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1、纳米材料基础 北京航空航天大学 于荣海 材料的电学性能1 电性材料 Conductor Semiconductor Dielectric ElectricalconductionHowmanymoveableelectronsarethereinamaterial carrierdensity Howeasilydotheymove mobility SemiconductivityElectronsandholesIntrinsicandextrinsiccarriersSemiconductordevices p njunctionsandtransistorsConductioninpo
2、lymersandionicmaterialsDielectricbehavior Electricalpotential V Current I volts amperes Ohm slaw I V RRistheelectricalresistance ohms Rduetointrinsicresistivity m geometry lengthL areaAthroughwhichthecurrentpasses R L A Inmostmaterials e g metals thecurrentiscarriedbyelectrons electronicconduction I
3、nioniccrystals thechargecarriersareions ionicconduction Electricalconductivity abilitytoconduct 1 Electricfieldintensity E V LOhm slawcanberewrittenintermsofthecurrentdensityJ I AJ E Electricalconductivityvariesbetweendifferentmaterialsbyover27ordersofmagnitude thegreatestvariationofanyphysicalprope
4、rty Metals 105 m 1Semiconductors 10 6 105 m 1Insulators 10 6 m 1 Atomsformasolid valenceelectronsinteract twoquantummechanicaleffects 1 Heisenberg suncertaintyprinciple constrainelectronstoasmallvolume raisestheirenergycalledpromotion 2 Pauliexclusionprinciplelimitsthenumberofelectronswiththesameene
5、rgy Result valenceelectronsformwideelectronenergybandsinasolid Bandsseparatedbygaps whereelectronscannotexist FermiEnergy EF highestfilledstateat0KConductionband partiallyfilledoremptybandValenceband highestpartiallyorcompletelyfilledband Semiconductorsandinsulators valencebandisfilled andnomoreelec
6、tronscanbeadded Pauli sprinciple Electricalconduction electronsgainenergyinanelectricfield Notpossibleinthesematerials forbiddenbandgap Probabilityanelectronreachestheconductionbandis exp Eg 2kT EgisbandgapProbability55Atroomtemperature 2kT 0 05eVEg 2 8eVAninsulator EnergyBandStructuresandConductivi
7、ty semiconductorsandinsulators Semiconductorsandinsulators electronsmustjumpacrossbandgapintoconductionbandtofindconductingstatesaboveEfEnergyneeded heatorradiationDifference semiconductorselectronsreachconductionbandatroomtemperature notininsulatorsElectronpromotedtoconductionbandleavesahole positi
8、vecharge inthevalenceband italsoparticipatesinconduction EnergyBandStructuresandConductivity metals Metals highestoccupiedbandispartiallyfilledorbandsoverlapConductionoccursbypromotingelectronsintoconductingstates startrightaboveFermilevel Energyprovidedbyanelectricfieldissufficienttoexcitemanyelect
9、ronsintoconductingstates EnergyBandStructuresandBonding metals semiconductors insulators Metals valenceelectronsforman electrongas Insulators valenceelectronstightlyboundto orsharedwith theindividualatoms ionic covalentSemiconductors mostlycovalentbonding EnergyBandStructuresandConductivity Semicond
10、uctorsandInsulators Metals ElectronMobility Forceonelectronis eE e chargeNoobstacles electronspeedsupinanelectricfield Vacuum TVtube orperfectcrystalRealsolid electronsscatteredbycollisionswithimperfectionsandthermalvibrations friction resistance netdriftvelocityofelectronsvd eE e electronmobility m
11、2 V s 1 FrictionTransferspartofenergysuppliedbyelectricfieldintolatticeasheat Electricheater E Scatteringevents Netelectronmotion ElectronMobility Electricalconductivityproportionaltonumberoffreeelectronsperunitvolume Ne andelectronmobility e Ne e e metal semi metal semi Nmetal Nsemi Conductivity Re
12、sistivityofMetals Totalresistivity tot Matthiessenrule total thermal impurity deformation IncreaseswithT withdeformation andwithalloying Conductivity ResistivityofMetals Influenceoftemperature Increasingthermalvibrationsanddensityofvacancies T o aTImpurities Solidsolution I Aci 1 ci ciisimpurityconc
13、entrationTwo phasealloy and phases i V V Plasticdeformation MaterialsofChoiceforMetalConductors Silver OneofbestforelectricalconductionbuthighcostCopper inexpensive abundant high butsoftCu Bealloy Precipitationhardening solidsolutionalloying coldworkingdecreaseconductivity Aluminum lowweight moreres
14、istanttocorrosionbuthalfasgoodasCuNickel chromiumalloy low highR resistanttohightemperatureoxidation Heatingelements Semiconductivity Intrinsicsemiconductors conductivitydefinedbyelectronicstructureofpurematerial Extrinsicsemiconductors electricalconductivityisdefinedbyimpurityatoms Intrinsicsemicon
15、ductors I Ne n CT3 2exp Eg 2kT Numberofelectronsinconductionband Egisbandgap Electronpromotedintotheconductionband hole positivecharge invalenceband Inelectricfield electronsandholesmoveinoppositedirectionandparticipateinconduction Si Eg 1 1eV oneoutofevery1013atomscontributesanelectrontoconductionb
16、andatroomT Intrinsicsemiconductors II n e e p e hp holeconcentration hholemobilityn electronconcentration e mobility e handn p n e e h p e e h n andp increaseexponentiallywithT eand hdecreaseaboutlinearlywithT Conductivityofintrinsicsemiconductorsincreaseswithtemperature differentfrommetals Intrinsicsemiconductors III Extrinsicsemiconductors definedbyimpurityatomsSiextrinsicatroomTifimpurityconcentrationisoneatomper1012 rememberestimateofnumberofelectronspromotedtoconductionbandat300K Differentc
