Chapter-4-Metal-oxide-semiconductor-FET-(MOSFET)

MOSFETMetalOxideSemiconductorFieldEffectTransistorsEBB424EDr.SabarD.HutagalungSchoolofMaterials&MineralResourcesEngineering,UniversitiSainsMalaysiaDifferenttypesofFETsJunctionFET(JFET)Metal-Oxide-SemiconductorFET(MOSFET)Metal-SemiconductorFET(MESFET)DifferenttypesofFETsJunctionFET(JFET)DifferenttypesofFETsMetal-Oxide-SemiconductorFET(MOSFET)DifferenttypesofFETsMetal-SemiconductorFET(MESFET)BasicMOSFET(n-channel)Thegateelectrodeisplacedontopofaverythininsulatinglayer.Thereareapairofsmalln-typeregionsjustunderthedrain&sourceelectrodes.Ifapplya+vevoltagetogate,willpushawaythe‘holes’insidethep-typesubstrateandattractsthemoveableelectronsinthen-typeregionsunderthesource&drainelectrodes.BasicMOSFET(n-channel)Increasingthe+vegatevoltagepushesthep-typeholesfurtherawayandenlargesthethicknessofthecreatedchannel.Asaresultincreasestheamountofcurrentwhichcangofromsourcetodrain—thisiswhythiskindoftransistoriscalledanenhancementmodedevice.Cross-sectionandcircuitsymbolofann-typeMOSFET.Ann-channelMOStransistor.Thegate-oxidethickness,TOX,isapproximately100angstroms(0.01mm).Atypicaltransistorlength,L=2l.Thebulkmaybeeitherthesubstrateorawell.Thediodesrepresentpn-junctionsthatmustbereverse-biasedBasicMOSFET(p-channel)Thesebehaveinasimilarway,buttheypasscurrentwhena-vegatevoltagecreatesaneffectivep-typechannellayerundertheinsulator.Byswappingaroundp-typeforn-typewecanmakepairsoftransistorswhosebehaviourissimilarexceptthatallthesignsofthevoltagesandcurrentsarereversed.Pairsofdeviceslikethiscarecalledcomplimentarypairs.Inann-channelMOSFET,thechannelismadeofn-typesemiconductor,sothechargesfreetomovealongthechannelarenegativelycharged(electrons).Inap-channeldevicethefreechargeswhichmovefromend-to-endarepositivelycharged(holes).Notethatwithan-channeldeviceweapplya+vegatevoltagetoallowsource-draincurrent,withap-channeldeviceweapplya-vegatevoltage.IllustratesthebehaviourofatypicalcomplimentarypairofpowerMOSFETsmadebyHitachiforuseinhi-fiamplifiers.StructureandprincipleofoperationAtopviewofMOSFET,wherethegatelength,L,andgatewidth,W.NotethatLdoesnotequalthephysicaldimensionofthegate,butratherthedistancebetweenthesourceanddrainregionsunderneaththegate.Theoverlapbetweenthegateandthesource/drainregionisrequiredtoensurethattheinversionlayerformsacontinuousconductingpathbetweenthesourceanddrainregion.Typicallythisoverlapismadeassmallaspossibleinordertominimizeitsparasiticcapacitance.Topviewofann-typeMOSFETMOSFET-BasicStructureI-VCharacteristicsofMOSFETI-VCharacteristicsofMOSFETI-VCharacteristicsofMOSFETIdealOutputCharacteristicsofMOSFETIdealTransferCharacteristicsofMOSFETTypesofMOSFETTypesofMOSFETEnhancementModeEnhancementModeDepletionModeDepletionModeSubthresholdregionChannelLengthMOSFETDimensions-TrendMOSFETscalingscenarioVoltageScalingPowerSupplyVoltageThresholdVoltageThresholdVoltageGateOxideThicknessChannelProfileEvolutionMOSFETCapacitancesMOSFETCapacitancesOverlapCapacitanceGateResistanceComponentsofCinandCoutNewmaterialsneededforscalingSincetheearly1980s,thematerialsusedforintegratedMOSFETonsiliconsubstrateshavenotchangedgreatly.Thegate“metal”ismadefromhighly-dopedpolycrystallineSi.Thegateoxideissilicondioxide.Forthesmallestdevices,thesematerialswillneedtobereplaced.NewGateOxideThecapacitanceperareaofthegateoxideisScaledMOSFETsrequirelargerCox,whichhasbeenachievedwithsmallertox.IncreasingKcanalsoincreaseCox,andotheroxides,“high-Kdielectrics”arebeingdeveloped,includingforexample,mixturesofHfO2andAl2O3.NewGateMetalThedopedpolycrystallinesiliconusedforgateshasaverythindepletionlayer,approximately1nmthick,whichcausesscalingproblemsforsmalldevices.Othersmetalsarebeinginvestigatedforreplacingthesilicongates,includingtungstenandmolybdenum.Removingthesubstrate:SilicononInsulator(SOI)Forhigh-frequencycircuits(about5GHzandabove),capacitivecouplingtotheSisubstratelimitstheswitchingfrequency.Also,leakageintothesubstratefromthesmalldevicescancauseextrapowerdissipation.Theseproblemsarebeingavoidedbymakingcircuitsoninsulatingsubstrates(eithersapphireorsilicondioxide)thathaveathin,approximately100nmlayerofcrystallinesilicon,inwhichtheMOSFETsarefabricated.SilicononInsulator(SOI)SOI—silicononinsulator,referstoplacingathinlayerofsiliconontopofaninsulatorsuchasSiO2.Thedeviceswillbebuiltontopofthethinlayerofsilicon.ThebasicideaofSOIistoreducedtheparasiticcapacitanceandhencefasterswitchingspeed.SilicononInsulator(SOI)Everytimeatransistoristurnedon,itmustfirstchargeallofitsinternal(parasitic)capacitancebeforeitcanbegintoconduct.Thetimeittakestochargeupanddischarge(turnoff)theparasiticcapacitanceismuchlongerthantheactualturnonandoffofthetransistor.Iftheparasiticcapacitancecanbereduced,thetransistorcanbeswitchedfaster—performance.SilicononInsulator(SOI)Oneofthemajorsourceofparasiticcapacitanceisfromthesourceanddraintosubstratejunctions.SOIcanreducedthecapacitanceatthesourceanddrainjunctionssignificantly—byeliminatingthedepletionregionsextendingintothesubstrate.SOICMOSSilicon-on-insulatorCMOSoffersa20–35%performancega