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Analog and Digital VLSI Design: Lecture 8: MOS Operation

This document discusses MOS operation and characteristics. It describes how MOS transistors can function as switches or capacitors depending on the gate voltage. The key current equation for an nMOS transistor is presented, showing current depends on gate-source and drain-source voltages. The linear and saturation regions of the I-V characteristics are explained. Non-ideal effects like subthreshold conduction and channel length modulation are also covered.

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Yash Gupta
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93 views14 pages

Analog and Digital VLSI Design: Lecture 8: MOS Operation

This document discusses MOS operation and characteristics. It describes how MOS transistors can function as switches or capacitors depending on the gate voltage. The key current equation for an nMOS transistor is presented, showing current depends on gate-source and drain-source voltages. The linear and saturation regions of the I-V characteristics are explained. Non-ideal effects like subthreshold conduction and channel length modulation are also covered.

Uploaded by

Yash Gupta
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PPTX, PDF, TXT or read online on Scribd
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Analog and Digital VLSI Design

EEE F313/INSTR F313

Lecture 8: MOS Operation


MOS as a Switch
For PMOS
Assert low switch
VG (A) VT, PMOS is on, else it will be off (open circuit)
it may be written as: y x. A

For NMOS
Assert high switch
VG (A) 0, NMOS is on, else it will be off (open circuit)
it may be written as: y x. A
MOS Capitance
A
Vg < 0 -- C
t
MOS ox

Polysilicon Gate ox

Silicon di oxide insulator


+ + + + + + + +
+ - + - + - + +
+ + - + - - + +
p body - -

Accumulation
MOS as a Capacitor
For 0<V <V , electrons (minority) are attracted,
G T

and holes are repelled, so negative charges


are created by acceptor ions (immobile) =>
depletion region is formed

For V >V , hole density further reduces and


G T

due to the law of mass-action, electron


density now starts increasing
IV Characteristics
Idealized model

Too many effects are neglected

In Linear region, Ids depends on


How much charge is in the channel?
How fast is the charge moving?
MOS Current Equation

For an nMOS: I f (V ,V )
ds gs ds

Inversion charge/area: Q C (V V V ( x))


n ox gs t

Charge is carried by e-

()
=

nMOS I-V
6
2.5
5

4
2

= 0 V <
3

2 1.5

Cut off 1
1
0
0 0.5 1 1.5 2 2.5


= V >
2 V <
Linear, Resistive

2 V >
= V >
2
Saturation
nMOS I-V
If Vds << (Vgs -Vt )

Voltage dependent linear resistor (Vgs)

When Vds is moderately high

Voltage dependent linear resistor (Vgs, Vds)

When, Vds >= (Vgs -Vt )

Voltage dependent current source


Non-ideal Effects
Subthreshold conduction
Ids is not zero for Vgs<Vt

Channel length modulation


Ids depends upon Vds beyond saturation

Velocity saturation
The drift velocity saturates at some electric field (Ec)
Non Ideal Effects
Mobility Degradation
- + - +
G
S D

- - n+- -- -- -- - -- --- - -- -- -- -- -- -- - - - - - - - n+ - -
+ +
+ + p- Body + - + +
+ + - + + -
+ + + + - + - - + + + +
+ +
- -
> eff
Non Ideal Effects
Tunneling Leakage gate current
G

n+ - - - - - - - - - - - - - -- n+
Non Ideal Effects
Body effect

= 0 + ( + )

Vt0 threshold voltage is at the body potential


Non Ideal Effects
Temperature Dependence
Transistor characteristics influenced by temperature
e.g. mobility, Vt

Geometry Dependence
Mismatch between assumed and actual sizes

Parasitics
CKV

Thank You

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