L10 Kvs MOSFET Full

8/12/2019 L10 Kvs MOSFET Full

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ESc201 : Introduction to Electronics

Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

r. . . r vas avaDept. of Electrical Engineering

IIT Kanpur

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Introduction

Classification of MOSFET

P channelEnhancement typeDe letion t e

N channelEnhancement typeDepletion type

2

Widely used in IC circuits


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MOSFET

e a x e em con uc or e ec rans s or

3 An NMOSFET


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Device Structure of Enhancement-Type NMOS

L: 1 to 10 m

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Thickness of oxide layer: 0.02 to 0.1 m


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Device Structure of Enhancement-Type NMOS

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Symbols

NMOSFET

a Circuit s mbol for the n-channel enhancement-t e MOSFET.

(b) Modified circuit symbol with an arrowhead on the source terminal todistinguish it from the drain and to indicate device polarity (i.e., n channel).

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(c) Simplified circuit symbol to be used when the source is connected to the body

or when the effect of the body on device operation is unimportant.


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Symbols

NMOSFET

MostlyUsed

PMOSFET

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Physical Operation

rea ng an n c anne

Drain current controlled b v

Drain current controlled by vGS

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Creating a Channel for Current Flow

The enhance ent-t e N O transistor with a ositi evoltage applied to the gate.

An n channel is induced at the to of the substrate

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beneath the gate.


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Drain Current Controlled by Small Voltage v DS

An NMOS transistor with v GS >V t and with a small v DS applied.

e c anne e s un orm.The device acts as a resistance.

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.

Drain current is proportional to ( v GS V t ) v DS .


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v DS is increased

Operation of the enhancement NMOS transistor as v DS isincreased.

The induced channel acquires a tapered shape.

Channel resistance increases as v is increased

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Drain current is controlled by both of the two voltages.


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Channel Pinch- Off

Channel is inched off

Inversion layer disappeared at the drain pointDrain current is nt disappeared

Drain current is saturated and only controlled byGS

Triode re ion and saturation re ion

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Drain Current Controlled by vGS

v GS creates the channel.

Increasing v GS will increase the conductance ofthe channel.

At saturation region only the v GS controls thedrain current.

At subthreshold region, drain current has theexponential relationship with v GS

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Transfer Characteristics

I D(mA) N-MOSFET

For

V DS V GS V T

Cut On

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V GS(V)

V t: Threshold Voltage 14


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I-V Characteristics

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Regions of Transistor Operation

Cut off region ( v GS < V t ) Input voltage less than threshold voltage

-

iD(mA)

CutOff

On

16vGS(V)V T: Threshold Voltage 16


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r o e reg on v GS > T an v DS < v GS T Linear relationship between i DS and v DS reflects resistive

behaviour for small v DS

-

G S T

D S G S T

v

v v V

><

iD(mA)

CutOff

On

17vGS(V)V T: Threshold Voltage 17


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Transistor is on Drain bias is above saturation voltage G S T v V > Amplifier should operate in this region

-

D S G S T v v V >

iD

(mA)

CutOff

On

18V T: Threshold Voltage 18


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Different values of v GS (> V t ) provides different i DSand vDSCharacteristics

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The Switch Current Source MOSFET Model

MOS Device Open State Closed State

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The Switch Current Source MOSFET Model

When v > V and v > v V the amount of current provided by the source is

( )2 D GS T i v V = Unit of K: A/V 2

'where n n oxW

K k C = = W: gate width; L: gate Length

k n: Constant related to MOSFET properties (A/V 2)

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n Cox: Capacitance per unit area of parallel plate capacitor bygate electrode and channels


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Example-3: Assume: K= 1mA/V 2 and V T = 1V

+

_

GS operate in saturation region

G S T v V > 5 1G S v> D S G S T v v> 6 VG S v 1 V 6 VG S v< The first solution is not consistent with our initial assumption

of operation in the saturation mode V t (=2 V)

GSV 3 V=5 V

S V 3 V = R D=

D

S

S

SS II

R ==.

VD = 1 V

=+= K 553R SVS

31

mA0.4-5 V


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Example 5 (DC Analysis of MOSFET Circuits)

Desi n the circuit shown in fi ure for MOSFET to o erate in saturation with drain voltage of 0.1 V. Determine R D .The MOSFEThas V t = 1 V and kn `W/L = 1 mA/V 2 . Neglect r 0.

VDD = + 5 V G S T v V v v V >>

R DIDT G D

V v>VD = +0.1 V ( )GSV 5 V >V T = MOSFET is ON

V4.91.05VGD ==

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T G D

MOSFET is not in saturation


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Example 6 (DC Analysis of MOSFET Circuits)

saturation region with I D = 0.4 mA. The MOSFET has V t = 2 V, nC0 x =

20 A/V 2 , L = 10 m and W = 100 m. Neglect r 0 .

10 V v V >S G S T v v V >

VDVGD= 0

T G S D S V v v> .

T G D

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GD = GD t


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ID = 0.4 mA, V t = 2 V, nC0 x = 20 A/V 2 L = 10 m and W = 100 m.

GS )2V(101020

2mA0.4 = 2tGS00D )VV(L

C2

I = x2

10 V

GSGS

VGS = 0 or 4 V

G S T v V >GD tV V

L10 Kvs MOSFET Full

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