MOSFET: Principles, Device Physics, Operation & Applications (A Comprehensive Lecture by Dr. G. S. Virdi, Ex-Chief Scientist, CSIR-CEERI Pilani)

This lecture on Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) is designed to give a complete and deep understanding of MOSFET device structure, physics, operation modes, characteristics, and practical applications in modern electronics.

The lecture is delivered by Dr. G. S. Virdi, Ex-Chief Scientist at CSIR-Central Electronics Engineering Research Institute (CEERI), Pilani, with extensive 45+ years of R&D experience in microelectronics, semiconductor devices, and digital/analog circuit design.

The content provides clear explanation of MOSFET fundamentals to advanced concepts including:

MOSFET structure and fabrication basics

Enhancement & depletion mode MOSFETs

MOSFET IV characteristics & channel formation

Biasing techniques in amplifier circuits

Small-signal models and design parameters

MOSFET as amplifier and switch

Internal capacitances & high-frequency behavior

CMOS inverter fundamentals & characteristics

This comprehensive lecture is highly beneficial for:
✔ Engineering and Science students (B.Tech, B.E., M.Sc., M.Tech)
✔ Researchers and professionals in electronics and semiconductor field
✔ Learners preparing for advanced nanoelectronics and IC design

This educational resource will be uploaded on SlideShare for global accessibility and knowledge enhancement in the domain of semiconductor electronics.

1. MOSFETs
Dr.G.S.Virdi
Ex.Chief Scientist
CSIR -Central Electronics Engineering Research Institute
Pilani -333031,India

2. FET
2
FieldEffectTransistor(FET)
•Theconductivity(orresistivity)ofthepathbetweentwocontacts,the
sourceandthedrain,isalteredbythevoltageappliedtothegate.
–Deviceisalsoknownasavoltagecontrolledresistor.
Dr.G.S.VIRDI

3. MOSFETs
3
Ametal–oxide–semiconductorfield-effecttransistor(MOSFET,MOS-
FET,orMOSFET)isafield-effecttransistorwherethevoltage
determinestheconductivityofthedevice.
Theabilitytochangeconductivitywiththeamountofappliedvoltagecan
beusedforamplifyingorswitchingelectronicsignals.
MOSFETsarenowevenmorecommonthanBJTs(bipolarjunction
transistors)indigitalandanalogcircuits.
Dr.G.S.VIRDI

4. FET
4
n-channel
EnhancementMode
(nMOSFET)
p-channel
EnhancementMode
(pMOSFET)
n-channel
DepletionMode
(nMOSFET)
p-channel
DepletionMode
(pMOSFET)

5. MOSFET: Structure
TypicallyL=0.03μmto1μm,W=0.05
μmto100μm,andthethicknessofthe
oxidelayer(tox)isintherangeof1to
10nm.
5
perspectiveview;
Crosssection.
Dr.G.S.VIRDI

6. MOSFET: Operation
6
OperationwithZeroGateVoltage:
•Withzerovoltageappliedtothegate,twoback-to-backdiodesexistin
seriesbetweendrainandsource.
•n+drainregionandthep-typesubstrate,
•p-typesubstrateandthen+sourceregion.
•Theseback-to-backdiodespreventcurrentconductionfromdrainto
sourcewhenavoltageV
DSisapplied.
•Thepathbetweendrainandsourcehasaveryhighresistance(ofthe
orderof 10
12
Ω).
Dr.G.S.VIRDI

7. MOSFET: Operation
7
ChannelforCurrentFlow:
•Source&Drainaregroundedand
appliedapositivevoltagetothe
gate(V
GS).
V
GS• repellsthefreeholesfromthe
•PositiveV
GSattractselectronsfrom
then+sourceanddrainregions
intothechannelregion
regionofthesubstrateunderthe
gate(thechannelregion).
•Theseholesarepusheddownward
intothesubstrate,leavingbehinda
carrier-depletionregion.
•Whenasufficientnumberofelectrons
accumulatenearthesurfaceofthe
substrateunderthegate,annregionis
ineffectcreated,connectingthesource
anddrainregions,
Dr.G.S.VIRDI

8. MOSFET: Operation
8
•Thechanneliscreatedbyinverting
thesubstratesurfacefromptype
tontype.Hencetheinduced
channelisalsocalledaninversion
layer.
ChannelforCurrentFlow:
•Ifavoltageisappliedbetween
drainandsource,currentflows
throughthisinducednregion.
•Theinducednregionthusformsa
channelforcurrentflowfrom
draintosource
•Thisiscalledann-channel
MOSFEToranNMOStransistor.
TheexcessofV
GSoverV
tistermedthe
effectivevoltageortheoverdrive
voltageandisthequantitythat
determinesthechargeinthechannel.
Here,V
GS−V
t≡V
OV
Note:Ann-channelMOSFETisformedina
p-type
substrateDr.G.S.VIRDI

