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TheMetal-Oxide-SemiconductorField-EffectTransistor(MOSFET)

IntroductionRecallthatinLab3westudiedthecurrentversusvoltagepropertiesofaforwardbiaseddiode.ThediodeconsistedofaPNsemiconductorjunction(inpracticeweusedanNPNBJTtransistorbutfocusedourattentionontheBase-EmitterPNjunction).BycapturingthecurrentversusvoltagecharacteristicsofthejunctionwewereabletodetermineBoltzmann’sconstant,andwewerealsoabletousethetemperaturedependencyofthediode’sreversebiascurrent,Io,todeterminethebandgapofthesilicondiode.ThedevelopmentofthePNjunctionsetthestageforthefabricationofmanysubsequent“active”semiconductordevices.Oneofthemostimportantsuchdevicesisthemetal-oxide-semiconductorfieldeffecttransistor(MOSFET).Whereastransistorsingeneralareimportantbecauseoftheirversatility,behavingasswitches,amplifiers,oroscillatorsdependingontheirconfiguration,MOSFETsinparticularexhibitextremelybeneficiallow-powerswitchingpropertieswhencomparedagainstalternativetransistortypes.Assuchtheyhavebecomeabsolutelyfundamentalincomputingandmemoryapplications,wheretheirphysicaldimensionshavebeenscaleddowntoallowthefabricationofmanymillionsofMOSFETtransistorsonindividualsemiconductorchipsorintegratedcircuits.InthislabwewillmeasureseveralcharacteristicsofadiscreteMOSFETtransistorandcomparethepropertiestotheexpectedparametersprovidedbythemanufacturer.MOSFETStructureFigure1illustratestheprofileofann-channelMOSFET.Twon-dopedregions,thedrainandthesource,areembeddedinap-typesemiconductorsubstrate.Onthesurfaceofthesemiconductorisalayerofinsulator(SiO2inthecaseofSisubstrates).Openingsetchedormaskedintotheinsulatorallowthedepositionofmetallicelectrodescontactingn-dopeddrainandsourceregions.Notethatbeneaththegateelectrodetheinsulatorlayerremainsintact,isolatingthegateandsubstrate.Voltagesappliedtothis“gate”electrodecontroltheflowofcurrentbetweenthesourceanddrainbyintroducingordepletingchargecarrierstatesinthesubstrateregionbeneaththe

Figure1.Physicalstructureofanenhancementmoden-channelMOSFET,andschematicsymbol[1].

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gate.TheoryofOperationTheMOSCapacitorTheMOSFET’sbehaviorandoperationisbasedinlargepartonthephysicsoftheMOScapacitor.We’llinitiallyignorethepresenceofthesourceanddrain,leavingonlythecapacitor-likegate,insulator,andsemiconductorstructure.We’llbrieflyconsiderhowavoltageappliedtothegatemodifiestheenergybandsofthesemiconductorsubstrateinthevicinityoftheareabeneaththegate,andhowthisinturninfluencesthecarrierdensitiesanddepletionregionstherein.WhenVGis0V,themetalandsemiconductorFermi-levelsalign.Thesemiconductor’sFermi-levelisqφFelectronvoltsbelowitsintrinsiclevel,Ei,andisindicativeofhowstronglyp-typethesubstrateis,aswerecallthattheconcentrationofmajoritycarriersindopedp-typesemiconductorisgivenby 𝑝 = 𝑁! exp[−(

!!!!!! !

)].

WhenVG<0Visappliedatthegate,theFermi-levelinthemetalincreasesbyqVG.Thishastheeffectofdepositingnegativechargesatthegate,whichinturnattractsadditionalholestotheoxide-semiconductorinterface.Assuch,thesemiconductorbandsarebentneartheinterfaceandtheFermi-level,EF,andvalenceband,Ev,areclosertoeachotherinenergyduetotheincreasedmajoritycarrierdensity.Thedeviceissaidtobeinan“accumulation”state.Itscapacitanceisgivenby 𝐶!" =

Є!"!!",

where Є!"isthepermittivityoftheinsulatorlayeranddoxistheinsulatorthickness.WhenVG>0V,thingsbecomemoreinteresting.Theredistributionofcarrierstatescausethebandsofthesemiconductortobendneartheinterfacesuchthatthe

Fermi-levelandEibecomefartherapart.Thatis,VG>0Vcausespositivechargetobuildatthegate,andinturninducesareductionor“depletion”stateinthep-typesemiconductorin

Figure2:IdealMOSunderatVG=0Vgatevoltage.(Commonlycalled“flatband”condition.)

