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SemiconductorMaterials•1.1EnergyBandsandCarrierConcentration•1.1.1SemiconductorMaterials•Solid-statematerialscanbegroupedintothreeclasses—insulators(绝缘体）,semiconductors,andconductors.Figure1-1showstheelectricalconductivitiesδ(andthecorrespondingresistivitiesρ≡1/δ)associatedwith(相关）someimportantmaterialsineachofthreeclasses.Insulatorssuchasfused（熔融）quartzandglasshaveverylowconductivities,intheorderof1E-18to1E-8S/cm;固态材料可分为三种：绝缘体、半导体和导体。图1－1给出了在三种材料中一些重要材料相关的电阻值（相应电导率ρ≡1/δ)。绝缘体如熔融石英和玻璃具有很低电导率，在10-18到10-8S/cm;andconductorssuchasaluminumandsilverhavehighconductivities,typicallyfrom104to106S/cm.Semiconductorshaveconductivitiesbetweenthoseofinsulatorsandthoseofconductors.Theconductivityofasemiconductorisgenerallysensitivetotemperature,illumination（照射）,magneticfield,andminuteamountofimpurityatoms.Thissensitivityinconductivitymakesthesemiconductoroneofthemostimportantmaterialsforelectronicapplications.导体如铝和银有高的电导率，典型值从104到106S/cm;而半导体具有的电导率介乎于两者之间。半导体的电导率一般对温度、光照、磁场和小的杂质原子非常敏感。在电导率上的敏感变化使得半导体材料称为在电学应用上为最重要的材料。Thestudyofsemiconductormaterialsbeganinearlynineteenthcentury.Overtheyearsmanysemiconductorshavebeeninvestigated.Table1showaportion（部分)oftheperiodic(周期）tablerelatedtosemiconductors.Theelementsemiconductors,thosecomposedofsinglespeciesofatoms,suchassilicon(Si)andgermanium(Ge),canbefoundinColumnⅣ.However,numerouscompoundsemiconductorsarecomposedoftwoormoreelements.Forexample,galliumarsenide(GaAs)isaⅢ-Ⅴcompoundthatisacombination（合成）ofgallium(Ga)fromColumnⅢandarsenic(As)fromColumnⅤ.早在19世纪人们已经开始研究半导体材料。多年来人们研究了很多半导体材料。表1给出了与半导体相关的周期表中的部分元素。由单种元素组成的单质半导体如硅和锗在第Ⅳ族。而大量的化合物半导体有两个甚至更多元素组成。如GaAs是Ⅲ-Ⅴ化合物是由Ⅲ族的Ga和Ⅴ族的As化合而得。Priortotheinventionofthebipolartransistor（双极二极管）in1947,semiconductorswereusedonlyastwo-terminal（电极）devices,suchasrectifiers（整流器）andphotodiodes（光敏二极管）.Intheearly1950s,germaniumwasthemajorsemiconductormaterial.在1947年双极晶体管发明之前，半导体仅用作双极型器件如整流器和光敏二极管。早在20世纪50年代，锗是主要的半导体材料。However,germaniumprovedunsuitableinmanyapplicationsbecausegermaniumdevicesexhibitedhighleakagecurrents（漏电流）atonlymoderatelyelevatedtemperatures.Inaddition,germaniumoxideiswatersolubleandunsuitedfordevicefabrication.Sincetheearly1960ssiliconhasbecomeapracticalsubstitute（实际取代）andhasnowvirtuallysupplanted（事实上替代）germaniumasamaterialforsemiconductorfabrication(结构）然而锗不太适合在很多方面应用因为温度适当提高后锗器件会产生高的漏电流。另外，锗的氧化物是水溶性的不适合器件制作。所以20世纪60年代实际上锗被硅所取代，事实上硅替代锗成为半导体制作的材料之一。