9. MOSFET
9
•GateandChannelregionoftheMOSFETformaparallel-plate
capacitor,withtheoxidelayeractingasthecapacitordielectric.
•Positivegatevoltagecausespositivechargetoaccumulateonthetop
plateofthecapacitor(thegateelectrode).
•Negativechargeonthebottomplateisformedbytheelectronsinthe
inducedchannel.
•Thisfieldcontrolstheamountofchargeinthechannelhenceit
determinesthechannelconductivitywhenavoltageV
DSisapplied.
•Thisistheoriginofthename“field-effecttransistor”(FET).
Dr.G.S.VIRDI

10. MOSFET: Operation
10
ApplyingaSmallV
DS:
•ThevoltageV
DScausesa
currenti
Dtoflowthroughthe
inducednchannel.
•Currentiscarriedbyfree
electronstravelingfromsource
todrain
•AnNMOStransistorwithV
GS>
V
tandwithasmallV
DSapplied.
Thedeviceactsasaresistance
whosevalueisdeterminedby
V
GS.
•Specifically,the channel
conductanceisproportionalto
V
GS–V
t,andthusi
Dis
proportionalto(V
GS–V
t)V
DS. depletionregionisnot
shownforsimplicity.
Dr.G.S.VIRDI

11. MOSFET: Operation
11
•ApplyingaSmallV
DS:
•Thei
D–V
DScharacteristicsof
theMOSFETwhenthevoltage
appliedbetweendrainand
source,vDS,iskeptsmall.
•Thedeviceoperatesasa
linearresistancewhosevalue
iscontrolledbyV
GS.

12. MOSFET: Operation
12
OperationoftheenhancementNMOS
transistorasV
DSisincreased.Theinduced
channelacquiresataperedshape,andits
v
DSresistanceincreasesas isincreased.
Here,v
GSiskeptconstantatavalue>V
t;
V
GS=V
t+V
OV.
Thedraincurrenti
DVsV
DSforan
enhancement-typeNMOStransistor
operatedwith
V
GS=V
t+V
OV.
Dr.G.S.VIRDI

13. MOSFET:p-Channel
13
Physicalstructureof
thePMOStransistor
Dr.G.S.VIRDI

14. FET
14
Circuitsymbol
forthen-channel
enhancement-
typeMOSFET
Modifiedsymbol
nchannel.
Simplifiedcircuit
symbol
Dr.G.S.VIRDI

15. MOSFET:I
D-V
DSCharacteristics
15
Dr.G.S.VIRDI

16. MOSFET:I
D-V
DSCharacteristics
16
Thei
D−v
DScharacteristics
foranenhancement-type
NMOStransistor
Dr.G.S.VIRDI

17. BiasinginMOSCircuits

18. BiasinginMOSAmplifierCircuits
18
•AnessentialstepinthedesignofaMOSFETamplifiercircuitisthe
establishmentofanappropriatedcoperatingpointforthetransistor.
•Thisstepisalsoknownasbiasingorbiasdesign.
•Anappropriatedcoperatingpointorbiaspointischaracterizedbya
stableandpredictabledcdraincurrentI
Dandbyadcdrain-to-source
voltageV
DSthatensuresoperationinthesaturationregionforall
expectedinput-signallevels.
•TypesofBiasing:
–BiasingbyFixingV
GS
–BiasingbyFixingVGandConnectingaResistanceintheSource
–BiasingUsingaDrain-to-GateFeedbackResistor
–BiasingUsingaConstant-CurrentSource
Dr.G.S.VIRDI

19. BiasinginMOSAmplifierCircuits
19
BiasingbyFixingV
GS:
•ThemostcommonapproachtobiasingaMOSFETistofixitsgate-to-
sourcevoltageV
GStothevaluerequiredtoprovidethedesiredI
D.
•ThisvoltageisderivedfromthepowersupplyvoltageV
DDthroughthe
useofanappropriatevoltagedivider.
•IndependentofhowthevoltageV
GSmaybegenerated,thisisnota
goodapproachtobiasingaMOSFET.
Becauseweknowthat,
AndthethresholdvoltageV
Otheoxide-capacitanceC
OX,andtransistor
aspectratioW/Lvarywidelyamongdevicesofsamesizeandtype.
Dr.G.S.VIRDI

20. BiasinginMOSAmplifierCircuits
20
•BiasingbyFixingV
GS:
•BiasingbyfixingV
GSisnota
goodtechnique.
•Figuretwoi
D-v
GScharacteristic
curvesrepresentingextreme
valuesinabatchofMOSFETs
ofthesametype.
•ForthefixedvalueofV
GS,the
resultantspreadinthevalues
ofthedraincurrentcanbe
substantial.
Dr.G.S.VIRDI

