微电子英语PPT

Intheprecedingsectionweconsideredthedriftcurrent,thatisthetransportofcarrierwhenanelectricfieldisapplied.Anotherimportantcurrentcomponentcanexistifthereisaspatialvariationofcarrierconcentrationinthesemiconductormaterial,thatis,thecarriertendtomovefromaregionofhighconcentrationtoaregionoflowconcentration.Thiscurrentcomponentiscalleddiffusioncurrent.前面部分我们提到了漂移电流，就是外加电场下载流子的运输。如果半导体中载流子浓度发生空间变化，也就是说载流子倾向于从浓度高的地方向浓度低的地方移动，那么另外一种电流成分也存在。这种电流成分被称为扩散电流。Letusconsidertheeffectofdroppingatinyamountofhighlyconcentratedredinkinaglassofwater.Supposethatitiscarefullydonesothatthewaterisnotdisturbed.Wewilobservethattheinkspotisslowlyenlarged.Physically,theinkdropletcontainsalargenumberofmoleculesofcolorpigments.Theindividualmoleculesmovearoundbecauseofthermalenergy.让我们来考虑将一滴高浓度的红色墨水滴入一杯水中。假定这个过程非常小心以至于不会扰乱杯中的水。我们将会观察到墨水滴慢慢变大。从物理上来说，这个墨水滴包含了大量的色素分子。每个单独的分子由于热能到处移动。TheirmotionisrandomandtheyexperiencecollisionsjustlikeelectronsscatteringdescribedinSection2.1.Intrulyrandommovements,halfthescatteredcolorpigmentswillmovetowardthecenteroftheinkdropletandtheotherhalfwillmoveawayfromthecenter.Now,thenumberofmoleculesisfixed.Themigrationofthemoleclesreducesthedensityatthecenterofthecolorspotsothatthespotbecomesenlargedwithalightercolor.它们的移动是任意的并且在这个过程中会经历撞击就像2.1中描述的电子被驱散是一样的。在真正的任意的移动中。半数的被驱散的的色素分子会向着墨水滴中心移动而另外一半会远离中心。现在分子被混合在了一起。分子的迁移降低了色素中心的密度，这样这滴墨水会以更浅的颜色扩大。Letthecurveatt1representthecolor-pigmentconcentrationoftheinkdropletshortlyafteritreachesthewater.Ithasapeakdensityofunityandasmallhalf-width.Astimegoesby,moremoleculesmigrateawaysothatthedensityatthecentergoesdownandthehalf-widthincreases.Eventually,thewholeglassofwaterwillbepermeatedwithredinktobecomeuniformlightredliquid.曲线t1处代表墨水滴刚接触水面时色素的浓度。它有最高的结合密度和一个较小的半径。随着时间增加，越来越多的分子迁移走导致中心处密度减小而半径增加。最终，这整杯水将被红墨水渗入而变成一杯均匀的浅红色的液体。Noticethatthemovementofcolorpigmentsisrelatedtotheconcentrationgradientsothatthemoleculeshaveanettendencytomovefromhighconcentrationtolowconcentration.Thisisthebasicphysicsofparticlediffusion.Whentheconcentrationbecomesuniformthroughouttheliquid,themoleculescontinuetomovearound.注意到色素分子的移动跟浓度梯度有关，也就是说分子最终倾向由高浓度从低浓度移动。这也是微粒扩散的基本原理。单整杯液体的浓度变成一致时，分子继续到处运动。However,thisrandommotionrequiresthatmovementsinalldirectionsarethesamesothattheconcentrationuniformityremainsunchanged.Macroscopically,thedensityofthecolorpigmentsisthesamethroughoutsothatthediffusionstops.Therefore,theprogressofdiffusionresultsfromnonuniformdistributionofthecolor.然而，这种任意运动要求更个方向的移动是一样的，这样的话浓度一致性保持不变。宏观来说，色素的密度整体是不变的这样扩散也就停止了。