001MOSFET+Funda

8/8/2019 001MOSFET+Funda

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H ow it works?

When V GS = 0; And V DS = a +ve voltage w.r.t. Source;

IDS = 0.

p-substrate (Bulk)

n+ n+

SD

G

B

+V DS+V GS

I DS


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H ow it works?

When V GS = a +ve voltage w.r.t. Source; And V DS = a +ve voltage w.r.t. Source;

IDS starts flowing.

p-substrate (Bulk)

n+ n+

SD

G

B

+V DS+V GS

I DS Inversionchannel


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T h res h old voltage

The voltage applied between the gate andthe source of a MOSFET below which the

drain-to-source current I DS effectively dropsto zero.

This is denoted by V T .

Therefore the effective voltage appliedacross the gate, V G = (VGS -VT)


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R egions of Operation

Depending upon t h e biasing, a MOSFET maybe operating in one of t h e 3 regions:

1. C ut -off region:

2. Linear region:

3. Saturation region:


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1. C ut -off region:

V GS < V T ;

N o channel formed, hence irrespective of the valueof VDS,

IDS = 0;


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2. Linear region:

V GS >= V T ; V DS < (V GS -V T )

IDS = QnC ox W/L[(VGS -VT) VDS - V2DS /2]

B

p-substrate (Bulk)

n+ n+

SD

G+V DS+V GS

Inversionchannel


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3. Saturation region:

V GS >= V T ; V DS >= (V GS -V T)

IDS = QnCox W/L(VGS -VT)2

p-substrate (Bulk)

n+ n+

SD

G+V DS+V GS

ChannelPinch-off


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V - I Ch aracteristics

Linear


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Secondary effects

1. Body Effect

2. Ch annel - lengt h Modulation

3. Mobility Variation

4. DrainP

unch

th

rough

5 . I mpact I onization


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1. Body Effect

VT = VT0 + K[(2* b+|VSB|) - (2* b)]Where, K= (tox/I ox)(2q I SiN A)

* b= kT/q ln(NA /Ni);

Ni carrier concentration in Intrinsic silicon.

D

G

S BVSB+ -

VDD

N o body effect

M1

M2D

D

S

SG

G

VDD

M2 h as body effect

Vout


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2. Ch annel - lengt h Modulation

Since channel pinch-off takes place in saturationregion, Leff = (L-x)

@ IDS = QnCox W/L(VGS -VT)2(1+ PVDS ),where P is the channel-length modulation factor = 1/Leff xx/xVDS

p-substrate (Bulk)

n+ n+

SD

G+V DS+V GS

L

L eff x


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3. Mobility Variation

IDS = QnCox W/L(VGS -VT)2(1+ PVDS )

Mobility varies with the type of charge carrier.

Mobility decreases with increasing doping concentration.

Mobility decreases with increasing temperature.


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4. Drain P unc h t h roug h

When the drain is at a high enough voltage w.r.t. thesource, the depletion region around the drain mayextend to the source, thus causing current to flowirrespective of the gate voltage (i.e., even if it iszero). This is known as drain punchthrough.

p-substrate (Bulk)

n+ n+

SD

G+V DS+V GS

Depletionregion


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5 . I mpact I onization

Hot electrons impacting the drain, dislodging holesthat are then swept toward the negatively chargedsubstrate and appear as a substrate current. This

is known as im pact ioniz at ion .

These hot electrons can penetrate the gate oxide,causing a gate current. Eventually this can lead todegradation of MOS device parameters (VT,subthreshold current, transconductance), which inturn can lead to the failure of circuits.

001MOSFET+Funda

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