Figure3:ForVG<0Vthedeviceisinastateof"accumulation."Positivechargeonthegateelectrodedrawselectronstotheoxide-semiconductorboundaryandtheenergybandsofthematerialbend(EFsremainsflat).

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theareaneartheoxide-semiconductorinterface.Theeffectisanalogoustothedepletionregionbetweenapn+junction1,thewidthofwhichis[2],𝑊 = [!Є! !!

!!! ]!!.

Here,φsisthepotentialdifferenceacrossthedepletionregion(i.e.energybandbending,asshowninFigure6),whereVG=Vi+φsandVirepresentsthepotentialdifferenceacrosstheoxideinsulatorlayer.Asaresult,thecapacitancebecomeslikeaseriescombinationoftheoxidecapacitanceandthatduetothedepletionregionwidth, 𝐶! =

Є!!,

whereЄ!isthepermittivityofthedepletionlayer.

AsVGbecomesevenlarger,eventuallyEFbecomesgreaterthanEi.Inthissituationtheregionofsemiconductorneartheoxideinterfacebecomes“inverted,”meaningthatconductionbandcarrierstatesbecomefilledwithminorityelectronsformingann-channel.(RealizethatwhenweadddrainandsourceelectrodestothisMOScapacitor,wecaninducecurrentconductionacrossthenewlyformedn-channeljustbyapplyingaVDS!)Atthispointthedepthofthesemiconductordepletionlayerisatamaximum,andthetotalcapacitanceofthedeviceisataminimum,

𝐶 = !!!"

+ !!!

!!= !!"!!

(!!"!!!).

Thegatevoltagerequiredtoinducethisinversionstateisdefinedasthedevice’s“thresholdvoltage,”VT.VTrepresentsthepointatwhichtheMOSFETbecomesconductive,anditcanbecontrolledindevicefabricationbytailoringthematerialparametersandphysicaldimensionsoftheMOSFET.AsVGbecomesgreaterthanVTtheinvertedMOScapacitorentersastateof“stronginversion,”whereφs=2φF.

1Recallthatforabiasedpnjunctionthedepletionwidthisgivenby𝑊 = [2Є!

!!!!!

(!!!!!!!!!

) ]!!,whereVo-Vis

thechangeinthepotentialbarrierduetobiasvoltageV,andVoisthepotentialbarrieratequilibrium.

Figure5:VG>>0Vcausesastateofinversion,whereachannelofn-typecarrierformsattheoxidesemiconductorboundary.

Figure4:VG>0Vcausesastateof"depletion"neartheoxide-semiconductorboundary.

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MeasurementsofVT,Cox,andCdcanthusbeusedtocharacterizemanyofthephysicalparametersofthedevice,likeoxidethicknessanddopingdensities.(Thatis,C-Vmeasurementscanserveasausefulcharacterizationandreliabilitytestingtool.)TheMOSFETTransistorSohavingconsideredthestatesoftheMOScapacitor,whatoftheMOSFETtransistorphysics?Withtheadditionofn-dopedsourceanddrainregions(orp-dopedregionsforap-channelMOSFET),recognizethatwe’veintroducedtwopnjunctionsintothedevicestructure.IfweconsidertheMOScapacitor’saccumulationanddepletionstatesaswevaryVG,thesubstrateremainsp-typeandallthatisalteredistheconcentrationofmajorityp-typecarriersintheareabeneaththegate.Assuch,voltageappliedfromdraintosource,VDS,isequivalenttoreversebiasingthedrain-substratenpjunction.Thepotentialbarrierisincreased,thewidthofthedepletionboundarybetweendrainandsubstrateiswidened,andnodraincurrentflows.However,asVGbecomesgreaterthanVTwe’veseenthatinversionoccurs,andsuddenlythedrainandsourcebecomeconductivelyconnectedviatheinducedn-channel.CurrentcanflowfreelyifavoltageVDSisapplied.TheMOSFETisswitchedintoitsconductivemode.WewouldstillliketoconsiderhowthedevicebehavesinthisinversionstateaswevaryVDS.WhileVDSissmall,thecurrentfromdraintosourceisobservedtoberoughlyproportionaltoVDS,andtheMOSFETactssomewhatlikeanohmicresistiveload.Inthisscenariothedeviceisparticularlyusefulforswitchingapplications.(MillionsofMOSFETswitchesareusedincomputermemoryandprocessors.)AsVDSisincreasedforagivenVG>VTeventuallyallcarriersgeneratedinthechannellayerarequicklysweptfromdraintosource.Aspartofyourreportyou’llcommentonhowthisisreflectedinthemeasurementsyou’llmakeofIDversusVDS.

