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Introduction toCMOS VLSI

Design

Introduction

ASHWANI MISHRA

NARAINA COLLEGE OF ENGINEERING & TECHNOLOGY

KANPUR

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CMOS VLSI Design0: Introduction Slide 2

Introduction

Integrated circuits: many transistors on one chip.

Very Large Scale Integration (VLSI): very many

Complementary Metal Oxide Semiconductor

Fast, cheap, low power transistors Today: How to build your own simple CMOS chip

CMOS transistors

Building logic gates from transistors

Transistor layout and fabrication

Rest of the course: How to build a good CMOS chip

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CMOS VLSI Design0: Introduction Slide 3

Silicon Lattice

Transistors are built on a silicon substrate

Silicon is a Group IV material

Forms crystal lattice with bonds to four neighbors

Si SiSi

Si SiSi

Si SiSi

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CMOS VLSI Design0: Introduction Slide 4

Dopants

Silicon is a semiconductor

Pure silicon has no free carriers and conducts poorly

Adding dopants increases the conductivity

Group V: extra electron (n-type) Group III: missing electron, called hole (p-type)

As SiSi

Si SiSi

Si SiSi

B SiSi

Si SiSi

Si SiSi

-

+

+

-

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CMOS VLSI Design0: Introduction Slide 5

p-n Junctions

A junction between p-type and n-type semiconductor

forms a diode.

Current flows only in one direction

p-type n-type

anode cat ode

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CMOS VLSI Design0: Introduction Slide 6

nMOS Transistor

Four terminals: gate, source, drain, body

Gate oxide body stack looks like a capacitor

Gate and body are conductors

SiO2 (oxide) is a very good insulator Called metal oxide semiconductor(MOS)

capacitor

Even though gate is

no longer made of metal

n

p

GateSource Drain

bulk Si

SiO2

Polysilicon

n

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CMOS VLSI Design0: Introduction Slide 7

nMOS Operation

Body is commonly tied to ground (0V)

When the gate is at a low voltage:

P-type body is at low voltage

Source-body and drain-body diodes are OFF No current flows, transistor is OFF

n

p

GateSource Drain

bulk Si

SiO2

Polysilicon

n

D

0

S

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CMOS VLSI Design0: Introduction Slide 8

pMOS Transistor

Similar, but doping and voltages reversed

Body tied to high voltage (VDD)

Gate low: transistor ON

Gate high: transistor OFF Bubble indicates inverted behavior

SiO

n

GateSource Drain

bulk Si

olysilicon

p+ p+

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CMOS VLSI Design0: Introduction Slide 9

Transistors as Switches

We can view MOS transistors as electrically

controlled switches

Voltage at gate controls path from source to drain

g

s

d

g = 0

s

d

g = 1

s

d

g

s

d

s

d

s

d

nMOS

pMOS

OFFON

ONOFF

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CMOS VLSI Design0: Introduction Slide 10

CMOS Inverter

A Y

0

1

V

A Y

A Y

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CMOS VLSI Design0: Introduction Slide 11

CMOS Inverter

A Y

0

1 0

V

A Y 0

GND

ON

OFF

A Y

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CMOS VLSI Design0: Introduction Slide 12

CMOS Inverter

A Y

0 1

1 0

VDD

A 0 Y 1

GND

OFF

ON

A Y

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CMOS VLSI Design0: Introduction Slide 13

CMOS NAND Gate

A B Y

0 0

0 1

1 0

1 1

A

B

Y

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CMOS VLSI Design0: Introduction Slide 14

CMOS NAND Gate

A B Y

0 0 1

0 11 0

1 1

A=0

B=0

Y=1OFF

ON ON

OFF

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CMOS VLSI Design0: Introduction Slide 15

CMOS NAND Gate

A B Y

0 0 1

0 1 11 0

1 1

A=0

B=1

Y=1OFF

OFF ON

ON

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CMOS VLSI Design0: Introduction Slide 16

CMOS NAND Gate

A B Y

0 0 1

0 1 11 0 1

1 1

A=1

B=0

Y=1ON

ON OFF

OFF

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CMOS VLSI Design0: Introduction Slide 17

CMOS NAND Gate

A B Y

0 0 1

0 1 11 0 1

1 1 0

A=1

B=1

Y=0ON

OFF OFF

ON

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CMOS VLSI Design0: Introduction Slide 18

CMOS NOR Gate

A B Y

0 0 1

0 1 01 0 0

1 1 0

A

B

Y

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CMOS VLSI Design0: Introduction Slide 19

CMOS Fabrication

CMOS transistors are fabricated on silicon wafer

Lithography process similar to printing press

On each step, different materials are deposited or

etched Easiest to understand by viewing both top and

cross-section of wafer in a simplified manufacturing

process

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CMOS VLSI Design0: Introduction Slide 20

Inverter Cross-section

Typically use p-type substrate for nMOS transistors

Requires n-well for body of pMOS transistors

n

p substrate

p

n well

A

YGND VDD

n p

SiO2

n diffusion

p diffusion

polysilicon

metal1

nMOS transistor pMOS transistor

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CMOS VLSI Design0: Introduction Slide 21

Inverter Mask Set

Transistors and wires are defined by masks

Cross-section taken along dashed line

GND VDD

Y

s strate ta ell ta

n OS transistor OS transistor

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CMOS VLSI Design0: Introduction Slide 22

Summary

MOS Transistors are stack of gate, oxide, silicon

Can be viewed as electrically controlled switches

Build logic gates out of switches

Draw masks to specify layout of transistors

Now you know everything necessary to start

designing schematics and layout for a simple chip!