide raises the device's threshold voltage, whereas thinner dielectric lowers the threshold voltage.

17. What does the "timescale 1 ns/ 1 ps" specify in Verilog code?

Verilog's timescale 1ns/1ps denotes that your time scale is ns with a resolution of 1ps.

18. What are the two types of procedural blocks in Verilog?

Blocks in the procedure called procedural blocks contain Verilog code that describes the behavior of a circuit. You can find many of these procedural blocks in a module. There are two kinds: Initial and always.

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

Main steps to take to resolve setup and hold violations in VLSI

  • The optimization and reorganization of the logic between the flops are carried out. Logics are combined to aid in the resolution of this problem
  • There is a way to modify the flip-flops to provide less setup delay and faster device setup services
  • Modifying the launch-flop to have a better hold on the clock pin, which provides CK->Q, making the launch-flop fast and aiding in the resolution of setup violations
  • You can modify the clock network to reduce the delay or slowing of the clock that captures the flip-flop action
  • You can add a delay/buffer to the function to allow for less delay

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

The greater the number of stacks, the slower is the gate. The number of gates in the stack of NOR and NAND gates is usually the same as the number of inputs plus one. As a result, the input is limited to four.

21. What are the different types of skews used in VLSI?

Skews are classified into three types: local, global, and useful.

  • The difference between the launching and destination flip-flops is dealt with by the local skew, which defines a time path between them
  • The global skew is the time difference between the earliest component arriving at the flip and the latest within the same domain
  • Even though the delays are not measured, the clock is usually uniform for both components
  • The useful skew defines the delay in recording the capturing flip flop paths and thus aids in the creation of an environment with the exact requirements for the timing path launch and capture; You must meet the holding requirement in this case

22. What do you understand by multiplexer?

A multiplexer is a digital combinational circuit that selects and outputs a single input from multiple input lines. It performs the same function as a switch. As a result, we can refer to it as a digital switch.

23. What do you understand by an SCR?

SCR (silicon control rectifier) is a type of thyristor that can switch between ON and OFF states via bias or gate input control. Its symbol is similar to that of a diode. As the name implies, it is made up of silicon that controls power and converts AC voltage to DC.

24. What do you understand by DCMs? Why are they used?

DCM, or Digital Clock Manager, is a fully digital control system that uses feedback to maintain clock signal characteristics with high precision. DCM accomplishes this despite normal variations in operating temperature and voltage.

25. What is slack in VLSI?

  • Slack is the difference between the desired and actual arrival times of a signal
  • Slack time determines whether the design is operating at the desired frequency [for a timing path]
  • Positive slack indicates that the design is meeting the deadline but can still be improved
  • Zero slack indicates that the design is operating at the desired frequency
  • Negative slack demonstrates that the design did not meet the specified timings at the specified frequency
  • Slack must always be positive, and negative slack indicates a timing violation

26. What is the usage of defparam?

The keyword defparam is used to modify the parameter values at any module instance in the design. At compile time, the defparam overrides the parameter value.

27. How many transistors do a static RAM use?

For the same amount of storage, static RAM requires approximately six times the number of transistors as dynamic RAM. In most cases, dynamic RAM employs one or two transistors per bit. Add transistors for address decoding, bus interface, and so on.

28. What are the different ways to prevent Antenna Violation?

Three fundamental techniques for preventing antenna violations are:

  • Metal hopping: reduce the amount of charge accumulation by reducing the area of metal interconnect connected to a transistor's gate.
  • Floating gate attachment: increase the gate area so that the ratio (metal area)/(gate area) is less than the maximum metal to gate area ratio allowed. You can accomplish this by connecting the floating gates to the appropriate net.
  • Antenna diode: the addition of an antenna diode provides an alternative path to discharge the accumulated charges on the transistor's gate.

29. What is the function of tie-high and tie-low cells?

Tie-high and tie-low connect the gate's transistors using either power or ground. The gates are connected using power or ground, and the power bounces from the ground, turning them on and off.

30. What is the primary function of metastability in VHDL?

When digital circuits attempt to synchronize asynchronous digital data, they experience metastability. In this case, a slight deviation causes the outputs to revert to one of the two stable states.

31. What is the way to stop metastability in VLSI?

Most metastable conditions manifest themselves in one of two ways:

  • You are sampling a signal that is not generated by the FPGA
  • You are crossing time zones

Both of these situations get easily remedied. You can simply "double-flop" your data whenever encountering a situation that may introduce metastability.

32. What is MTBF in VLSI?

Mean Time To Failure (MTTF) - the time it takes for a component or instrument to fail and be replaced. Mean Time Between Failures (MTBF) - the time it takes for an element or instrument to get repaired after it has failed.

33. What is the difference between the Mealy and Moore state machine?

A Mealy machine has fewer states than a Moore machine. The Mealy machine changes its output based on its current state and the current input. Its output is placed on the transition. Every Mealy machine responds to inputs relatively quickly.

Moore Machine Model

Mealy Machine Model

The Moore model includes machines with an entry action, and the output is determined solely by the machine's state.

The Mealy model only uses input actions, and the output gets determined by the program's state and previous inputs.

Hardware systems are designed using the Moore model.

Both hardware and software systems are designed using the Mealy model.

Because the program is written only in the state, the Moore machine's output is solely dependent on the state. The Mealy machine's output is affected by both the state and the input.

The Mealy machine's output is a combination of both input and state.

When we change the signal, the state variables also delay.

The Moore machine has no glitches, and its output is determined solely by states rather than the input signal level.

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34. What is the difference between Synchronous and Asynchronous Reset?

Synchronous reset means that the reset is sampled concerning the clock. In other words, enabling reset will have no effect until the next active clock edge.

Asynchronous reset samples reset independently of CLK. When reset is enabled, it takes effect immediately and does not check or wait for clock edges.

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Conclusion

Practice these VLSI interview questions and ace your interview in no time. If you are finding difficulty in getting a job, then sign-up for our Master’s course in Full Stack Java Developer with a Job Guarantee.  Get job-ready with HIRIST and HackerEarth. Complete the course to get an assured job with an average salary of 6.7 LPA. Apply Now!

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