21. BiasinginMOSAmplifierCircuits
21
BiasingbyFixingV
GandConnectinga
ResistanceintheSource:
•Anexcellentbiasingtechniquefordiscrete
MOSFETcircuitsconsistsoffixingthedcvoltage
atthegate,V
G,andconnectingaresistanceinthe
sourcelead,asshowninfigure.
Wecanwrite,
V
G=V
GS+R
SI
D
Dr.G.S.VIRDI

22. BiasinginMOSAmplifierCircuits
22
BiasingbyFixingV
GandConnectinga
ResistanceintheSource:
•IfV
G>>V
GS,I
Dwillbedeterminedbythevalues
ofV
GandR
S.
•IfV
G>V
GS,resistorR
sprovidesnegative
feedback,whichwillstabilizethevalueofthebias
currentI
D.
•Fromequation,whenI
Dincreases&V
Gis
constant,V
GSwilldecrease.Whichwillfurther
decreaseI
D.
•ThustheR
sworkstokeepI
Dasconstantas
possible.
•ThisnegativefeedbackactionofR
sgivesitthe
namedegenerationresistance.
V
G=V
GS+R
SI
D
Dr.G.S.VIRDI

23. BiasinginMOSAmplifierCircuits
23
BiasingbyFixingV
G andConnectingaResistanceintheSource:
•Figureshowsthei
D–v
GScharacteristics
fortwodevicesthatrepresentthe
extremesofabatchofMOSFETs.
•Astraightlinethatrepresentsthe
constraintimposedbythebiascircuit—
namely.
•Theintersectionofthisstraightlinewith
thei
D–v
GScharacteristiccurveprovides
thecoordinates(I
DandV
GS)ofthebias
point.
•Inthiscase,thevariabilityobtainedinI
D
ismuchsmaller.Also,notethatthe
variabilitydecreasesasV
GandR
sare
madelarger.
Dr.G.S.VIRDI

24. BiasinginMOSAmplifierCircuits
24
BiasingbyFixingV
GandConnectinga
ResistanceintheSource:
Practicalimplementationusingasingle
supply:
•Thecircuitutilizesonepower-supplyV
DDand
derivesV
Gthroughavoltagedivider(R
G1,R
G2).
•SinceI
G=0,R
G1andR
G2canbeselectedtobe
verylarge(intheMΩrange),allowingthe
MOSFETtopresentalargeinputresistancetoa
signalsource
Dr.G.S.VIRDI

25. BiasinginMOSAmplifierCircuits
25
BiasingUsingaDrain-to-GateFeedback
Resistor:
•Asimpleandeffectivebiasingarrangementutilizing
afeedbackresistorconnectedbetweenthedrain
andthegateisshowninfigure.
•HerethelargefeedbackresistanceR
G(usuallyin
theMΩrange)forcesthedcvoltageatthegateto
beequaltothatatthedrain(becauseI
G=0).
Thuswecanwrite
V
GS=V
DS=V
DD–R
DI
D
Whichcanberewrittenintheform
V
DD=V
GS+R
DI
D
Dr.G.S.VIRDI

26. BiasinginMOSAmplifierCircuits
26
•BiasingUsingaDrain-to-GateFeedback
Resistor:
•IfI
Dincreasesduetoanyreason,thenV
GSmust
decrease.
•ThedecreaseinV
GSinturncausesadecreasein
I
D.
•Thusthenegativefeedbackordegeneration
providedbyR
GworkstokeepthevalueofI
Das
constantaspossible.
V
GS= V
DS=V
DD–R
DI
D
V
DD= V
GS+R
DI
D
Dr.G.S.VIRDI

27. BiasinginMOSAmplifierCircuits
27
BiasingUsingaConstant-CurrentSource:
•ThemosteffectiveschemeforbiasingaMOSFET
amplifieristhatusingaconstant-currentsource,as
showninfigure.
•HereR
G(usuallyinMΩrange)establishesadc
groundatthegateandpresentsalargeresistance
toaninputsignalsourcethatcanbecapacitively
coupledtothegate.
•ResistorR
Destablishesanappropriatedcvoltage
atthedraintoallowfortherequiredoutputsignal
swingwhileensuringthatthetransistoralways
remainsinthesaturationregion.
Dr.G.S.VIRDI

28. SmallSignalOperationModel

29. Small-SignalOperationandModels
29
•Considertheconceptualamplifiercircuit
showninfigure.
•HeretheMOStransistorisbiasedby
applyingadcvoltageV
GS,andtheinput
signaltobeamplified,v
gs,issuperimposed
onthedcbiasvoltageV
GS.
•Theoutputvoltageistakenatthedrain.
Conceptualcircuitto
studytheoperation
oftheMOSFETasa
small-signalamplifier
Dr.G.S.VIRDI

