FieldEffectTransistors(FETs)场效应晶体管（FET）

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1、FieldEffectTransistors(FETs)Inafieldeffecttransistor,currentflowthroughasemiconductorchanneliscontrolledbytheapplicationofanelectricfield(voltage)perpendiculartothedirectionofcurrentflow.WeconsidertheMOSFET(MOST)-themetaloxidesemiconductor(fieldeffect)

2、transistor.FieldEffectTransistors(FETs)InadepletionmodeMOSFETachannelisbuiltinsothatconductionoccurswithnocontrolvoltageapplied.Weconsiderann-channelenhancementmodedevicewhichhasno‘built-in’conductionchannel.N-channelenhancementmodeMOSFET(schematic)Oxi

3、de(SiO2)n+n+p-typesiliconGate(G)Drain(D)Source(S)SymbolforN-channelenhancementmodeMOSFETNoteconnectiontosubstrateshownSymbolforN-channelenhancementmodeMOSFETNoteconnectiontosubstrateshownGSDN-channelenhancementmodeMOSFET:outlineofoperationVGS=0,littleo

4、rnocurrentcanflowSD,backtobackp-njunctions.AsapositiveVGSisapplied,holesinthep-regionarerepelled.AsVGSincreasesfurther,electronsareattractedfromthesubstratetowardsthepositivegate.Aninversionlayerofmobileelectronsformsnearthesiliconsurface.N-channelenh

5、ancementmodeMOSFETwithinversionlayerSource(S)Oxide(SiO2)n+n+p-typesiliconGate(G)Drain(D)Inversionlayer(conductionchannel)N-channelenhancementmodeMOSFET:outlineofoperationThisinversionlayeriseffectivelyn-type.Electronscancarrycurrentinancontinuousn-type

6、path(orchannel)fromsourcetodrain.TheconductionchannelformswhenVGSattainsathresholdvoltage,VTN-channelenhancementmodeMOSFETThevalueofVTisdeterminedbythedeviceprocessandthelevelofthep-type(substrate)doping.Notethatann-channeldeviceisformedwithap-typesubs

7、trate.N-channelenhancementmodeMOSFETAsVGSincreasesfurther,moreelectronsaredrawnintotheinversionlayerandthechannelresistancedecreases(Thedeviceoperatesasavoltagecontrolledresistor).However……N-channelenhancementmodeMOSFETThevoltagebetweengateandchannelva

8、riesfromVGSatthesourceendtoVGS-VDSatthedrainend.Thusasthevoltageatthedrainend,VDS,isincreasedtheeffectivegate-channelvoltage(verticalfield)isdecreased.N-channelenhancementmodeMOSFETThiscausesthecarrierdensityatthedrainendoftheinversionl