Metal Oxide Semiconductor Field Effect Transistor

Basicinformation

Metal-Oxide-SemiconductorField-EffectTransistor,MOSFET),abbreviatedasmetaloxidehalffieldeffecttransistor,isafieldeffecttransistorthatcanbewidelyusedinanaloganddigitalcircuits.Accordingtotheir"channel"polarity,metaloxidesemiconductorfieldeffecttransistorscanbedividedintoN-channeltypewiththemajorityofelectronsandP-channeltypewiththemajorityofholes.TheyareusuallycalledItisN-typemetaloxidehalffieldeffecttransistor(NMOSFET)andP-typemetaloxidehalffieldeffecttransistor(PMOSFET).

JudgingfromthenameoftheMOSFET,itwillactuallygivepeoplethewrongimpression.BecauseMOSFETisthefirstletterMoftheEnglishword"metal",itdoesnotexistinmostofthecurrentcomponentsofthesamekind.Earlymetaloxidehalffieldeffecttransistorgateusedmetalasamaterial,butwiththeadvancementofsemiconductortechnology,modernmetaloxidehalffieldeffecttransistorgatehasreplacedmetalwithpolysilicon.

MetalOxideHalfFieldEffectTransistorisconceptually"Insulated-GateFieldEffectTransistor"(Insulated-GateFieldEffectTransistor,IGFET).ThegateinsulatinglayerofIGFETmaybeothermaterialsthantheoxidelayerusedinMOSFETs.SomepeopleprefertouseIGFETswhenreferringtofieldeffecttransistorcomponentswithpolysilicongates,butmostoftheseIGFETsrefertometaloxidehalffieldeffecttransistors.

TheoxidelayerintheMOSFETislocatedaboveitschannel.Dependingonitsoperatingvoltage,thethicknessofthisoxidelayerisonlytenstohundredsofangstroms(Å).Usuallythematerialissilicondioxide(SiO2),butsomenewadvancedprocessescanalreadyusesiliconoxynitride(SiON)astheoxidelayer.

Today'ssemiconductorcomponentmaterialsareusuallysiliconasthefirstchoice,butsomesemiconductorcompanieshavedevelopedprocessesthatuseothersemiconductormaterials.ThemostfamousisthesilicongermaniumprocessdevelopedbyIBMusingamixtureofsiliconandgermanium.(SiGeprocess).Itisapitythatmanysemiconductormaterialswithgoodelectricalproperties,suchasgalliumarsenide(GaAs),cannotgrowagood-qualityoxidelayeronthesurface,sotheycannotbeusedtomakeMOSFETcomponents.

WhenalargeenoughpotentialdifferenceisappliedbetweenthegateandsourceoftheMOSFET,theelectricfieldwillinducechargesonthesurfaceofthesemiconductorundertheoxidelayer.Aso-called"inversionchannel"isformed.Thepolarityofthechannelisthesameasitsdrainandsource.Assumingthatthedrainandsourcearen-type,thenthechannelwillalsoben-type.Afterthechannelisformed,theMOSFETcanallowcurrenttopass,anddependingonthevoltageappliedtothegate,theamountofcurrentthatcanflowthroughthechanneloftheMOSFETwillalsochangeunderitscontrol..

CircuitSymbols

CircuitsymbolscommonlyusedinMOSFETscomeinmanyforms,themostcommondesignisthataverticallinerepresentsthechannel(Channal),twoThewiringparalleltothechannelrepresentsthesource(Source)andthedrain(Drain),andthewiringperpendiculartothechannelontheleftrepresentsthegate(Gate),asshowninthefigurebelow.Sometimesthestraightlinerepresentingthechannelisreplacedwithadottedlinetodistinguishbetweenenhancementmode(alsoknownasenhanced)MOSFETanddepletionmode(depletionmode).,Alsoknownasdepletiontype)MetalOxideHalfFieldEffectTransistor.

Becausethemetaloxidehalffieldeffecttransistorontheintegratedcircuitchipisafour-terminalcomponent,inadditiontothesource(S),drain(D),andgate(G),thereisalsoaBase(BulkorBody).IntheMOSFETcircuitsymbol,thedirectionofthearrowextendingfromthechanneltotherightcanindicatethatthedeviceisann-typeorp-typeMOSFET.ThearrowdirectionalwayspointsfromthePterminaltotheNterminal,sothearrowpointingfromthechanneltothebaseterminalisap-typeMOSFET,orPMOSforshort(representingthechannelofthiscomponentasp-type);otherwise,itrepresentsthebaseelectrodep-type,andthechannelisn-type,thisdeviceisann-typeMOSFET,referredtoasNMOS.InageneraldistributedMOSFETcomponent,thebaseandsourceareusuallyconnectedtogether,sothedistributedMOSFETisusuallyathree-terminalcomponent.TheMOSFETsinintegratedcircuitsusuallyusethesamecommonbulk,sothepolarityofthebaseisnotmarked,andacircleisaddedtothegateofthePMOStoshowthedifference.