30. Small-SignalOperationandModels
30
•DCBiasPoint:
•ThedcbiascurrentI
Dcanbefoundbysettingthe
signalv
gstozero;
Thus,
Here,Itisassumedthatλ=0
Here,V
OV=V
GS−V
tistheoverdrivevoltageat
whichtheMOSFETisbiasedtooperate.
Thedcvoltageatthedrain,V
DS,willbe
V
DS= V
DD−R
DI
D
Dr.G.S.VIRDI

31. Small-SignalOperationandModels
31
•DCBiasPoint:
•Toensuresaturation-region
musthave
V
DS>V
OV
operation,we
•Furthermore,sincethetotalvoltageatthe
drainwillhaveasignalcomponent
superimposedonV
DS,V
DShastobe
sufficientlygreaterthanV
OVtoallowforthe
requirednegativesignalswing.
Dr.G.S.VIRDI

32. Small-SignalOperationandModels
32
SignalCurrentintheDrainTerminal:
•Considerthesituationwiththeinputsignalv
gs
applied.
•Thetotalinstantaneousgate-to-sourcevoltage
willbe
v
GS=V
GS+v
gs
resultinginatotalinstantaneousdraincurrent
i
D,
dcbiascurrentI
D
currentcomponentthatisdirectlyproportionaltotheinputsignalv
gs
representsnonlineardistortion.
Dr.G.S.VIRDI

33. Small-SignalOperationandModels
33
•SignalCurrentintheDrainTerminal:
•ToreducethenonlineardistortionintroducedbytheMOSFET,theinput
signalshouldbekeptsmallsothat
resultingin
or,equivalently,
Ifthissmall-signalconditionissatisfied,theni
Dcanbeexpressedas
i
D≈I
D+i
d
where
i
d=k
n(V
GS−V
t)v
gs
Dr.G.S.VIRDI

34. Small-SignalOperationandModels
34
Theparameterthatrelatesi
dandv
gsistheMOSFETtransconductance
g
m,
orintermsoftheoverdrivevoltageV
OV,
g
m=k
nV
OV
Dr.G.S.VIRDI

35. Small-SignalOperationandModels
35
shows a•Figure
graphical
ofthe
interpretation
small-signal
operationof the
MOSFETamplifier.
•Notethatg
misequalto
theslopeofthei
D–v
GS
characteristicatthebias
point,
Small-signaloperation
of theMOSFETamplifierDr.G.S.VIRDI

36. Small-SignalOperationandModels
36
VoltageGain:
•Totalinstantaneousdrainvoltagev
DSasfollows:
v
DS=V
DD−R
Di
D
•Underthesmall-signalcondition,wehave
v
DS=V
DD−R
D(I
D+i
d)
•whichcanberewrittenas
v
DS=V
DS−R
di
d
•Thusthesignalcomponentofthedrainvoltageis
V
ds=−i
dR
D=−g
mv
gsR
D
whichindicatesthatthevoltagegainisgivenby
Theminussignindicates that theoutput signalv
dsis180°outof phasewithrespect
totheinputsignal v
gs.
Conceptualcircuitto
studyoperationofthe
MOSFET
Dr.G.S.VIRDI

37. Small-SignalOperationandModels
•Foroperationinthesaturation(active)regionat
alltimes,theminimumvalueofv
DSshouldnot
fallbelowthecorrespondingvalueofv
GSby
morethanV
t.
•Themaximumvalueofv
DSshouldbesmaller
thanV
DD;otherwisetheFETwillenterthecutoff
regionandthepeaksoftheoutputsignal
waveformwillbeclippedoff.
37
VoltageGain:
•Theinputsignalisassumedtohaveatriangular
waveformwithanamplitudemuchsmallerthan
2(V
GS–V
t),thesmall-signalconditiontoensure
linearoperation.
Conceptualcircuitto
studyoperationof
theMOSFET
Dr.G.S.VIRDI

38. Small-SignalOperationandModels
Dr.G.S.VIRDI
38
VoltageGain:
Conceptualcircuittostudy
operationoftheMOSFET
Totalinstantaneousvoltagesv
GSandv
DS

39. Small-SignalOperationandModels
39
Small-SignalEquivalent-CircuitModels:
•TheFETbehavesasavoltage-controlledcurrentsource.
•Itacceptsasignalv
gsbetweengateandsourceandprovidesacurrent
g
m.v
gsatthedrainterminal.
•Theinputresistanceofthiscontrolledsourceisveryhigh—ideally,
infinite.
•Theoutputresistance—isalsohigh.
Dr.G.S.VIRDI

