Omar_Elementary Solid State Physics_Lecture9

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1、Chapter7Weshalldiscuss:•thep-njunction=⇒explainitsrectificationpropertyandshowhow:•thejoiningof2p-njunctionsleadstothejunctiontransistorwhichactsasanamplifier.7.2Thep-njunction:TheRectifierThep-njunctionis:•aspecimenmadeofasingle–crystalsemiconductorinwhichtherearetwoad-jacen

2、tregions(seeFig.7.1):–ann-type–ap-typethen-regionisdopedwithdonorimpurities;thep-regionwithacceptorimpurities.Nd=donorconcentrationNn=acceptorconcentration.Notethatthewholejunctionisonesinglepieceofcrystalinwhich:•thetwosidesaredopeddifferentlyratherthantwodifferentpiecesjoi

3、nedto-gether.Themostinterestingelectricalproperty:•apotentialdifference,thecontactpotential,developsacrossthejunction,be-cause•electronsbegintodiffuseacrossjunctionfromntopregion•holesdiffusefromptonregion.Duetolargeconcentrationgradientatthejunction:⇓pregionacquiresnetnegati

4、vechargebecause•electronsenterthatregion•holesleavethatregion.⇓CONTACTPOTENTIALDEVELOPSNote:RefertotheFig.7.2whereenergyonp-sideishigherthanenergyonn-sidebyeφwhereφ=potentialdifference=contactpotential.ContactPotentialimpedesflowofdiffusioncurrents:dφelectricfield=E0=−dxactsto

5、opposefurtherdiffusionoftheelectronsandholes,andonly•thoseelectronswithkineticenergy>eφcandiffusefromn-sidetop-side.Aschargetransfercontinues,potentialcontinuestoincrease=⇒diffusionfluxcontinuestodecreaseuntil:Diffusionfluxofelectronsfromntopsidebecomesbalancedbyanelectronfluxfro

6、mptonsidecalledthe“GenerationFlux”2WHATISTHESOURCEOFTHEGENERATIONFLUX?•Onp-side,electronsandholesarecreatedthermally.Theseelectronsandholesrecombine,BUTanelectronfluxflowingtonregionresults,andanysuchelectronreachingthejunctionis=⇒quicklysweptawaytonsidebythestrongelectricfie

7、ldinsidethejunction.Thus,therearetwoelectronfluxesacrossthejunction:1.Afluxfromntopsideduetolargeelectronconcentrationonn-side.ThisiscalledtherecombinationfluxJnr,becauseelectronsflowingintothepregioneventuallycombinewiththeholesthere.2.ThegenerationfluxJng,flowingfromptonside.E

8、QUILIBRIUMREQUIRES:Jnr0=Jng0.Bysimilararguments:•holesinthevalencebandflowacrossjunction−→