Figure7:TheMOSFETinprofile.VG>>0Vcauses“stronginversion”andann-channelforms,linkingaconductivepathfromdraintosource.TheMOSFETisin“on”state.

Figure6:Theonsetof"stronginversion,"whereφs=2φFintheneutralpsubstrate.

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ExperimentInthisexperimentyouwillcharacterizethebehaviourofaMOSFETtransistorbymeasuringdraincurrent,ID,undervariousVGandVDSconditions,andyouwillcomparetheresultsobservedagainstvaluesprovidedfrommanufacturerdatasheets.RecallthatinLab3wemeasuredtheI-VcurveforaPNjunctionbymanuallyvaryingacurrentsupplyandrecordingtheresultantcurrentandvoltageacrossthediode.Inthislaboratorywewillneedtocollectsimilarcurves.HoweverwewillautomatetheprocesssomewhatbybuildinganI-VcurvetracingtoolusinganArduinomicrocontrollerandcertainvoltagesupplyandcurrentsensing“breakoutboards.”2InitiallyyouwillwireyourequipmentandwriteashortprogramtoacquireagraphofIDasafunctionofVDSforvariousgatevoltages,VG.Aswell,you’llreviseyourprogramtoacquireagraphofIDvsVGforafixedlargevalueofVDS.Fromthisgraphyou’lldeterminethetransconductancegainofyourtransistor(oftenwrittengfs)andcompareagainstthedatasheetspecification.FinallybasedonyourcharacterizationofyourMOSFETtransistor,youwilldesignasimpletransistorswitchtoprovidepowertoasmallloaddevicelikealightbulbormotor.ComponentsMOSFETtransistor(2N7000,ZVN2106,orsimilar)Arduino(USBBoarduino,AdafruitMetroMini,orsuitablealternativeversion)AdafruitMCP474512-bit5VDACbreakoutboard(2required)AdafruitINA219DCHigh-SideCurrentSensorbreakoutboardMCP6002dualop-amp(CMOS)TIP41CBJTtransistorandexternalpowersupplyElectricalprototypingboardJumperwires[Ifamplifyingthe5VDACsignal,extracomponents:*Externalpowersupply10kOhmresistorsx3]

2Note,theobjectiveofthelabistodeterminepropertiesoftheMOSFETtransistor.TheArduinoI-Vcurvetraceryouwillbuildandprogramservesasatooltohelpachievethatendgoal.Howeverthetoolshouldnotbeamainfocusofyourattentionwhencomposingyourreport.

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I-VCurveTracerCircuitLayout

Figure8:LayoutofelectricalcomponentsandwiringforI-Vcurvetracingapplication.(VersionshownusesMetroMiniArduino.Seeappendixforalternativeversionandconfigurations.)