40. Small-SignalOperationandModels
40
Small-SignalEquivalent-CircuitModels:
Small-signalmodelsfortheMOSFET:
Neglectingthedependenceofi
D
onv
DSintheactiveregion
includingthe
modulation,
resistance
effectofchannel-length
modeled byoutput
Dr.G.S.VIRDI

41. Small-SignalOperationandModels
Mr.A.B.Dr.G.S.VIRDI
41
Small-SignalEquivalent-CircuitModels:
•IntheanalysisofaMOSFETamplifiercircuit,
thetransistorcanbereplacedbythe
equivalent-circuitmodel shownin Figure.
•Therestofthecircuitremainsunchanged
exceptthatidealconstantdcvoltagesources
arereplacedbyshortcircuits.
•Mostseriousshortcomingofthismodelisthat
itassumesthedraincurrentinsaturationto
beindependentofthedrainvoltage,but
actuallydraincurrentdependsonv
DSina
linearmanner.
•Suchdependencewasmodeledbyafinite
resistancer
obetweendrainandsource,
Neglectingthe
dependenceofi
Donv
DSin
theactiveregion
includingtheeffectofchannel-
lengthmodulation,

42. Small-SignalOperationandModels
42
Small-SignalEquivalent-CircuitModels:
Dr.G.S.VIRDI

43. MOSFETasAmplifier&Switch

44. MOSFETasAmplifier
44
•Inthesaturationregion,theMOSFETactsasavoltage-controlled
currentsource:Changesinthegate-to-sourcevoltagev
GSgivesriseto
changesinthedraincurrenti
D.
•ThusthesaturatedMOSFET canbeusedtoimplementa
transconductanceamplifier
Dr.G.S.VIRDI

45. MOSFETasAmplifier
45
Large-Signal Operation:
TheTransferCharacteristic:
•Groundedsourceterminaliscommonto
boththeinputandoutput.
•Here,changesinv
1(v
GS=v
1)giveriseto
changesini
D,weareusingaresistorR
Dto
obtainanoutputvoltagev
0
v
0=v
DS=V
DD–R
D.i
D
Inthiswaythetransconductanceamplifier
isconvertedintoavoltageamplifier.
•Todeterminethevoltagetransfer
characteristicoftheCSamplifier,wewill
assumev
jtobeintherangeof0toV
DD.
Basic structureof
common-sourceamplifier
Dr.G.S.VIRDI

46. MOSFETasAmplifier
Mr.A.B.Shinde 46
Large-SignalOperation-TheTransferCharacteristic:
Basic structureof
common-sourceamplifier
Transfercharacteristicoftheamplifier
Dr.G.S.VIRDI

47. MOSFETasAmplifier
47
Large-SignalOperation:
TheTransferCharacteristic:
v
DS= V
DD–R
D.i
D
•Straightlineoni
D-v
DScharacteristics
curvesshowsthei
D-v
DSrelationship.
•Sincev
GS=v
1,forv
1<V
tthetransistor
willbecutoff,i
Dwillbezero,andv
0=
v
DS=V
DD(pointA).
•AsV
iexceedsV
tthetransistorturns on,
i
Dincreases,andv
0decreases.
•Thiscorrespondstopointsalongthe
segmentoftheloadlinefromAtoB.
•Wehaveidentifiedaparticularpointin
thisregionofoperationandlabeledit
Q.ItisobtainedforV
GS=V
IQandhas
thecoordinatesV
0Q=V
DSQandI
DQ.
Transfercharacteristic
oftheamplifier
Dr.G.S.VIRDI

48. Dr.G.S.VIRDI
MOSFETasAmplifier
48
Large-SignalOperation:
TheTransferCharacteristic:
•Saturation-regionoperationcontinues
untilv
0decreasesbelowV
t.
•Atthispoint,v
DS=v
GS-V
Dandthe
MOSFETentersitstrioderegion.
•ThisisreferstopointBingraph.
PointBisdefined byv
0B=v
1B–V
t.
•ForV
i>V
IB,thetransistorisdriven
deeperintothetrioderegion.
•Thecharacteristiccurvesinthetriode
regionarebunchedtogether,theoutput
voltagedecreasesslowlytowardszero.
•Herewehaveidentifiedaparticular
operatingpointCobtainedforv
1=V
DD.
•ThecorrespondingoutputvoltageV
OCwill
usuallybeverysmall. Transfercharacteristic
oftheamplifier

49. MOSFETasAmplifier
49
Large-SignalOperation-TheTransferCharacteristic:
Basic structureof
common-sourceamplifier
TransferCharacteristics
Dr.G.S.VIRDI

