Getting shortlisted for an interview is just the first step towards landing your dream job. You must prepare well for the interview, which normally determines whether you will be granted the position or not. What then is the best means of preparation?
It would be best to go through several common interview questions in the related field to be safe. Brainstorming an answer to a possible question beforehand will help you answer it well when asked. This article will look at 20 common questions in VLSI interviews that you should have at your fingertips.
Let’s get right into it!
1. Tell Us About The Procedural Blocks in Verilog
Unlike other fields, VLSI interviews are mainly made up of technical questions. You should therefore know how to approach these lest you mess up your chance. Do not beat around the bush but give the interviewer a concise and straight-to-the-point answer.
There are two main types of procedural blocks for Verilog, the initial and always blocks. The former only runs once at zero, whereas the latter executes itself repeatedly in a loop fashion. It always gets executed hence the name always.
2. Do You Know the Skews Used in VLSI? Would you please Tell Us About the Different Types?
You should know that there are only three different skews in VLSI. The best approach to this question is to first identify the role of the skew before explaining the different available types to the interviewer.
Skews are used in the clock to reduce delays and processes more understandable. There exist three types of skews: local, global and useful. The local skew deals with the difference between the launching and destination flip flop, defining a time path between them.
The global skew relates to the delay between the earliest component arriving at the flip compared with the latest within the same domain. The clock is usually uniform for both components, even though the delays are not measured.
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The useful skew defines the delay in recording the capturing flip flop paths and thus helps create an environment with the exact requirement for the launch and the capture of the timing paths. The holding requirement must be met in this case.
3. You Must Have Used Other Programming Languages in Your Career. Can You Tell Us How They Are Different from Verilog?
The interviewer wants you to compare Verilog with other programming languages. It would help if you, therefore, convinced them that you have a deep understanding of both. Also, make sure that your answer is concise and straight to the point.
I have used both Verilog and other normal programming languages and therefore understand their differences. Verilog has several threads and simulation time concepts that the normal programming languages lack. Verilog also has basic circuit concepts such as network connections.
4. Do You Know the Three Regions of Operation of MOSFET? How Are These Used?
This is a technical question that seeks to unravel just how good you are in this field. While answering, first start by explaining what MOSFET does before you delve into how the regions are used. Make sure that your answer is correct.
MOSFET, also known as metal oxide semiconductor field-effect transistor, is a special type of transistor which plays a big role in amplifying or switching electronic signals. It has three regions of operation, namely triode, saturation, and cut-off regions. The triode and cut-off regions function as switches, whereas the saturation region acts as an amplifier.
5. You Must Have Dealt with Static RAM. Therefore, Could You Tell Us How Many Transistors It Use
The interviewer is assessing your knowledge of a particular item in Verilog. Your answer should therefore convince them that you have vast experience and knowledge in this field. Given that this is a technical question, therefor is only one that you must get right.
A static RAM uses a total of six transistors. It is also quite special since both the read and write operations use the same port. (You can explain a bit further. However, do not steer away from the main point)
6. Tell Us the Different Gates That Call for The Application of The Boolean Logic
This will mostly be asked as a follow-up question once you have answered something on the Boolean logic. All in all, you still need to show the interviewer that you understand several concepts in this field. Your answer must be detailed enough.
There are three main types of gates where Boolean Logic is applicable. These are NOT, AND, and OR gates. The first (NOT) gate has one output and input. The AND gate has one output since it combines two outputs. For the OR gate, either of the value shows the same output. Any other functional gates work in combination with these three. They include the XNOR gate, NAND gate, NOR gate, and the XOR gate.
7. What Do You Understand by Verilog?
The interviewer will most probably ask this at the start of the interview or before it progresses further. Verilog is an important aspect of VLSI that you need to know. Make sure that your answer is correct.
Verilog is a hardware description language used to describe electronic circuits and systems. The circuit components are usually prepared inside a module, complete with both behavioral and structural statements. The structural statements consist of the logic gates, microprocessors, and counters, whereas behavioral statements have programming aspects such as stimulus vectors, if-then statements, and loops.
8. What Do You Understand by The Depletion Region?
By now, you must have noticed that there are several concepts in VLSI that you need to have at your fingertips. This question, like most of our recommendations, is testing your knowledge of a given concept. Make sure that you correctly and clearly answer the question. Avoid guesswork.
A depletion region is created when a positive voltage is transmitted across a gate, pushing back/ repelling the free holes from the substrate area under the gate. It is also known as a carrier depletion region owing to how it is created.
9. Why Do VLSI Circuits Use MOSFETS Instead of BJTs?
The interviewer is testing your knowledge of VLSI circuits. This is a technical question that needs to be answered correctly. Use your experience and knowledge of these circuits to convince the interviewer that you are good at your job.
There are three main reasons why VLSI circuits use MOSFETS instead of BJTs. First, MOSFETS are usually more compact than BJT since they can easily occupy tiny silicon areas on IC chips. Second, they are easier to manufacture, and finally, only circuits that use MOSFETS can work with digital and memory ICs.
10. How Can a Binary Number Give a Signal, Or Better Yet, Convert a Digital Signal?
This is yet another technical question seeking your knowledge on signal creation. First, start by explaining binary numbers before delving into what you have been asked. Make sure that you get it right.
A binary number is made up of either 0 or 1. Number 1 normally stands for the ON state, whereas number 0 represents the OFF state. These numbers can combine several machines into one or a circuit and operate them through arithmetic calculations and sorting operations.
