半导体-二极管-转换器-semiconductors-diodes-transistorsVIP

Semiconductors,diodes,transistors

(HorstWahl,QuarkNetpresentation,June2001)ElectricalconductivityEnergybandsinsolidsBandstructureandconductivitySemiconductorsIntrinsicsemiconductorsDopedsemiconductorsn-typematerialsp-typematerialsDiodesandtransistorsp-njunctiondepletionregionforwardbiasedp-njunctionreversebiasedp-njunctiondiodebipolartransistoroperationofbipolarpnptransistorFETELECTRICALCONDUCTIVITYinorderofconductivity:superconductors,conductors,semiconductors,insulatorsconductors:materialcapableofcarryingelectriccurrent,i.e.materialwhichhas“mobilechargecarriers”(e.g.electrons,ions,..) e.g.metals,liquidswithions(water,moltenioniccompounds),plasma

insulators:materialswithnoorveryfewfreechargecarriers;e.g.quartz,mostcovalentandionicsolids,plasticssemiconductors:materialswithconductivitybetweenthatofconductorsandinsulators;e.g.germaniumGe,siliconSi,GaAs,GaP,InPsuperconductors:certainmaterialshavezeroresistivityatverylowtemperature.somerepresentativeresistivities():R=L/A,R=resistance,L=length,A=crosssectionarea;resistivityat20oC

resistivityinm

resistance(in)(L=1m,diam=1mm)aluminum 2.8x10-8 3.6x10-2brass 8x10-8 10.1x10-2copper 1.7x10-8 2.2x10-2platinum 10x10-8 12.7x10-2silver 1.6x10-8 2.1x10-2carbon 3.5x10-5 44.5germanium 0.45 5.7x105silicon 640 6x108porcelain 1010-1012 1016-1018teflon 1014 1020blood 1.5 1.9x106fat 24 3x107ENERGYBANDSINSOLIDS:Insolidmaterials,electronenergylevelsformbandsofallowedenergies,separatedbyforbiddenbands valenceband=outermost(highest)bandfilledwithelectrons(“filled”=allstatesoccupied)conductionband=nexthighestbandtovalenceband(emptyorpartlyfilled)“gap”=energydifferencebetweenvalenceandconductionbands,=widthoftheforbiddenbandNote: electronsinacompletelyfilledbandcannotmove,sinceallstatesoccupied(Pauliprinciple);onlywaytomovewouldbeto“jump”intonexthigherband-needsenergy;electronsinpartlyfilledbandcanmove,sincetherearefreestatestomoveto.Classificationofsolidsintothreetypes,accordingtotheirbandstructure:insulators:gap=forbiddenregionbetweenhighestfilledband(valenceband)andlowestemptyorpartlyfilledband(conductionband)isverywide,about3to6eV;semiconductors:gapissmall-about0.1to1eV;conductors:valencebandonlypartiallyfilled,or(ifitisfilled),thenextallowedemptybandoverlapswithitBandstructureandconductivityINTRINSICSEMICONDUCTORSsemiconductor=materialforwhichgapbetweenvalencebandandconductionbandissmall;

(gapwidthinSiis1.1eV,inGe0.7eV).atT=0,therearenoelectronsintheconductionband,andthesemiconductordoesnotconduct(lackoffreechargecarriers);atT>0,somefractionofelectronshavesufficientthermalkineticenergytoovercomethegapandjumptotheconductionband;

fractionriseswithtemperature;

e.g.at20oC(293K),Sihas0.9x1010conductionelectronspercubiccentimeter;at50oC(323K)thereare7.4x1010.electronsmovingtoconductionbandleave“hole”(covalentbondwithmissingelectron)behind; underinfluenceofappliedelectricfield,neighboringelectronscanjumpintothehole,thuscreatinganewhole,etc.

holescanmoveundertheinfluenceofanappliedelectricfield,justlikeelectrons; bothcontributetoconduction.inpureSiandGe,thereareequallymanyholes(“p-typechargecarriers”)asthereareconductionelectrons(“n-typechargecarriers”);puresemiconductorsalsocalled“intrinsicsemiconductors”.Intrinsicsilicon:DOPEDSEMICONDUCTORS:“dopedsemiconductor”:(also“impure”,“extrinsic”)=semiconductorwithsmalladmixtureoftrivalentor pentavalentatoms;n-typematerial

donor(n-type)impurities:

dopantwith5valenceelectrons(e.g.P,As,Sb)4electronsusedforcovalentbondswithsurroundingSiatoms,oneelectron“leftover”;leftoverelectronisonlylooselybound

onlysmallamountofenergyneededtoliftitintoconductionband(0.05eVinSi)