50. MOSFETasAmplifier
Mr.A.B.Shinde 50
Large-SignalOperation:
TheTransferCharacteristic:
PointCobtainedforv
i=V
DD.
Thecorrespondingoutputvoltage
V
OCwillusuallybeverysmall.
Thispoint-by-pointdeterminationof
thetransfercharacteristicresultsin
thetransfercurveshowninfigure.
Observethatwehavedelineated
itsthreedistinctsegments,each
correspondingtooneofthethree
regionsofoperationofMOSFET
Q
1.
TransferCharacteristics
Dr.G.S.VIRDI

51. MOSFETasAmplifier
51
MOSFETasaSwitch:
•WhentheMOSFETisusedasaswitch,itisoperatedattheextreme
pointsofthetransfercurve.
•Thedeviceisturnedoffbykeeping,v<V
t.Here,v
0=V
DD.
•TheswitchisturnedonbyapplyingavoltageclosetoV
DD.Here,v
0is
verysmall.
•Thecommon-sourceMOScircuitcanbeusedasalogicinverterwith
the"low"voltagelevelcloseto0Vandthe"high"levelclosetoV
DD.
Dr.G.S.VIRDI

52. MOSFETasAmplifier
52
MOSFETasaSwitch:
OperationasaLinearAmplifier
•TooperatetheMOSFETasanamplifier,saturation-modeismaintained.
•Thedeviceisbiasedatasomewhereneartothemiddleofthetransfer
curve.Thevoltagesignaltobeamplifiedv
tisthensuperimposedonthe
dcvoltageV
IQ.
•Bykeepingv
tsufficientlysmalltorestrictoperationtoanalmostlinear
segmentofthetransfercurve,theresultingoutputvoltagesignalv
0will
beproportionaltov
t.
•Thatis,theamplifierwillbeverynearlylinear,andv
Qwillhavethesame
waveformasv
texceptthatitwillbelargerbyafactorequaltothe
voltagegainoftheamplifieratQ.
Dr.G.S.VIRDI

53. MOSFETasAmplifier
53
MOSFETasaSwitch:
OperationasaLinearAmplifier
Thusthevoltagegainisequaltotheslopeofthetransfercurveatthe
biaspointQ.
Theslopeisnegative,hencethebasicCSamplifierisinverting.
Iftheamplitudeoftheinputsignalv,theoutputsignalwillbecome
distortedsinceoperationwillnolongerberestrictedtoanalmostlinear
segmentofthetransferCurve.
Dr.G.S.VIRDI

54. MOSFETasAmplifier
54
MOSFETasaSwitch:
AnalyticalExpressionsforthe
TransferCharacteristic:
•Fromthei-vrelationshipswecansee
that,theMOSFEToperatesinthree
regions—cutoff,saturation,and
triode.
•Cutoff–RegionSegment,XA:
Here,v
i<V
t,andv
0=V
DD.
•Saturation–RegionSegment,A
QB:
Here,v
i,≥V
tand
v
0 ≥v
i-V
t
•Triode-RegionSegment,BC:
Here,v
i≥V
tandv
0≤v
i–V
t
Transfercharacteristic
oftheamplifier
Dr.G.S.VIRDI

55. SingleStageMOSAmplifier

56. SingleStageMOSAmplifier
56
•TheBasicStructure:
•Figureshowsthebasiccircuittoimplement
thevariousconfigurationsofdiscrete-circuit
MOSamplifiers.
•Duetoeffectivenessandsimplicityconstant-
currentbiasingtechniqueisusedforbiasing
theMOStransistor.
•Figureindicatesthedccurrentandthedc
voltagesresultingatvariousnodes.
Dr.G.S.VIRDI

57. SingleStageMOSAmplifier
57
CharacterizingAmplifiers:
1.Theamplifieriswithasignalsourcehavinganopen-circuitvoltagev
sig
andaninternalresistanceR
sig.Thesearetheparametersofanactual
signalsource.Similarly,R
Lisanloadresistance.
2.ParametersR
i,R
0,A
vs,A
is,andG
mpertaintotheamplifierproper;thatis,
theydonotdependonthevaluesofR
sigandR
L.Bycontrast,R
in,R
out,
A
v,A
i,G
v0,andG
vmaydependononeorbothofR
sigandR
L.
3.Asmentionedabove,fornonunilateralamplifiers,R
inmaydependonR
L,
andR
outmaydependonR
sig.Nosuchdependenciesexistforunilateral
amplifiers,forwhichR
in=R
iandR
out=R
0.
4.Theloadingoftheamplifieronthesignalsourceisdeterminedbythe
inputresistanceR
in.ThevalueofR
indeterminesthecurrentthatthe
amplifierdrawsfromthesignalsource.Italsodeterminestheproportion
ofthesignalv
sigthatappearsattheinputoftheamplifier.
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58. SingleStageMOSAmplifier
58
•CharacterizingAmplifiers:
•Figureshowsanamplifierfedwithasignalsourcehavinganopen-
circuitvoltagev
sigandaninternalresistanceR
sig.
•Thesecanbetheparametersofanactualsignalsource,
•Theamplifierisshownwithaload resistanceR
Lconnectedtotheoutput
terminal.
•Here,R
Lcanbeanactualloadresistanceortheinputresistanceofa
succeedingamplifierstageinacascadeamplifier.
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59. SingleStageMOSAmplifier
59
•CharacterizingAmplifiers:
•Figureshowstheamplifiercircuitwiththeamplifierblockreplacedbyits
equivalent-circuitmodel.
•TheinputresistanceR
inrepresentstheloadingeffectoftheamplifier
inputonthesignalsource.
R
inandR
sigformsavoltagedividerthatreducesv
sigtothevaluev
i
Dr.G.S.VIRDI

