二阶效应MOSFET的非理想特性Word格式文档下载.docx

二阶效应MOSFET的非理想特性Word格式文档下载.docx

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∙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