mosfield‐ effecttransistors

7/25/2019 MOSFIELD EFFECTTRANSISTORS

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NTUEE Electronics L.H.Lu 51

CHAPTER5MOSFIELDEFFECTTRANSISTORS(MOSFETs)

ChapterOutline

5.1 DeviceStructureandPhysicalOperation

5.2 CurrentVoltageCharacteristics

5.3 MOSFETCircuitsatDC

5.4 ApplyingtheMOSFETinAmplifierDesign

5.5 SmallSignalOperationandModels

5.6 BasicMOSFETAmplifierConfigurations

5.7 BiasinginMOSAmplifierCircuits

5.8 DiscreteCircuitMOSAmplifiers

5.9 TheBodyEffectandOtherTopics


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5.1DeviceStructureandPhysicalOperation

DevicestructureofMOSFET

MOSmetaloxidesemiconductorstructure

MOSFETisafourterminaldevice:gate(G),source(S),drain(D)andbody(B)

Thedevicesize(channelregion)isspecifiedbychannelwidth(W)andchannellength(L)

TwokindsofMOSFETs:nchannel(NMOS)andpchannel(PMOS)devices

Thedevicestructureisbasicallysymmetricintermsofdrainandsource

Sourceanddrainterminalsarespecifiedbytheoperationvoltage



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Operationwithzerogatevoltage

TheMOSstructureformaparallelplateplatecapacitorwithgateoxidelayerinthemiddle

Twopnjunctions(SBandDB)areconnectedasbacktobackdiodes

Thesourceanddrainterminalsareisolatedbytwodepletionregionswithoutconductingcurrent

TheoperatingprincipleswillbeintroducedbyusingthenchannelMOSFETasanexample

Creatingachannel

for

current

flow

Positivechargesaccumulateingateasapositivevoltageappliestogateelectrode

Electricfieldformsadepletionregionbypushingholesinptypesubstrateawayfromthesurface

ElectronsaccumulateonthesubstratesurfaceasgatevoltageexceedsathresholdvoltageVtTheinducednregionthusformsachannelforcurrentflowfromdraintosource

ThechanneliscreatedbyinvertingthesubstratesurfacefromptypetontypeinversionlayerThefieldcontrolstheamountofchargeinthechannelanddeterminesthechannelconductivity



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Applyingasmalldrainvoltage

ApositivevGS>VtisusedtoinducethechannelnchannelenhancementtypeMOSFET

FreeelectronstravelfromsourcetodrainthroughtheinducednchannelduetoasmallvDSThecurrentiDflowsfromdraintosource(oppositetothedirectionoftheflowofnegativecharge)

Thecurrentisproportionaltothenumberofcarriersintheinducedchannel

Thechanneliscontrolledbytheeffectivevoltageoroverdrive

voltage:vOVvGSVt

Theelectronchargeinthechannelduetotheoverdrivevoltage:|Q|=CoxWLvOVGateoxidecapacitanceCoxisdefinedascapacitanceperunitarea

MOSFETcanbeapproximatedasalinearresistorinthisregionwitharesistancevalueinverselyproportionaltotheexcessgatevoltage


vDS(mV)

iD(mA)

100 200

0.1

0.2

0.3

0.4

vGS=Vt+1V

vGS=Vt+2V

vGS=Vt+3V

vGS=Vt+4V

vGSVt


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Operationasincreasingdrainvoltage

AsvDSincreases,thevoltagealongthechannelincreasesfrom0tovDSThevoltagebetweenthegateandthepointsalongthechanneldecreasesfromvGSatthesource

endto(vGSvDS)atthedrainend

SincetheinversionlayerdependsonthevoltagedifferenceacrosstheMOSstructure,increasingvDSwillresultinataperedchannel

TheresistanceincreasesduetotaperedchannelandtheiDvDScurveisnolongerastraightline

AtthepointvDSsat=vGSVt ,thechannelispinchedoffatthedrainside

IncreasingvDSbeyondthisvaluehaslittleeffectonthechannelshapeandiDsaturatesatthisvalue

Trioderegion:vDS


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DerivationoftheIVrelationship

InducedchargeinthechannelduetoMOScapacitor:

EquivalentresistancedR alongthechannel:

IVderivations:

Processtransconductance parameter(A/V2):kn=nCox Aspectratio:W/L

Transconductance parameter(A/V2):kn=nCox(W/L)