60. SingleStageMOSAmplifier
60
•CharacterizingAmplifiers:
•Thesecondparameterincharacterizingamplifierperformanceisthe
open-circuitvoltagegainA
vo,definedas
ThelastparameteristheoutputresistanceR
o.Fromfigure,R
oisthe
resistanceseenlookingbackintotheamplifieroutputterminalwithv
iset
tozero.
AsR
oisdeterminedwithv
i=0,thevalueofR
odoesnotdependonR
sig.
Dr.G.S.VIRDI

61. SingleStageMOSAmplifier
61
•CharacterizingAmplifiers:
Outputvoltagev
o
Voltagegainoftheamplifier,A
v
Overallvoltagegain,G
v
Dr.G.S.VIRDI

62. SingleStageMOSAmplifier
62Dr.G.S.VIRDI

63. SingleStageMOSAmplifier
63
CSAmplifier:
•Figureshowsacommon-source(CS)amplifierfedwithasignalsource
v
sighavingasourceresistanceR
sig.
•AnalyzethiscircuittodetermineR
in,A
vo,andR
o.Here,assumeR
Dis
partoftheamplifier;thusifaloadresistanceR
Lisconnectedtothe
amplifieroutput,R
LappearsinparallelwithR
D.Insuchacase,wewish
todetermineA
vandG
vaswell.
•ReplacingtheMOSFETwithitshybrid-πmodel(withoutr
o),weobtain
theCSamplifierequivalentcircuitasshowninsecondfigure.
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64. SingleStageMOSAmplifier
64
CSAmplifier:
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65. MOSFETInternalCapacitances

66. MOSFETInternalCapacitances
66
•Various internal
capacitances,areshown
forn-channelMOSFET
operating in the
saturationregion.
•Therearefourinternal
capacitances:
C
gs• andC
gd,resultfrom
the gate-capacitance
effect;
•C
sbandC
db,arethe
depletioncapacitancesof
thepnjunctionsformed
bythesourceregionand
thesubstrate,andthe
drainregionandthe
substrate,respectively.
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67. MOSFETInternalCapacitances
67
•Thepolysilicongateformsaparallel-platecapacitorwiththechannel
region,whereoxidelayerworksasdielectric.
•Thegate(oroxide)capacitanceperunitgateareaisdenotedC
ox.When
thechannelistaperedandpinchedoff,thegatecapacitanceisgivenby
2/3WLC
ox.
•Therearetwoothersmallcapacitancesresultingfromtheoverlapofthe
gatewiththesourceregion(orsourcediffusion)andthedrainregion(or
draindiffusion).
•EachoftheseoverlapshasalengthL
ovandthustheresultingoverlap
capacitancesC
ovaregivenby
Typically,L
ov=0.05to0.1L.
Wecannowexpressthegate-to-sourcecapacitanceC
gsas
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68. MOSFETInternalCapacitances
68
•Forthegate-to-draincapacitance,wenotethatthechannelpinch-offat
thedrainendcausesC
gdtoconsistentirelyoftheoverlapcomponent
C
ov,
Thedepletion-layercapacitancesofthetworeverse-biasedpnjunctions
formedbetweeneachofthesourceandthedraindiffusionsandthep-
typesubstrate.
Thus,forthesourcediffusion,wehavethesource-bodycapacitance,
C
sb,
whereC
sb0isthevalueofC
sbatzerobody-sourcebias,V
SBisthe
magnitudeofthereverse-biasvoltage,andV
0isthejunctionbuilt-in
voltage(0.6Vto0.8V).
Dr.G.S.VIRDI