Themainreasonswenowusesiliconarethatsilicondevicesexhibitmuchlowerleakagecurrents,andhigh-qualitysilicondioxidecanbegrownthermally.Thereisalsoaneconomicconsideration.Devicegradesiliconcostsmuchlessthananyothersemiconductormaterial.siliconintheformofsilicaandsilicates（硅酸盐）comprises25%oftheEarth’scrust（地表）,andsiliconissecondonlytooxygeninabundance（分布）.Atpresent,siliconisoneofthemoststudiedelementsintheperiodictable;andsilicontechnologyisbyfarthemostadvancedamongallsemiconductortechnologies我们用硅材料的主要原因有硅器件存在非常低的漏电流且能够通过热法生长出高质量的二氧化硅。器件级硅成本远少于其它半导体材料。硅以硅石和硅酸盐形式存在并占地球地表层的25％，而且硅元素在分布中排在氧之后的第二位。当今硅是在元素周期表中研究最多的元素；硅技术是在所有半导体技术中最先进的。Manyofthecompoundsemiconductorshaveelectricalandopticalpropertiesthatareabsent（缺少）insilicon.Thesesemiconductors,especiallygalliumarsenide(GaAs),areusemainlyformicrowaveandphotonicapplications.Althoughwedonotknowasmuchaboutthetechnologyofcompoundsemiconductoraswedoaboutthatofsilicon,compoundsemiconductortechnologyhasadvancedpartlybecauseoftheadvancesinsilicontechnology.Inthisbookweareconcernedmainlywithdevicephysicsandprocessingtechnologyofsiliconandgalliumarsenide.有很多化合物半导体具有硅所缺少的电光性能。这些半导体特别是GaAs主要用作微波和光学应用。虽然我们了解化合物半导体技术不如硅材料的多，但化合物半导体技术由于硅技术的发展而发展。在本书中我们主要介绍硅和砷化镓的器件物理和制备技术。CrystalStructureThesemiconductormaterialswewillstudyaresinglecrystals,thatis,theatomsarearrangedinathree-dimensionalperiodicfashion.Theperiodicarrangement（排布）ofatomsinacrystaliscalledalattice（晶格）.Inacrystal,anatomneverstray（偏离）farfromasingle,fixedposition.Thethermalvibrationsassociatedwiththeatomarecenteredaboutthisposition.Foragivensemiconductor,thereisaunitcell（晶胞）thatisrepresentativeoftheentirelattice;byrepeatingtheunitcellthroughoutthecrystal,onecangeneratetheentirelattice.我们研究的半导体材料是单晶，也就是说，原子是按照三维周期形式排列。在晶体中原子的周期排列称为晶格。在晶体里，一个原子从不远离它确定位置。与原子相关的热运动也是围绕在其位置附近。对于给定的半导体，存在代表整个晶格的晶胞，通过在晶体中重复晶胞组成晶格。Figure1-2showssomebasiccubic-crystalunitcells.Figure1-2(a)showsasimplecubic（立方）crystal;eachcornerofthecubiclatticeisoccupiedbyanatomthathassixequidistant（等距）nearestneighboringatoms.Thedimensionaiscalledthelatticeconstant.Onlypolonium（钋）iscrystallizedinthesimplecubiclattice.Figure1-2(b)isabody-centeredcubic（体心立方）(bcc)crystal,whereinadditiontotheeightcorneratoms,anatomislocatedatcenterofthecube.图1－2给出一些立方晶体晶胞。图1－2（a）给出了一个简单的立方晶体；立方晶格的每个角由一个原子占据，所以有6个等距原子。a的大小称为晶格常数。只有金属钋明确是单立方晶体。图1－2（b）是体心立方晶体，除了8个角原子外，一个原子在其立方中心上。Inabcclattice,eachatomhaseightnearest-neighboringatoms.Crystalsexhibitingbcclatticesincludethoseofsodium（钨）andtungsten（钠）.Figure1-2(c)showsaface-centeredcubic(fcc)（面心立方）crystalthathasoneatomateachofthesixcubicfacesinadditionto（还有）theeightcorneratoms.Inanfcclattice,eachatomhas12nearestneighboringatoms.Al