VLSI & ASIC Miscellaneous interview questions

1)Explain zener breakdown and avalanche breakdown?

A thermally generated carrier (part of reverse saturation current) falls down the junction barrier and acquires energy from the applied potential. This carriers collides with a crystal ion and imparts sufficient energy to disrupt a covalent bond.In addition to the original carrier, a new electron-hole pair has been generated. These carriers may also pick up sufficient energy and creates still another electron-hole pair. This cumulative process is called the Avalanche breakdown.
A reverse electric field at the junction causes a strong force to be applied on a bounded electron by the field to tear it out of its covalent bond. The new hole-electron pair which is created increases the reverse current, called zener breakdown.

2)What is Instrumentation Amplifier(IA) and what are all the advantages?

An instrumentation amplifier is a differential op-amp circuit providing high input impedances with ease of gain adjustment by varying a single resistor

3) What is the fundamental difference between a MOSFET and BJT ?

In MOSFET,current flow is either due to electrons(n-channel MOS) or due to holes(p-channel MOS)
- In BJT, we see current due to both the carriers.. electrons and holes. BJT is a current controlled device and MOSFET is a voltage controlled device.

4) What is the basic difference between Analog and Digital Design?

Digital design is distinct from analog design. In analog circuits we deal with physical signals which are continuous in amplitude and time. Ex: biological data, sesimic signals, sensor output, audio, video etc.

Analog design is quite challenging than digital design as analog circuits are sensitive to noise, operating voltages, loading conditions and other conditions which has severe effects on performance. Even process technology poses certain topological limitations on the circuit. Analog designer has to deal with real time continuous signals and even manipulate them effectively even in harsh environment and in brutal operating conditions.
Digital design on the other hand is easier to process and has great immunity to noise. No room for automation in analog design as every application requires a different design. Where as digital design can be automated. Analog circuits generally deal with instantaneous value of voltage and current(real time). Can take any value within the domain of specifications for the device.consists of passive elements which contribute to the noise( thermal) of the circuit . They are usually more sensitive to external noise more so because for a particular function a analog design
uses lot less transistors providing design challenges over process corners and temperature ranges. deals with a lot of device level physics and the state of the transistor plays a very important role Digital Circuits on the other hand deal with only two logic levels 0 and 1(Is it true that according to quantum mechanics there is a third logic level?) deal with lot more transistors for a particular logic, easier to design complex designs, flexible logic synthesis and greater speed although at the cost of greater power. Less sensitive to noise. design and analysis of such circuits is dependant on the clock. challenge lies in negating the timing and load delays and ensuring there is no set up or hold violation.

5)What is ring oscillator? And derive the freq of operation?

Ring oscillator circuit is a coupled inverter chain with the output being connected to the input as feedback. The number of stages(inverters) is always odd to ensure that there is no single stable state(output value). sometimes one of the stages consists of a logic gate which is used to initialise and control the circuit. The total time period of operation is the product of 2*number of gates and gate(inverter) delay. And frequency of operation will be inverse of time period.
Application: used as prototype circuits for modeling and designing new semiconductor processes due to simplicity in design and ease of use. Also forms a part of clock recovery circuit.

6)What are RTL, Gate, Metal and FIB fixes? What is a "sewing kits"?

There are several ways to fix an ASIC-based design. >From easiest to most extreme:

RTL Fix -> Gate Fix -> Metal Fix -> FIB Fix

First, let's review fundementals. A standard-cell ASIC consists of at least 2 dozen manufactured layers/masks. Lower layers conists of materialsmaking up the actual CMOS transistors and gates of the design. The upper 3-6 layers are metal layers used ti connect everything together. ASICs, of course, are not intended to be flexible like an FPGA, however, important "fixes" can be made during the manufacturing process. The progression of possible fixes in the manufacturing life cycle is as listed above.

An RTL fix means you change the Verilog/VHDL code and you resynthesize. This usually implies a new Plance&Route. RTL fixes would also imply new masks, etc. etc. In other words - start from scratch.

A Gate Fix means that a select number of gates and their interconections may be added or subtracted from the design (e.g. the netlist). This avoids resynthesis. Gate fixes preserve the previous synthesis effort and involve manually editing a gate-level netlist - adding gates, removing gates, etc. Gate level fixes affect ALL layers of the chip and all masks.

A Metal Fix means that only the upper metal interconnect layers are affected. Connections may be broken or made, but new cells may not be added. A Sewing Kit is a means of adding a new gate into the design while only affecting the metal layers. Sewing Kits are typically added into the initial design either at the RTL level or during synthesis by the customer and are part of the netlist. A Metal Fix affects only the top layers of the wafers and does not affect the "base" layers.

Sewing Kits are modules that contain an unused mix of gates, flip-flops or any other cells considered potentially useful for an unforseen metal fix. A Sewing Kit may be specified in RTL by instantiating the literal cells from the vendor library. The cells in the kit are usually connected such that each cell's output is unconnected and the inputs are tied to ground. Clocks and resets may be wired into the larger design's signals, or not.

A FIB Fix (Focussed Ion Beam) Fix is only performed on a completed chip. FIB is a somewhat exotic technology where a particle beam is able to make and break connections on a completed die. FIB fixes are done on individual chips and would only be done as a last resort to repair an otherwise defective prototype chip. Masks are not affected since it is the final chip that is intrusively repaired.

Clearly, these sorts of fixes are tricky and risky. They are available to the ASIC developer, but must be negotiated and coordinated with the foundry. ASIC designers who have been through enough of these fixes appreciate the value of adding test and fault-tolerant design features into the RTL code so that Software Fixes can correct mior silicon problems!

Verilog FAQ

Synthesis FAQ

Digital FAQ

Timing FAQ


Cmos FAQ

Misc FAQ