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Chapter: VLSI Design : Circuit Characterization and Simulation

Important Short Questions and Answers: Circuit Characterization and Simulation

VLSI Design - Circuit Characterization and Simulation - Important Short Questions and Answers: Circuit Characterization and Simulation

1. Draw the circuit of a nMOS inverter.


2. Give the expression for pull-up to pull-down ratio ( Zpu/Zpd) for an nMOS inverter driven by another nMOS inverter.


3. Draw the circuit of a CMOS inverter.


4. What are the advantages of CMOS inverter over the other inverter configurations?

a. The steady state power dissipation of the CMOS inverter circuit is negligible.


b. The voltage transfer characteristic (VTC) exhibits a full output voltage wing between 0V and VDD. This results in high noise margin.


5. What are stick diagrams?


Stick diagrams are used to convey layer information through the use of a color code. A stick diagram is a cartoon of a chip layout. They are not exact models of layout. The stick diagram represents the rectangles with lines which represent wires and component symbols.


6. What are the different color codes used for single poly silicon nMOS technology?


·           n-diffusion (n-diff.) and other thinoxide regions -green


·           Polysilicon (poly.) - red


·           Metal 1 (metal) - blue Implant - yellow


·           Contacts - black or brown (buried)


7. What are design rules?


Design rules are the communication link between the designer specifying requirements and the fabricator who materializes them. Design rules are used to produce workable mask layouts from which the various layers in silicon will be formed or patterned.


8. Define a superbuffer.


A superbuffer is a symmetric inverting or noninverting gate that can supply or remove large currents and switch large capacitive loads faster than a standard inverter.


9.What are BiCMOS Gates?


When bipolar and MOS technology are merged, the resulting circuits are referred to as biCMOS circuits. High gain vertical npn transistors with their collectors tied to the positive rail, and medium-gain lateral npn transistors are both compatible with conventional CMOS processing. BiCMOS gates can be used to improve the performance of line drivers and sense amplifiers.


10. what is the special feature of twin-tub process?


In twin-tub process, threshold voltage, body effect n and p devices are independently optimized.


11. what are the advantage of twin-tub process?


Advantages of twin-tub process are


i.          Separate optimized wells are available.

ii.        Balance performance is obtained for n and p transistors.



12. what is SOI? What is the material used as Insulator?


SOI means Silicon-on-Insulator. In this process, Sapphire or SiO2 is used as insulator.


13. what are the various etching process used in SOI process?


Various etching process used in SOI are


i.          Isotropic etching process.


ii.        Anisotropic etching process.


iii.     Preferential etching process.


14.   what are the advantages and disadvantages 0f SOI process? Advantages of SOI process

i.          There is no well formation in this process.


ii.        There is no field-Inversion problem.


iii.     There is no body effect problem.


Disadvantages of SOI process


i.          It is very difficult to protect inputs in this process.


ii.        Device gain is low.


iii.     The coupling capacitance between wires always exist.


15.   what are the possible modes in nMOS enhancement transistor?


i.          accumulation mode


ii.        depletion mode


ii.        Inversion mode


16.   In saturation region, what are the factors that affect Ids?


i.          distancebetween source and drain.


ii.        channel width


iii.     Threshold oltage


iv.     thickness of oxide layer


v.        dielectric constant of gate insulator


vi.     Carrier mobility.


17.What is Body effect?


The threshold voltage VT is not a constant w. r. to the voltage difference between the substrate and the source of MOS transistor. This effect is called substrate-bias effect or body effect.


18.What is Channel-length modulation?


The current between drain and source terminals is constant and independent of the applied voltage over the terminals. This is not entirely correct. The effective length of the conductive channel is actually modulated by the applied VDS, increasing VDS causes the depletion region at the drain junction to grow, reducing the length of the effective channel.


19. Define Threshold voltage in CMOS?


The Threshold voltage, VT for a MOS transistor can be defined as the voltage applied between the gate and the source of the MOS transistor below which the drain to source current, IDS effectively drops to zero.


20. Define Rise time


Rise time, tr is the time taken for a waveform to rise from 10% to 90% of its steady-state value.


21. Define Fall time


Fall time, tf is the time taken for a waveform to fall from 90% to 10% of its steady-state value.


22. Define Delay time


Delay time, td is the time difference between input transition (50%) and the 50% output level. This is the time taken for a logic transition to pass from input to output.


23. What are two components of Power dissipation?


There are two components that establish the amount of power dissipated in a CMOS circuit. These are:


i)  Static dissipation due to leakage current or other current drawn continuously from the power supply.


ii)  Dynamic dissipation due to - Switching transient current – Charging and discharging of load capacitances.


24. Give some of the important CAD tools.


Some of the important CAD tools are:

i) Layout editors

ii)  Design Rule checkers (DRC)

iii)     Circuit extraction

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