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8/8/2019 Lecture 01 Upload

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VLSI

design

Lecture 1:

MOS Transistor Theory


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CMOS VLSI Design3: CMOS Transistor Theory Slide 2

Outline

Introduction

MOS Capacitor

nMOS I-V Characteristics

pMOS I-V Characteristics


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Introduction

Treatment of transistors as something beyond idealswitches

An ON transistor passes a finite amount of current

Depends on terminal voltages Derive current-voltage (I-V) relationships

Transistor gate, source, drain all have capacitance

I = C ((V/(t) -> (t = (C/I) (V

Capacitance and current determine speed


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MOS Capacitor

Gate and body form MOS capacitor

Operating modes

Accumulation

Depletion Inversion

polysilicon gate

(a)

silicon dioxide insulator

p-type body+-

Vg < 0

(b)

+-

0 < Vg < Vtdepletion region

(c)

+-

Vg > Vt

depletion regioninversion region


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Terminal Voltages

Mode of operation depends on Vg, Vd, Vs Vgs = Vg Vs Vgd = Vg Vd Vds = Vd Vs = Vgs - Vgd

Source and drain are symmetric diffusion terminals By convention, source is terminal at lower voltage

Hence V ds u 0

nMOS body is grounded. First assume source is 0 too.

T

hree regions of operation Cutoff

Linear

Saturation

s

s

s

-

- -


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nMOS Cutoff

No channel

Ids = 0

+-

Vgs

= 0

n+ n+

+-Vgd

p-type body

b

g

s d


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nMOS Linear

Channel forms

Current flows from d to s

e- from s to d

Ids increases with Vds Similar to linear resistor

+-

Vgs > Vt

n+ n+

+-

Vgd = Vgs

+-

Vgs > Vt

n+ n+

+-

Vgs > Vgd > Vt

Vds = 0

0 < Vds < Vgs-Vt

p-type body

p-type body

b

g

s d

b

g

s dIds


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I-V Characteristics

In Linear region, Ids depends on

How much charge is in the channel?

How fast is the charge moving?


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Channel Charge

MOS structure looks like parallel plate capacitorwhile operating in inversion

Gate oxide channel

Qchannel =

n n

p-t pe od

gd

gate

source

-

gs

-drain

ds

channel-

g

s d

g

n n

p-t pe od

W

L

tox

SiO2 gate oxide(good insulator, Iox = 3.9)

polysilicongate


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Channel Charge


Gate oxide channel

Qchannel = CV C =

n n

p-t pe od

Vgd

gate

source

-

Vgs-

drain

Vds

channel-

Vg

Vs Vd

Cg

n n

p-t pe od

W

L

tox

SiO2 gate oxide(good insulator, Iox = 3.9)

polysilicongate


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Channel Charge


Gate oxide channel

Qchannel = CV

C = Cg = IoxWL/tox = CoxWL

V =

-t

V

t

s rc

-

V s-

r i

V s

c l-

V

Vs V

-t

W

L

t x

SiO2 t xi( i s l t r, Iox = 3.9)

polysilicongate

Cox = Iox / tox


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Channel Charge


Gate oxide channel

Qchannel = CV

C = Cg = IoxWL/tox = CoxWL

V = Vgc Vt = (Vgs Vds/2) Vt

-t d

Vgd

g t

s rc

-

Vgs-

dr i

Vds

c l-

Vg

Vs Vd

g

-t d

W

L

t x

SiO2 g t xid(g d i s l t r, Iox = 3.9)

polysilicongate

Cox = Iox / tox


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Carrier velocity

Charge is carried by e-

Carrier velocity v proportional to lateral E-fieldbetween source and drain

v=


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Carrier velocity



v= QE Q called mobility

E =


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Carrier velocity



v= QE Q called mobility

E = Vds/L

Time for carrier to cross channel:

t=


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Carrier velocity



v

= QE Q called mobility E = Vds/L

Time for carrier to cross channel:

t= L / v


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nMOS Linear I-V

Now we know

How much charge Qchannel is in the channel

How much time teach carrier takes to cross

dsI !


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nMOS Linear I-V

Now we know

How much charge Qchannel is in the channel

How much time teach carrier takes to cross

channel

ox 2

2

ds

ds gs t ds

ds gs t ds

It

W VC V V V

L

VV V V

Q

F

!

!

!

ox=

WF


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nMOS Saturation I-V

If Vgd < Vt, channel pinches off near drain

When V ds > Vdsat = Vgs Vt Now drain voltage no longer increases current

dsI !


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nMOS Saturation I-V



2dsat

ds gs t dsat I F !


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nMOS Saturation I-V



2

2

2

dsatds gs t dsat

gs t

I F

F

!

!


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nMOS I-V Summary

2

cutoff

linear

saturatio

0

2

2n

gs t

dsd s gs t ds ds dsat

gs t ds dsat

V V

V I V V V V V

V V V V

F

F

!

"

Shockley1st order transistor models


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Example

0.6 Qm process (Example)

From AMI Semiconductor

tox = 100

Q = 350 cm2

/V*s Vt = 0.7 V

Plot Ids vs. Vds Vgs = 0, 1, 2, 3, 4, 5

Use W/L = 4/2 P

14

2

8

3.9 8.85 10350 120 /

100 10ox

W W WC A V

L L L F Q Q

y ! ! !

0 1 2 3 4 5 0

0 .5

1

1 .5

2

2 .5

Vds

Ids

(mA)

Vgs = 5

Vgs = 4

Vgs = 3

Vgs

= 2

Vgs = 1


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pMOS I-V

All dopings and voltages are inverted for pMOS

Mobility Qp is determined by holes

Typically 2-3x lower than that of electrons Qn

120 cm2

/V*s in AMI 0.6 Qm process Thus pMOS must be wider to provide same current

In this class, assume Qn / Qp = 2

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