Principleofoperation

Thecoreofthemetaloxidehalffieldeffecttransistor

Metal-oxidelayer-semiconductorstructure

MetaloxidehalffieldeffectTransistorisstructuredwithametal-oxidelayer-semiconductorcapacitorasthecore(mostlyMOSfield-effecttransistorsusepolysiliconinsteadofmetalasitsgatematerial),andmostoftheoxidelayerismadeofsilicondioxide.Thebaseofthesilicon,andaboveitisthepolysiliconasthegate.Suchastructureisexactlyequaltoacapacitor.Theoxidelayeristhedielectricmaterialinthecapacitor,andthecapacitancevalueisdeterminedbythethicknessoftheoxidelayerandthedielectricconstantofsilicondioxide(dielectricconstant).)Todecide.ThegatepolysiliconandbasesiliconbecomethetwoendpointsoftheMOScapacitor.

WhenavoltageisappliedacrosstheMOScapacitor,thechargedistributionofthesemiconductorwillalsochange.ConsideraMOScapacitorformedbyap-typesemiconductor(theholeconcentrationisNA),whenapositivevoltageVGBisappliedtothegateandbaseterminals(asshowninthefigure),Theconcentrationofholeswilldecrease,andtheconcentrationofelectronswillincrease.WhentheVGBisstrongenough,theelectronconcentrationnearthegateendwillexceedthehole.Inthep-typesemiconductor,theareawheretheelectronconcentration(negativelycharged)exceedsthehole(positivelycharged)concentrationistheso-calledinversionlayer(inversionlayer).

ThecharacteristicsoftheMOScapacitordeterminetheoperatingcharacteristicsoftheMOSFET,butacompleteMOSFETstructurealsorequiresasourcethatprovidesamajoritycarrierandThedrainthatacceptsthesemajoritycarriers.

StructureofMOSFETs

Asmentionedabove,thecoreofMOSFETsistheMOScapacitorlocatedinthecenter,andtheleftandrightsidesaretheMOScapacitors.Sourceanddrain.Thecharacteristicsofthesourceanddrainmustbethesamen-type(ieNMOS)orthesamep-type(iePMOS).The"N+"markedonthesourceanddrainoftheNMOSintheleftpicturerepresentstwomeanings:(1)NmeansdopedimpuritypolarityinthesourceanddrainregionsisN;(2)"+"Itmeansthatthisregionisaheavilydopedregion,whichmeansthattheelectronconcentrationinthisregionismuchhigherthanotherregions.Thesourceanddrainareseparatedbyanareaof​​oppositepolarity,whichistheso-calledbase(orbase)area.IfitisNMOS,thenthedopingofthebaseregionisp-type.Onthecontrary,forPMOS,thesubstrateshouldben-type,whilethesourceanddrainarep-type(andheavilydopedP+).Thedopingconcentrationofthematrixdoesnotneedtobeashighasthesourceordrain,sothereisno"+"intheleftpicture.

ForthisNMOS,onlythesurfaceareaof​​thesemiconductordirectlybelowtheMOScapacitorisactuallyusedasachannelforcarrierstopassthrough.Whenapositivevoltageisappliedtothegate,negativelychargedelectronswillbeattractedtothesurface,formingachannel,allowingthemajorityofn-typesemiconductorcarriers—electronstoflowfromthesourcetothedrain.Ifthisvoltageisremoved,oranegativevoltageisapplied,thenthechannelcannotbeformed,andthecarrierscannotmovebetweenthesourceanddrain.

AssumingthattheobjectofoperationischangedtoPMOS,thenthesourceanddrainarep-type,andthesubstrateisn-type.WhenanegativevoltageisappliedtothegateofthePMOS,theholesonthesemiconductorwillbeattractedtothesurfacetoformachannel,andthemajorityofthesemiconductor-holescanflowfromthesourcetothedrain.Assumingthatthisnegativevoltageisremoved,orapositivevoltageisadded,thechannelcannotbeformed,andthecarrierscannotbemovedbetweenthesourceandthedrain.

Inparticular,thesourceintheMOSFETmeans"providingasourceofmajoritycarriers".ForNMOS,mostcarriersareelectrons;forPMOS,mostcarriersareholes.Incontrast,thedrainistheterminalthatacceptsmajoritycarriers.

OperationmodeofMOSFET

Accordingtothe"bias"appliedonthegate,source,anddrainoftheMOSFET."(Bias)isdifferent,acommonenhancementmoden-type.