“n-typesemiconductor”,hasconductionelectrons,noholes(apartfromthefewintrinsicholes)example:dopingfraction

of10-8

SbinSiyieldsabout5x1016conductionelectronspercubiccentimeteratroomtemperature,i.e.gainof5x106overintrinsicSi.p-typematerialacceptor(p-type)impurities:

dopantwith3valenceelectrons(e.g.B,Al,Ga,In)

only3ofthe4covalentbondsfilled

vacancyinthefourthcovalentbond

hole“p-typesemiconductor”,hasmobileholes,veryfewmobileelectrons(onlytheintrinsicones).advantagesofdopedsemiconductors:can”tune”conductivitybychoiceofdopingfractioncanchoose“majoritycarrier”(electronorhole)canvarydopingfractionand/ormajoritycarrierwithinpieceofsemiconductorcanmake“p-njunctions”(diodes)and“transistors”DIODESANDTRANSISTORSp-nJUNCTION:

p-njunction=semiconductorinwhichimpuritychangesabruptlyfromp-typeton-type;“diffusion”=movementduetodifferenceinconcentration,fromhighertolowerconcentration;inabsenceofelectricfieldacrossthejunction,holes“diffuse”towardsandacrossboundaryinton-typeandcaptureelectrons;electronsdiffuseacrossboundary,fallintoholes(“recombinationofmajoritycarriers”);

formationofa“depletionregion”

(=regionwithoutfreechargecarriers)

aroundtheboundary;chargedionsareleftbehind(cannotmove):negativeionsleftonp-side

netnegativechargeonp-sideofthejunction; positiveionsleftonn-side

netpositivechargeonn-sideofthejunction

electricfieldacrossjunctionwhichpreventsfurtherdiffusion.PnjunctionFormationofdepletionregioninpn-junction:DIODEdiode=“biasedp-njunction”,i.e.p-njunctionwithvoltageappliedacrossit“forwardbiased”:p-sidemorepositivethann-side;“reversebiased”:n-sidemorepositivethanp-side;forwardbiaseddiode:thedirectionoftheelectricfieldisfromp-sidetowardsn-side

p-typechargecarriers(positiveholes)inp-sidearepushedtowardsandacrossthep-nboundary,

n-typecarriers(negativeelectrons)inn-sidearepushedtowardsandacrossn-pboundary

currentflowsacrossp-nboundaryForwardbiasedpn-junctionDepletionregionandpotentialbarrierreducedReversebiaseddiodereversebiaseddiode:appliedvoltagemakesn-sidemorepositivethanp-side

electricfielddirectionisfromn-sidetowards p-side

pusheschargecarriersawayfromthep-n

boundary

depletionregionwidens,andnocurrentflows

diodeonlyconductswhenpositivevoltageappliedtop-sideandnegativevoltageton-side

diodesusedin“rectifiers”,toconvertacvoltagetodc.ReversebiaseddiodeDepletionregionbecomeswider, barrierpotentialhigherTRANSISTORS(bipolar)transistor=combinationoftwodiodesthatsharemiddleportion,called“base”oftransistor;othertwosections:“emitter''and“collector”;usually,baseisverythinandlightlydoped.twokindsofbipolartransistors:pnpandnpntransistors“pnp”meansemitterisp-type,baseisn-type,andcollectorisp-typematerial;in“normaloperationofpnptransistor,applypositivevoltagetoemitter,negativevoltagetocollector;operationofpnptransistor:ifemitter-basejunctionisforwardbiased,“holesflow”frombatteryintoemitter,moveintobase;someholesannihilatewithelectronsinn-typebase,butbasethinandlightlydoped

mostholesmakeitthroughbaseintocollector,holesmovethroughcollectorintonegativeterminalofbattery;i.e.“collectorcurrent”flowswhosesizedependsonhowmanyholeshavebeencapturedbyelectronsinthebase;thisdependsonthenumberofn-typecarriersinthebasewhichcanbecontrolledbythesizeofthecurrent(the“basecurrent”)thatisallowedtoflowfro