microelectronics - mosfet lecture - iit kanpur

8/12/2019 Microelectronics - MOSFET Lecture - IIT Kanpur

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EE210: Microelectronics-I

Lecture-27 : MOSFET-1

B. Mazhari

G-NumberB. Mazhari, IITK

1Dept. of EE, IIT Kanpur


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Transistor

+

IO

+

VoVIN

-

urren O s muc more sens ve o IN an O

O O

in oV V


3/25

Field Effect Principle O Oin o

I I

V V

-2V

- - - - - - - - - -- - - - - - -

+ - + -+ - + - + - + - + - + -

N-Type+ -

+ -+ - + - + - + - + - + -+ - + - + - + - + - + -

Modulation of conductivity using electric field

Transconductance


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N-Type

Oxide

N-Type


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Transistors

Transistor

BipolarJunction FieldEffectTransistorTransistor(BJT) (FET)

MetalOxidesemiconductor(MOSFET)


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Depletion-Mode Transistor

DrainSource

0V 5V

-

n+n+ N-type

-

0V

In a depletion-mode transistor, a channel exists without any gate voltage beingapplied and current flows when drain voltage is applied.

current to reduce.


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Channel exists at zero gate voltage and is depleted by gate voltage

DrainSource-1V

-2V-3V

Oxide

n+n+

p-type

Channel is completely pinched off and current ~zero


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NMOS Enhancement mode transistor: Inversion ModeTransistor

G

SiO

-S

N+N

+

P-Silicon

B


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Gate

ource0V 5V

n+n+

p-type

Body0V

No channel exists when gate voltage is zero and current is

zero as we .


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Sourcee Drain

+

.+ + + + + + + + + + +

Oxide

+

+

+

+

n+ n+ + + + +

-

+ + + +


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Sourcee Drain

+

.+ + + + + + + + + + +

Oxide

- -n+ n+ + + + +

-

+ + + +

Depletion width increases in p n

But something interesting happens: electron density at the surface also

increases


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At a sufficiently large voltage (>VTHN) a channel of electrons

forms at the Si/SiO2 interface.

GateDrainSource + + + + + + + + + + + + + +

Oxide

- - - - - - - - - - - - - - - - - - -

p-type

Body

Depletion charge

(inversion layer)


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Conductivity modulation at the surface?

V

Gate

+ + + + + + +

+

+

+

+

+

+

+

+ + + + + + +

p-type + + + + + +

16 3

2

10A

p N cm

n

A

n cm

N


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Field Effect 2 20 31 10s sn p n cm

VG T

1016

1015

1010

108

104

Ps:

104

105

1010

1012

n : 1016

1017

InvertedStrong

Inversion.

Surface P-Type

Intrinsic

N-Type

Surface carrier density can be changed from P-type to N-type


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Surface Carrier Density

ps ns

NAps

ni

ns

VV


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Flat band condition

V V

VG

Oxide

Whenever two different

material are brought into

+ + + + + + +

+ + + + + + +

,

potential difference

develops like in a pn

-

+

+

+

+

+

+

+unc on. us even

when no gate voltage is

applied, there is a

Body

voltage across the mos

capacitor.

V = V Flat-band condition meanin no NET volta e across the ca acitor.

Uniform hole density everywhere


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Depletion

butG FB G THN V V V V

Gate

Oxide

+ + + + + + + Depletion region

p-type +

+

+

+

+

+

+

+ + + + + +

Body

Holes are depleted from the surface S Bp p

Although n electron density is also very smallS Bn


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Strong Inversion G THN V V

Gate

Oxide

- - - - - - - - - - - - - - - - - - - - - - - - -

Depletion region

p-type

+

+

+

+

+

+

+ + + + + + +

Body

Electrons are accumulated at the surface S An N


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Accumulation

G FBV V

Gate

Oxide

+ + + ++

+ + + + + + + extra holes

p-type

+

+

+

+

+

+

+ + + + + + +

Body

Holes are accumulated at the surface S Bp p


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MetalOxideSemiconductorFieldEffectTransistor:

MOSFET

Depletion Mode Enhancement Mode

N-Channel P-Channel N-Channel P-Channel

G GG B

S S

SS


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Simplified Symbols and structure

G

D

G

D

B

S

S/BNMOS

Gatera nSource

OxideDrain

Oxide

Gate

SourceBody

n+n+n+n+p+

p-type

Body

p-type


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Simplified Symbols and structure

DrainGate

D D Oxide

+ +G

S/B

B

PMOS -

Body

DrainGateSourceBod

Oxide

++n+

n-t e


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G G

Oxide

+

B SB

++ + ++

DD

n-type

p-type

Body potential of NMOS transistors is same and is normally connected to

body-source PN junction is reverse biased.

ac can e a r cate n a separate n v ua -we an t us

each pmos body terminal can have a distinct voltage. Normally body and

source terminals of PMOS are shorted together.


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B S

G

BS

D

G

D

n+ n+p+ p+ n+ n+

Two NMOS side-by-side-

B S

G

BS

D

G

D

p+p+p+ p+n+ n+

x ex e

n-type

p-type - - -

microelectronics - mosfet lecture - iit kanpur

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