Drain

current

of

MOSFETs: Trioderegion:

Saturationregion:

Onresistance(channelresistanceforsmallvDS):


)]([)( xvVvCxQ tGSoxI

]21)[(

)]([

)]([)(

2

0 0

DSDStGSoxnD

v L

DtGSoxn

tGSoxn

D

In

DD

vvVvLWCi

dxidvxvVvWC

xvVvWC

dxi

xWQ

dxidRidv

DS

)()()( xWQ

dx

Wxhxqn

dxdR

Inn

2)(2

1tGSnDsat

Vvki

]2

1)[( 2

DSDStGSnD vvVvki

)(/1 tGSnDS Vvkr


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ThepchannelenhancementtypeMOSFET

pchannelenhancedtypeMOSFETsarefabricatedonntypesubstratewithp+ sourceandp+ drain

Normally,sourceisconnectedtohighvoltageanddrainisconnectedtolowvoltage

Asanegativevoltageappliestothegate,theresultingfieldpusheselectronsinntypesubstrateawayfromthesurface,leavingbehindacarrierdepletionregion

As

gate

voltage

exceeds

a

negative

threshold

voltageVt ,

holes

accumulate

on

the

substrate

surfaceAptypechannel(inversionlayer)isinducedforcurrentflowfromsourcetodrain

NegativegatevoltageisrequiredtoinducethechannelenhancementtypeMOSFET

ComplementaryMOS(CMOS)

CMOStechnologyemploysbothPMOSandNMOSdevices

Ifsubstrateisptype,PMOStransistorsareformedinnwell(ntypebodyneeded)

Ifsubstrateisntype,NMOStransistorsareformedinpwell(ptypebodyneeded)

Thesubstrateandwellareconnectedtovoltageswhichreversebiasthejunctionsfordeviceisolation

Exercise5.1

(Textbook)

Exercise5.2(Textbook)



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5.2CurrentVoltageCharacteristics

Circuitsymbol

nchannelenhancementmodeMOSFET

Thecurrentvoltagecharacteristics

Cutoffregion:(vGSVt)

Trioderegion:(vGS>VtandvDSVtandvDSvGSVt)

large

signal

model

(saturation)


]2

1)[( 2DSDStGSoxnD vvVv

L

WCi

2)(2

1tGSoxnD Vv

L

WCi

0Di


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Channellengthmodulation

ThechannelpinchoffpointmovesslightlyawayfromdrainasvDS>vDSsatTheeffectivechannellength(Leff)reduceswithvDSElectronstraveltopinchoffpointwillbeswepttodrainbyelectricfield

ThelengthaccountedforconductanceinthechannelisreplacedbyLeff:

Finiteoutput

resistance

VA(Earlyvoltage)=1/isproportionaltochannellength:VA=VAL

VAisprocesstechnologydependentwithatypicalvaluefrom5~50V/m

DuetothedependenceofiDonvDS,MOSFETshowsfiniteoutput

resistanceinsaturationregion


)1()(2

1)(

2

1)(

2

1

)]([

2'2'2'

0 0

'

L

LVv

L

WkVv

LL

WkVv

L

Wki

dxidvxvVvWk

tGSntGSntGS

eff

nD

Vv L

DtGSn

tGS eff

)1()(2

1thatassuming 2' DStGSnDDS vVv

L

Wkiv

L

L

1

])(2

[][ 12

'1

D

A

D

tGSn

constantv

DS

Do

I

V

IVv

L

Wk

v

ir

GS


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Thebodyeffect

TheBSandBDjunctionshouldbereversebiasedforthedevicetofunctionproperly

Normally,thebodyofanchannelMOSFETisconnectedtothemostnegativevoltage

ThedepletionregionwidensinBSandBDjunctionsandunderthechannelasVSBincreases

Bodyeffect:Vtincreasesduetotheexcesschargeinthedepletionregionunderthechannel

The

body

effect

can

cause

considerable

degradation

in

circuit

performanceThresholdvoltage:

Currentequations:

Temperatureeffect

Vtdecreasesby~2mVforevery1Crise iDincreaseswithtemperature

kndecreaseswithtemperature iDdecreaseswithincreasingtemperature

Foragivenbiasvoltage,theoverallobservedeffectofatemperatureincreaseisadecreaseiniD