Arduino-basedI-VCurveTracer-ConfigurationandPrinciplesofOperationThegoalofthefirstpartoftheexperimentistocaptureI-Vcurveswherewemonitordraincurrent,ID,whilevaryingdrain-sourcebiasvoltage,VDS,forvariousgatevoltages,VGS.ThegoalofthesecondpartoftheexperimentistomeasureIDwhilevaryingVGSasVDSisheldatafixedvoltage.Theconfigurationofthecircuitisthesameforbothpartsoftheexperiment;onlycodechangesarerequired.Notethattransistorsareoftenstaticsensitive,soitisbesttoavoidtouchingtheirpins.Furthermore,itisimportanttoknowwhichpinofaMOSFETiswhich.FortheMOSFETsusedinthislab,thepinsareasintheFigure,thoughitisalwaysgoodtocheckthespecificationsheettobecertain.Toconfiguretheelectricalcomponents,oneDACboardwillbeusedtosupplyourVDSvoltage(dac_vds).Notethatwerouteourdac_vdsVoutpinthroughanMCP6002op-ampstage,followedbyaTIP41 Figure9:MOSFETpinout

for2N7000andZVN2106versions.

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npnBJTtransistor.Theop-ampstageservesto“isolate”anyDACoutputresistancefrominfluencingvoltageslevelsatlaterstagesofthecircuit(i.e.theDACcouldactasaparallelresistorandcreatea“voltagedivider”effect).However,despitethattheop-ampstageisolatesourdac_v_dsvoltage,theMCP6002chipcannot“source”enoughcurrenttoproperlydriveourMOSFET.Tocompensate,aTIP41transistorstageisaddedtoincreasethecurrentavailabletotheMOSFET.Thisisattheexpenseofasmalldropinourvoltageduetothebase-emitterPNjunction.AftertheBJTstage,awireconnectstotheV+inputontheINA219CurrentSensorboard,andawirefromtheV-outputoftheINA219connectstotheMOSFET’sdrain.TheMOSFET’ssourcepinisconnectedtoground.ThesecondDACboardisusedtosupplyourVGSvoltage(dac_v_g).Thereisnoneedto“isolate”thisDACbecausetheMOSFETgateprovidesextremelyhigh(effectivelyinfinite)insulation,orresistance,sonovoltagedividingeffectisexpected.TheV_outpinfromdac_vgsisthereforewireddirectlytothegatepinofourMOSFET.ArduinoCodeWewillwritetwoshortprogramstocapturetheMOSFETIVcurves.Program1ThefirstprogramshoulduseoneDACtosetvaluesofgatevoltage(~0V,0.5V,1.0V…,5V)anduseasecondDACtoscanthedrain-sourcevoltageinsmallincrements(stepsofafewmV).Foragivenvoltageincrement,applythevoltageforonlyafewmillisecondswhiletakingreadingsandthenresetto0Vtoallowthetransistortimetocool.Program2Modifyyourprogramsothatyouapplythelargestdrain-sourcevoltageavailable(maximumDACV_out).Nowstepthegatevoltagefrom0-5Vinsmallsteps(afewmV).AnalysisandDiscussionYouwillcaptureasetofIDvsVDSdatafromyourprogrambyopeninganArduinoserialterminal.CopyandpastethedataforagivenVGSconditionintoaspreadsheet.YoushouldbeabletoplotseveralIDvsVDScurvesonthesameplottoillustratetheeffectofchangingVGS.(You’llusetheplottohelpdesignabasicMOSFETtransistorswitch.)Onceyou’vemodifiedyourprogramtocaptureIDvsVGSforalargevalueofVDSyou’llsimilarlycaptureyourresultsbypastingthedatafromaserialterminalintoaspreadsheet.You’llusethisgraphtodeterminebothVTandgfsforyourtransistor,andyou’lldeterminewhetheritfallswithinmanufacturerspecification.

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CommentonwhetheryourIDvsVDScurvesandIDvsVGScurveareasexpected.ExplaintheshapeofthecurvesofIDvsVDSasVDSincreases.Whereappropriate,compareyourresultstomanufacturerspecification:determinetheslopeoftheplotofIDvsVGforVG>VT(linearregion).Compareagainstthetolerancesprovidedinthedatasheet.(Notethattransistorparameterscanvarydramatically,andthereforevaluesofVTandgfsmaybesignificantlydifferentthantypicalvaluesandstillfallwithinspecification.)Notethatinourcodewehaveintentionally“pulsed”ourMOSFETonforafewmillisecondsatatimetotakemeasurements,andthenpoweredVDSorVGoffforsomelengthoftime.Whyisthisrecommendedforourmeasurements?RefertotheMOSFETspecificationsheetinyouranswer.

