二阶效应MOSFET的非理想特性.docx

二阶效应MOSFET的非理想特性.docx

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二阶效应MOSFET的非理想特性

Contents

∙ShortChannelEffects

∙ChannelLengthModulation

oModelforchannellengthmodulation

oEarlyVoltage

∙MobilityDegradation（SurfaceScattering）

∙VelocitySaturation

∙BodyEffect（BackGateEffect）

oPhysicalProcess

oMathematicalFormulae

∙DrainInducedBarrierLowering（DIBL）

∙LeakageCurrentEffects

oSubthresholdConduction

oGateTunneling

oReverseBiasDiodeCurrent（JunctionLeakage）

∙MOSCapacitor

oModesofoperation

oCapacitance-Voltage（CV）characteristics

∙ParasiticComponents

oParasiticCapacitanceinMOSFET

oParasiticResistanceinMOSFET

∙Conclusion

ShortChannelEffects

Mostofthenon-idealeffectswewilldiscussinthispostareduetowhatiscommonlyregardedas“ShortChannelEffects”.

Generally,inordertoimprovetheperformanceandreducethecostofproduction,onewouldprefertoscaledownthesizeofthetransistors.Thisscalingdownalsoeliminatesmanystraycapacitancesthatarepresentintheoveralldevice.Ultimatelyincreasingthespeedofoperation.

Butwhenthechannellengthisscaleddowntotheorderofthedepletionlayer,acertainnumberofnon-idealeffectscomeintoplay.These second-ordereffects willbethemainfocusofthispost.

ChannelLengthModulation

Aswekeeponincreasingour  ,theregionforwhichtheinversionchargeiszerokeepsonincreasingforaconstantvalueof  maintained.Thusourchannellengthkeepsondecreasing.ThisphenomenoniscalledChannelLengthModulation.

SomeofusmightbefamiliarwithasimilareffectinthecaseofBJTknownas“BaseWidthModulation“.Thuswegeta  termintheexpressionfor  evenwhenweareoperatinginthesaturationregion.

Generally,thefabricationoftheMOSFETdevicesisdoneinawaysuchthatthechangeinlengthgivenby  islowwithachangein .

Figure1:

Reductioninthechannellengthduetochannellengthmodulation

Modelforchannellengthmodulation

Tomodelthechannellengthmodulation,weusethefollowingconstructs:

;here  isaparameterthatisusedtoquantifythechannellengthmodulationeffect.Wealwaysaimatkeepingthe  assmallaspossible.

Solving,weget ;then,usingbinomialapproximationweget:

Hence, .

Insertingtheaboveresultinthecurrentequationforsaturationmodeweget:

.

Hencefromtheequationonecanseetherelationof  with  isparabolicandwith  islinearforsaturationregionoperation.

Figure2:

OverallidealIVcharacteristicsofanNMOStransistorconsideringchannellengthmodulation

Intheplotoffigure2,wecanseetheeffectofChannelLengthModulation,hereevenwhen  .Therisein  islinearw.r.t.  insaturationregion.

Ideally,wewouldwantthecurrenttosaturateuponcethedrain-to-sourcevoltageexceedstheoverdrivevoltage.Hencetheleastthevariationofcurrentinthesaturationregionthebetteristhequalityoftransistoroperation.Theparameter  givestheslopeofthecurrentcurveinthesaturationregion.Thuswewouldwantour  tobeassmallaspossible（recallthatintheidealcase,itwaszero）.Practically  decreasesasourchannellength（L）increases.

EarlyVoltage

Figure3:

ExtrapolationofsaturationcurrentcurvestoobtainthevalueofEarlyvoltage

SupposewetakethedifferentIVcurvesintheirsaturationregionandextrapolatethemtowardsthenegativeaxisfor .Wewillobservethattheseextrapolatedlinescutthe  axisatacertainvaluegivenby .Thequantity  isalsoreferredtoasearlyvoltage .SomoreistheEarlyvoltage ,betteristheperformanceofourMOSFETinthesaturationregion.

MobilityDegradation（SurfaceScattering）

Practically,theelectronstravelingfromthesourcetodraininanNMOSdon’tfollowastraightpath.Formostcases,thelateralelectricfieldismuchmorethantheverticalelectricfield.Butstill,thereissomenon-zeroverticalelectricfieldpresentintheinversionchannel.

Figure4:

Effectivepathfollowedbytheelectronsduetosurfacescattering

Supposeanelectronisstartingfromthesourceterminaledge,asshowninfigure4.Thiselectronwillbeundertheinfluenceofboththeelectricfields.Thusitwillmovetowardsthedrainandalsowillbeattractedtowardstheoxidelayer（Keepinmindthatthepulltowardstheoxidelayerwillbeverylow）.Astheelectrongetsveryclosetothepracticallyunevenoxideinterface,itwillbescatteredandfollowapathtowardsthedownwarddirection.Againthiselectronwillbeattractedbytheelectricfield,andtheprocesscontinues.Thustrajectoryfortheelectronwillbemoreofazig-zagoneratherthanalinearone.

Thiseffectwillonlybedominantfortheelectronsthatareclosertotheoxideinterface.Astheverticalelectricfieldisweaker,mostoftheelectronswhichstartfarawayfromtheoxideinterfacewillreachthedrainbeforetheyencounterascatteringevent.Butaswescaledownthesizeofthetransisto