2)(2

1tGSoxnDsat

VvL

WCi

]2

1)[( 2

DSDStGSoxnD vvVv

L

WCi

)ln(and2

where

]22[0

i

Af

ox

SiA

fSBftt

n

N

q

kT

C

qN

VVV


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Breakdownandinputprotection

Weakavalanche

pnjunctionbetweenthedrainandsubstratesuffersavalanchebreakdownasVDSincreases

Largedraincurrentisobserved

Typicalbreakdownvoltage20~150V

Punch

through Occursatlowervoltage(~20V)forshortchanneldevices

Draincurrentincreasesrapidlyasthedraindepletionregionextendsthroughthechannel

Doesnotresultinpermanentdamagetothedevice

Gateoxidebreakdown

Gateoxidebreakdownoccurswhengatetosourcevoltageexceeds30V Permanentdamagetothedevice

InputProtection

ProtectioncircuitisneededfortheinputterminalsofMOSintegratedcircuits

Usingclampingdiodefortheinputprotection



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ThepchannelenhancementtypeMOSFET

ForaPMOS,thesourceisconnectedtohighvoltageandthedrainisconnectedtolowvoltage

ToinducethepchannelfortheMOSFET,anegativevGSisrequiredVt(thresholdvoltage)


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5.3MOSFETCircuitsatDC

DCanalysisforMOSFETcircuits

Assumetheoperationmodeandsolvethedcbiasutilizingthecorrespondingcurrentequation

Verifytheassumptionwithterminalvoltages(cutoff,triodeandsaturation)

Ifthesolutionisinvalid,changetheassumptionofoperationmodeandanalyzeagain

DCanalysisexample


VVVV DSGS 696.1and3 Assuming

MOSFET

in

saturation

VDS


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Example5.5(Textbook)


Exercise5.12

(Textbook)





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5.4ApplyingtheMOSFETinAmplifierDesign

MOSFETvoltageamplifier

MOSFETwitharesistiveloadRDcanbeusedasavoltageamplifier

Thevoltagetransfercharacteristic(VTC)

TheplotofvI(vGS)versusvO(vDS)

DCanalysisasvGSincreasesfrom0toVDD

Cutoff

mode:

(0

V

vGSVt)

Triodemode:(vGSfurtherincreases)


2)(2

1tGSnD Vvki

DtGSnDDDSO RVvkVvv 2)(21

]2

1)[(

2

DSDStGSnD vvVvki

0Di

DDDSO Vvv

DDSDStGSnDDDSO RvvVvkVvv ]2

1)[(

2


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BiasingtheMOSFETtoobtainlinearamplification

TheslopeintheVTCindicatesvoltagegain

MOSFETinsaturationcanbeusedasvoltageamplification

PointQisknownasbiaspointordcoperatingpoint

ThesignaltobeamplifiedissuperimposedonVBE vGS(t)=VGS+vgs(t)

ThetimevaryingpartinvGS(t)istheamplifiedsignal

Thecircuitcanbeusedasalinearamplifierif:

Aproperbiaspointischosenforgain

Theinputsignalissmallinamplitude

Thesmallsignalvoltagegain

TheamplifiergainistheslopeatQ:

Maximumvoltagegainoftheamplifier


DtGSnDDDS RVVkVV 2)(

2

1

DOVnDtGSnVv

GS

DSv RVkRVVk

dv

dvA

GSGS )(

||2/

|2/

||| maxvOV

DD

OV

DDv A

V

V

V

RIA


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DeterminingtheVTCbygraphicalanalysis

Providesmoreinsightintothecircuitoperation

Loadline:thestraightlinerepresentsineffecttheload

iD=(VDDvDS)/RDTheoperatingpointistheintersectionpoint

Locatingthe

bias

point

Q

Thebiaspoint(intersection)isdeterminedbyproperlychoosingtheloadline

TheoutputvoltageisboundedbyVDD(upperbound)andVOV(lowerbound)

Theloadlinedeterminesthevoltagegain

Thebiaspointdeterminesthemaximum upper/lowervoltageswingoftheamplifier



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5.5SmallSignalOperationandModels

TheDCbiaspoint

MOSFETinsaturation

Draincurrent:

Drainvoltage:

Thesmallsignalcircuitparametersaredeterminedbythebiaspoint

Thesignal

signal

operation

Thesmallsignaldraincurrent:

Thesmallsignalvoltagegain:


22

2

1)(

2

1OVntGSnD VkVVkI

OVDDDDDS VRIVV

dDgstGSntGSn

gsngstGSntGSntgsGSnD

gsGSGS

iIvVVL

WkVVL

Wk

vL

WkvVV

L

WkVV

L

WkVvV

L

Wki

vVv

)()(2

1

2

1)()(

2

1)(

2

1

'2'

2''2'2'

gstGSnd vVVL

Wki )('

DOVn

gs

dv

gsDOVnDdd

dDddDDdDDDDDDDD

RVL

Wk

v

vA

vRVL

WkRiv

vVRiVRiIVRiVv

)(


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Thesmallsignalparameters

Transconductance(gm):describeshowidchangewithvgs

Outputresistance(ro):describeshowidchangewithvds

DraincurrentvarieswithvDSduetochannellengthmodulation

FiniterotomodelthelineardependenceofiDonvDS Theeffectcanbeneglectedifroissufficientlylarge

Body

transconductance

(gmb):

describes

how

idchanges

with

vbs

ThebodyeffectoftheMOSFETismodeledbygmb

Can

be

neglected

if

body

and

source

are

connected

together


DntGSnVv

GS

D

gs

dm I

L

WkVV

L

Wk

v

i

v

ig

GSGS

'' 2)(

1][ 1D

A

D

constantv

DS

Do

IV

Ivir GS

2' )(2

1tGSnD Vv

L

Wki

mmb gg

SB

tm

BS

ttGSn

BS

t

t

D

vv

BS

Dmb

v

Vg

v

VVv

L

Wk

v

V

V

i

v

ig

DS

GS

)(

'

constantconstant

oxSiAFSBFtt CqNvVV /2where]22[0

SBFSB

t

Vv

V

22

+

vgs

G D

S

+

vbs

B

rogmvgs gmbvbs


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Thesmallsignalequivalentcircuitmodels

Hybridmodel

Tmodel


Neglectro

Neglectro


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5.6BasicMOSFETAmplifierConfiguration

Threebasicconfigurations

Characterizingamplifiers

TheMOSFETcircuitscanbecharacterizedbyavoltageamplifiermodel(unilateralmodel)

TheelectricalpropertiesoftheamplifierisrepresentedbyRin,RoandAvoTheanalysisisbasedonthesmallsignalorlinearequivalentcircuit(dccomponentsnotincluded)

Voltagegain:

Overallvoltagegain:


CommonSource(CS) CommonGete(CG) CommonDrain(CD)

vo

oL

L

i

ov A

RR

R

v

vA

vo

soL

L

sigin

inv

sigin

in

sig

ov A

RR

R

RR

RA

RR

R

v

vG


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Thecommonsource(CS)amplifier

CharacteristicparametersoftheCSamplifier

Inputresistance:

Outputresistance:

Opencircuitvoltagegain:

Voltage

gain: Overallvoltagegain:

CSamplifiercanprovidehighvoltagegain

Inputandoutputareoutofphaseduetonegativegain

Output

resistance

is

moderate

to

high

SmallRDreducesRoatthecostofvoltagegain


inR

DoDo RrRR ||

DmoDmvo RgrRgA )||(

)||()||||(LDmoLDmv

RRgrRRgA

)||()||||( LDsig

moLDm

sig

v RRRr

rgrRRg

Rr

rG


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Thecommonsource(CS)withasourceresistance

Characteristicparameters(byneglectingro)

Inputresistance:

Outputresistance:


Voltagegain:

Overallvoltagegain:

SourcedegenerationresistanceRsisadopted

Gainisreducedbythefactor(1+gmR

s)

Consideredanegativefeedbackoftheamplifier


inR

Do RR

sm

Dmvo

Rg

RgA

1

sm

LDmv

Rg

RRgA

1

)||(

sm

LDmv

Rg

RRgG

1

)||(


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Thecommongate(CG)amplifier

CharacteristicparametersoftheCGamplifier(byneglectingro)

Inputresistance:

Outputresistance:


Voltagegain:

Overallvoltagegain:

CGamplifiercanprovidehighvoltagegain

Inputandoutputareinphaseduetopositivegain

Input

resistance

is

very

lowAsingleCGstageisnotsuitableforvoltageamplification

Outputresistanceismoderatetohigh

SmallRDreducesRoatthecostofvoltagegain

Theamplifierisnolongerunilateralifroisincluded


min gR /1

Do RR

Dmvo RgA

)||(LDmv

RRgA

)||(1

1LDm

sigm

v RRgRg

G


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Thecommoncollector(CD)amplifier

CharacteristicparametersoftheCDamplifier(byneglectingro)

Inputresistance:

Outputresistance:

Voltagegain:

Overallvoltagegain:

CDamplifierisalsocalledsourcefollower.