69. MOSFETInternalCapacitances
69
•Similarly,forthedraindiffusion,wehavethedrain-bodycapacitance
C
db,
whereC
db0isthecapacitancevalueatzeroreverse-biasvoltageand
V
DBisthemagnitudeofthisreverse-biasvoltage.Notethatwehave
assumedthatforbothjunctions,thegradingcoefficientm=1/2.
Problem:Forann-channelMOSFETwitht
ox=10nm,L=1.0μm,
W=10μm,L
ov=0.05μm,C
sb0=C
db0=10fF,V
0=0.6V,V
SB=1Vand
V
DS=2V.Calculatethefollowingcapacitanceswhenthetransistoris
operatingin saturation:C
ox,C
ov, C
gs,C
gd,C
sb,andC
db.
Ans:C
ox=3.45fF/μm
2
;
C
gd=1.72fF;
C
ov =1.72fF;
C
sb=6.1fF;
C
gs =24.7fF;
C
db = 4.1fF
Dr.G.S.VIRDI

70. MOSFETHighFrequencyModel

71. MOSFETHighFrequencyModel
•Figureshowsthesmall-signalmodeloftheMOSFET,includingthefour
capacitancesC
gs,C
gd,C
sb,andC
db.
•Thismodelisusedtopredictthehigh-frequencyresponseofMOSFET
amplifiers.
High-frequency,equivalent-circuitmodelfortheMOSFET
71
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72. MOSFETHighFrequencyModel
72
•Whenthesourceisconnectedtothebody,themodelsimplifies
considerably,asshowninfigure.
•Inthismodel,C
gd,althoughsmall,playsasignificantroleindetermining
thehigh-frequencyresponseofamplifiersandthusmustbekeptinthe
model.
Theequivalentcircuitforsourceisconnectedtothesubstrate
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73. MOSFETHighFrequencyModel
73
•CapacitanceC
db,canusuallybeneglected,resultinginsignificant
simplificationofmanualanalysis.
•Theresultingcircuitisshowninfigure.
Theequivalent-circuitmodelwithC
dbneglected
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74. MOSFETHighFrequencyModel
74
•Figureshowsthehigh-frequencyTmodelinitssimplifiedform.
Thesimplifiedhigh-frequencyTmodel
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75. MOSFETHighFrequencyModel
75
MOSFETHigh-FrequencyModel:Summary
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76. MOSFETHighFrequencyModel
76
•Problem:Calculatef
Tforthen-channelMOSFETwhosecapacitances
werefoundinExercise10.3.Assumeoperationat100μAandthat
k
n=160μA/V2.
•Ans.3.4GHz
Dr.G.S.VIRDI

77. FrequencyResponseof
CSAmplifier

78. FrequencyResponseofCSAmplifier
78
Magnitudeofthegainofadiscrete-circuitMOSamplifier
versusfrequency
Dr.G.S.VIRDI

79. FrequencyResponseofCSAmplifier
79
•Figureshows,atlowerfrequencies,themagnitudeoftheamplifiergain
fallsoffduetocouplingandbypasscapacitors.Here,itisassumedthat
theirimpedancesweresmallenoughtoactasshortcircuits.
•Atmidbandfrequencies,asthefrequencyoftheinputsignalislowered,
thereactance1/jωCofeachofthesecapacitorsbecomessignificant,
thisresultsinadecreaseintheoverallvoltagegainoftheamplifier.
•Loweranduppercut-offfrequencyf
L&f
H,arethefrequenciesatwhich
thegaindropsby3dBbelowitsvalueinmidband.
•BW=f
H−f
L(discrete-circuitamplifiers)
•BW=f
H(integrated-circuitamplifiers)
Dr.G.S.VIRDI

80. CMOS Inverter

81. CMOS Inverter
81
TheCMOSinverterisconstructedbyusing
nMOS&pMOStransistors.
AsthepMOStransistorpassesstrong1and
weak0,itisconnectedtothesupplyvoltage
V
DDand
nMOStransistorpassesstrong0andweak
1,it isconnectedtotheground.
CMOS inverter
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82. CMOS Inverter
82
CircuitOperation:
•Considerthetwoextremecases:
•Whenv
iisatlogic-0level,whichis0V,and
whenv
iisatlogic-1level,whichisV
DDvolts.
•Inbothcases,foreaseofexpositionwe
shallconsiderthen-channeldeviceQ
Ntobe
thedrivingtransistorandthep-channel
deviceQ
Ptobetheload.
•Ascircuitissymmetric,thisassumptionis
arbitrary,andthereversewouldleadto
identicalresults.
Dr.G.S.VIRDI

83. CMOS Inverter
83
CircuitOperation:
Circuitwithvi=V
DD
equivalentcircuit
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84. CMOS Inverter
84
CircuitOperation:
Circuitwithvi=0V
equivalentcircuit
Dr.G.S.VIRDI

85. CMOS Inverter
85
Voltage-
Transfer
Characteristic:

86. 86Dr.G.S.VIRDI