因此，扩散过程起因于色素分子的不均匀分布。Inasemiconductor,thediffusionofelectronsorholesresultsfromtheirmovementfromhighconcentrationtolowconcentration.Becauseelectronsandholesarechargecarriers,theirmotiongivesrisetoacurrentflowknownasthediffusioncurrent.ThediffusionfluxobeysFick’slawwhereF=fluxofcarriers=numberspassingthrough1cm2/s;D=diffusionconstant;N=carrierdensity.Withacross-sectionalareaA,thecurrentisgivenbyqAFsothatthediffusioncurrentsofelectronsandholesarewhereandarethediffusionconstantforelectronsandholes,respectively.在半导体中，电子和空穴的扩散起因于他们由高浓度向低浓度移动。由于电子和空穴是电荷载流子，所以他们的移动引起电流流动就是扩散电流。扩散的变化遵循菲克第一定律，其中F是载流子通量；D是扩散系数；N是载流子密度。在部分混合的区域A，电流由qAF导出所以电子和空穴的扩散电流如下，其中和分别是电子空穴的扩散系数。dxdNDFdxdnqADJnndxdpqADJppnDpDnDpDThediffusionconstantwillbeshownlatertoberelatedtothemobiltybyEinsteinrelationshipForthelightlydopedsiliconatroomtemperature=38cm2/sand=13cm2/s.ThetotalelectronandholecurrentsareobtainedbyaddingthedriftanddiffusioncomponentTheseequationswillbeusedthroughoutthetexttocharacterizesemiconductordevices.扩散系数与迁移率的关系如下。对于室温下轻参杂的硅=38cm2/s，=13cm2/s。总的电子空穴电流通过漂移电流和扩散电流叠加得到。这些等式将在后面的用来描述半导体器件的特性。qkTDDnnpp)(dxdnDnEqAJnnn)(dxdpDpEqAJpppnDpDnDpDInthermalequilibriumtherelationshippn=ni2isvalid.Ifexcesscarriersareintroducedtoasemiconductorsothatpnni2,wehaveanone-equilibriumsituation.Theprocessofintroducingexcesscarriersiscalledcarrierinjection.Wecaninjectcarriersbyusingvariousmethodsincludingopticalexcitationandforwardbiasingap-njunction.在热平衡条件下关系式pn=ni2是成立的。如果过量载流子被引入半导体中导致pnni2，我们就得到了一个非平衡状态。引入过剩载流子的过程称为载流子的注入。我们能通过各种方法注入载流子包括光激发和正向偏置下的pn结。Incaseofopticalexcitation,weshinealightonasemiconductor.Ifthephotonenergyofthelightisgreaterthanthebandgapenergyofthesemiconductor,wherehisthePlankconstantandvistheopticalfrequency,thephotonisabsorbedbysemiconductorandaelectron-holepair(EHP)isgenerated.Theopticalexcitationincreasestheelectronandholeconcentrationabovetheirequilibriumvalues.Theseadditionalcarriersarecalledexcesscarriers.在光激发的情况下，我们用光照射半导体。如果光子的能量比半导体能带间隙能量大，光子将被半导体吸收并产生电子空穴对。光激发增加电子空穴的浓度至高于平衡时的标准。这些添加的载流子被称为过剩载流子hgEhgEThemagnitudeoftheexcesscarrierconcentrationrelativetothemajoritycarrierconcentrationdeterminestheinjectionlever.Weshalluseanexampletoclarifythemeaningofinjectionlevel.Themajoritycarrierconcentrationisapproximatelyequaltothedonorconcentration,thatis,nn0=1015cm-3.Theminoritycarrierconcentrationisgivenby,pn0=ni2/nn0=1.45*105cm-3.相对于多数载流子浓度来说过剩的载流子浓度决定了注入的级别。我们可以用一个例子来说明注入级别的意义。多数载流子的浓度近似等于施主浓度，当nn0=1015cm-3时，少数载流子的浓度这样得到，pn0=ni2/nn0=1.45*105cm-3。Inthisnotation,thefirsts