Inputresistanceisveryhigh

Outputresistanceisverylow

Thevoltagegainislessthanbutcanbecloseto1

CDamplifiercanbeusedasvoltagebuffer


inR

mo gR /1

1)1/()/1/( LmLmmLLv RgRggRRA

1)1/()/1/()( LmLmmLLv

RgRggRRG


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5.7BiasinginMOSAmplifierCircuits

DCbiasforMOSFETamplifier

Theamplifiersareoperatingataproperdcbiaspoint

Linearsignalamplificationisprovidedbasedonsmallsignalcircuitoperation

TheDCbiascircuitistoensuretheMOSFETinsaturationwithapropercollectorcurrentID

Biasingbyfixinggatetosourcevoltage

Fix

the

dc

voltage

VGSto

specify

the

saturation

current

of

the

MOSFET:

BiascurrentdeviatesfromthedesirablevalueduetovariationsinthedeviceparametersVtandn

Biasingbyfixinggatevoltageandconnectingasourceresistance

Thebiasconditionisspecifiedby: and

Draincurrenthasbettertolerancetovariationsinthedeviceparameters


22

)(2

1

)(2

1

tGntGSnD VVkVVkI

StGSnGSG RVVkVV 2)(

2

1 2)(

2

1tGSnD VVkI


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Biasingusingadraintogatefeedbackresistor

Asinglepowersupplyisneeded

RGensurestheMOSFETinsaturation(VGS=VDS)

MOSFEToperatingpoint:

ThevalueofthefeedbackresistorRGaffectsthesmallsignalgain

Biasingusingaconstantcurrentsource

TheMOSFETcanbebiasedwithaconstantcurrentsourceI

TheresistorRDischosentooperatetheMOSFETinactivemode

Thecurrentsourceistypicallyacurrentmirror

Current

mirror

circuit: MOSFETsQ1andQ2areinsaturation

ThereferencecurrentIREF=I =ID

Whenapplyingtotheamplifiercircuit,thevoltage

VD2hastobehighenoughtoensureQ2insaturation


2)(2

1tGSn

D

GSDD VVkR

VV

2

2

)(21

)(2

1

tGSnREF

tGSnGSDD

VVkI

VVkR

VV


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Exercise5.33

(Textbook)





8 Di Ci i MOS A lifi


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5.8DiscreteCircuitMOSAmplifiers

Circuitanalysis:

DCanalysis:

Removeallacsources(shortforvoltagesourceandopenforcurrentsource)

Allcapacitorsareconsideredopencircuit

DCanalysisofMOSFETcircuitsforallnodalvoltagesandbranchcurrents

FindthedccurrentID

andmakesuretheMOSFETisinsaturation

ACanalysis:

Removealldcsources(shortforvoltagesourceandopenforcurrentsource)

Alllargecapacitorsareconsideredshortcircuit

ReplacetheMOSFETwithitssmallsignalmodelforacanalysis

The

circuit

parameters

in

the

small

signal

model

are

obtained

based

on

the

value

of

ID


Completeamplifiercircuit DCequivalentcircuit ACequivalentcircuit


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Thecommonsource(CS)amplifier

Thecommonsourceamplifierwithasourceresistance



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Thecommongate(CG)amplifier

Thecommon

drain

(CD)

amplifier



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Theamplifierfrequencyresponse

Thegainfallsoffatlowfrequencybandduetotheeffectsofthecouplingandbypasscapacitors

ThegainfallsoffathighfrequencybandduetotheinternalcapacitiveeffectsintheMOSFETs

Midband:

Allcouplingandbypasscapacitors(largecapacitance)areconsideredshortcircuit

Allinternalcapacitiveeffects(smallcapacitance)areconsideredopencircuit

Midband gainisnearlyconstantandisevaluatedbysmallsignalanalysis

ThebandwidthisdefinedasBW=fHfL AfigureofmeritfortheamplifierisitsgainbandwidthproductdefinedasGB=|AM|BW



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Exercise5.38

(Textbook)





mosfield‐ effecttransistors

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