11. How does Boolean Logic control logical Gates?
This is a technical question that seeks to determine whether you know the Boolean concepts in VLSI. The best approach is to offer a comprehensive answer without beating around the bush. Also, make sure that your repose is correct.
This is explained in Boolean algebra, where the true state is usually represented by the number one, usually referred to as the logic one or logic high. The false state is denoted by zero and is referred to as logic zero or low. It is quite different in digital electronics, where the logic high is usually denoted by a voltage potential.
12. How Would You Solve Setup and Hold Violations In VLSI?
The interviewer wants you to name the steps required in solving setup and hold violations in this field. This can either be an operational or technical question. All in all, make sure that you offer a comprehensive answer.
Solving the setup and hold violations in VLSI require several steps. It would be best if you carried the logic optimization and restructuring between the flops to combine the logics, which will solve the problem. The flip-flops that offer lesser setup delays should then be modified to offer faster services. You can also modify the network of the clock to prevent the clock from slowing down.
13. How Can One Prevent Antenna Violation?
Antenna violation normally occurs during plasma etching, where the charges generated from one metal strip to the other converge in one place. A longer strip means more charge accumulation. The best approach to this interview question is to mention several reliable methods that one can use to avoid this.
There are several proven ways of preventing antenna violation. One can add reverse diodes at the gates used in the circuits, jog the metal above the one with the etching effect or create jogging in the metal line, made up of one or more metals on the protected layer.
14. Your Resume Shows That You Have Spent a Good Deal of Time in This Field and Worked in Several Companies Both as An Employee and a Freelancer. What Would You Prefer? Freelancing or Employment?
This is a general question in interviews that has nothing to do with the job listing. The interviewer wants to get you talking. Through the conversation, they may pick one or two things about you. Be honest, but do not talk badly of either.
I have indeed had the chance to witness both worlds. Both have their fair share of advantages and disadvantages. Freelancing allows you to take charge of your working patterns and methods, giving you the freedom most people want. However, it is usually uncertain as one can go for a long time without work and also require you to go out of your way most of the time. Employment, on the other hand, comes with a certain level of job security. You wake up every day knowing that you have a job and expect a given salary at the end of a given time.
Both have their advantages, but I would pick employment over freelancing, or if possible, a blend of both.
15. You Know What Tie-High and Tie-Low Cells Are. Could You Tell Us Their Functions?
Such questions require you to go straight to the point. Well, you can give a brief explanation of what the cells are, but do not allow it to take up most of your answer. Your main focus should be on the functions and not what they are.
These two cells are usually used to connect the gate transistors through power or the ground. The cells, therefore, step in to replace the power bounce on the ground, which causes the turning on and off of the transistor. They ensure smooth current flow from one cell to the other. The tie-highs are usually connected to the VDD, whereas the tie-lows to the VSS. When this connection is established, the transistor does not need any power to bounce from the ground and function well.
16. What Do You Understand by Metastability in VSDL?
This is yet another functional question that sets to determine whether you understand the different functions and states in VSDL. The interviewer is testing your knowledge, and therefore, your answer should convince them that you are vastly learned in VSDL.
This is an unknown state which can neither be expressed as one or zero. It is used to make systems that go against the setup or hold time requirements. The setup time requirement is that the data should be stable before the clock edge, and the hold time calls for stable data after the passing of the clock edge. The resultant data in such a system is usually asynchronous and clock synchronous.
17. How Can One Prevent Metastability?
Whenever this question is asked immediately after one similar to the previous one, treat it as a follow-up question. The interviewer has used the previous question as an opening query and a foundation for the current one, all in all, make sure that you offer the correct steps in your reply.
There are several ways to prevent metastability. One can use proper two or three-stage synchronizers in scenarios where data originates from the asynchronous domain to recover the stat event. These synchronizers can also be used between cross-locking domains to remove the delay caused by the data element and thus reduce metastability. Lastly, using faster flip-flops can work the magic as it allows faster transactions and removes delay time from one component to another.
18. What Are Some of The Things That One Can Do to Achieve Better Yield?
This is an operational question that seeks to understand what you would do if faced with a similar scenario. Mention the different measures that can be taken for better yield.
The best way to achieve better yield is to work on the manufacturability flaws. Some of the measures that one can take include checking areas in design with lithographic issues, such as those having sharp cuts; using redundant vias which reduce carriage and barrier breakage; placing the decoupling capacitances optimally to deal with the power surges, and creating powerful russet files with spacing and shorting rules.
19. Differentiate Between Latches and Flip Flop Designs
We have mentioned flip flops severally in our article. It is a base design, similar to latches. These two are some of the common designs in VSLI, and therefore, it should not shock you when asked to differentiate between them.
Latches are usually level-sensitive, whereas the latter is edge-sensitive. The latch-based design has room for time borrowing which lacks in a traditional flop, allowing them to be more efficient. However, flip flop designs are usually less complicated than the latched designs.
20. How Did You Learn About Us?
This is a straightforward question. Have you had eyes on the company for quite some time, or did you see it on TV? Be honest with the interviewer.
I have interacted with some of your former employees who are proud of this organization. They are all grateful for the chance they got, which helped them progress in their careers. I have also been keeping up with your company’s news. (There are several things that you can mention, e.g., social media)
These are some of the commonest questions in VLSI interviews. Make sure that you carefully go through our answers once more as you also try to come up with unique ones. Also, work on your posture, speech, and answering skills, as well as basic interview skills to increase your chances of landing the job.