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DesigningaSimpleMOSFETTransistorSwitchBasedonyourIDvsVDSplotandyourvalueofVT,conceptuallydesignasimpletransistorswitchasshowninFigure10.Theintentoftheswitchcircuitistoturnonaresistiveloadlikealightbulborsmallmotor.Assumethepowerdemandfromtheloadisequaltothelastdigitofyourstudentnumber(ifworkinginpairs,selectonepartner’sstudentnumber),i.e.forstudentnumber123426,useapowerloadof6W.Assumeasupplyvoltage,Vdd,of24V.RecallthatP=IV.Selectasuitablecurrent(andthereforeVG)fromyourIDvsVDSplottosupplytheappropriatepowertothecircuitload.HowmuchpowerwillbedissipatedacrosstheMOSFETdrain/source?WilltheMOSFETsurvive?Explain.References[1]Simpson,RobertE.(1987).IntroductoryElectronicsforScientistsandEngineers.2nded.[2]Streetman,BenG.(1995).SolidStateElectronicDevices.UpperSaddleRiver,NJ:Prentice-Hall,4thed.

Figure10:SchematicofsimpleMOSFETswitchwithmotorload.

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Appendix1:IntroductiontoArduino

Arduino is a computing platform that consists of a hardware device (a microcontrollerknown as the board) and a software package to operate it. Arduino users can writeprogramsthatreadinformationfromsensorsandcancontroloutputdeviceslikemotorsorlights. The platform is relatively easy to use even for individuals with minimalprogrammingexperience.

The Arduino programming language is based on C/C++ and is used to write code andcommunicatewiththeboard.AnArduinoprogram,alsoknownasasketch,iswritteninanopensource softwarepackageknownas the integrateddevelopmentenvironment (IDE).The Arduino IDE is available online and can be downloaded from the following link:https://www.arduino.cc/en/Main/Software.

FigureA1:InterfaceofArduinointegrateddevelopmentenvironment(IDE).

TheIDEallowsuserstoVerifytheirprogrambeforeitisuploadedtotheboardtoensurethattheyare freeofanysyntaxerrors.FigureA1aboveshowsthe interfaceof IDEandasimplesketchusedtoblinkalightemittingdiode(LED).

Whenyoursketchiscompleteandcompilessuccessfully, itcanbeuploadedtotheboardusingtheUploadbuttonshowninFigureA1.

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InstallingArduinoLibraries

Insomecaseswhenadditionalcircuitcomponentsareused,special libraryfilesmightberequired. Inthisexperiment,youwillbeusinganINA219DCcurrentsensorandanMCP4725digital-to-analogconverter(DAC).Eachofthese“breakoutboards”givetheArduinoextracapabilitiesbutrequireinstallationofanArduinodriverlibraryfile.

Toinstallorupdatealibrary,gotoSketch>IncludeLibrary>ManageLibraries.

Usethesearchfeaturetolocateandinstall/updateAdafruitMCP4725.DothesamefortheAdafruitINA219library.

UploadingYourCodeandRunningYourProgram

NowthatyouhaveinstalledtheArduinoIDEandtherequiredlibraryfiles,restarttheIDEandplugyourArduinoboardusingtheUSBcableprovided.

To upload your program you’ll need to tell the IDEwhich type of Arduino you’re using(typicallyandArdiunoUnoorDuemilanova).GototheToolsmenuandselectyourboardmodel.

You’llalsoneedtotelltheIDEwhichserialportyourArduinoboardisconnectedto.AgainthiscanbeselectedunderTools>SerialPortmenu.ForMacintoshusers, select theportthatbeginswith/dev/cu.usbserial-.WindowsusersmightselectaCOMportlisted.

Finally, upload the sketch to the board. Your Arduinowill continuously run your sketchuntil it is unplugged or a different sketch is uploaded. If you unplug your Arduino yoursketchwillremaininstalledandwillautomaticallyrestartnexttimeyoupowerup.

GobuildanI-Vcurvetracer.Havefun!

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Appendix2:AlternateI-VCurveTracerHardwareConfiguration

FigureA2:CircuitconfigurationusingalternateBoarduinoArduino.