专业英语阅读材料Word格式文档下载.docx

专业英语阅读材料Word格式文档下载.docx

《专业英语阅读材料Word格式文档下载.docx》由会员分享，可在线阅读，更多相关《专业英语阅读材料Word格式文档下载.docx（75页珍藏版）》请在冰点文库上搜索。

3.2Band22

ReadingMaterials23

Session4TheSemiconductorinEquilibrium27

4.1ChargeCarriersinSemiconductors27

4.2IntrinsicSemiconductor30

4.3ExtrinsicSemiconductors31

Readingmaterials34

Session5CarrierTransport38

5.1Overviewofcarriertransport38

5.2Lowfieldtransport40

5.3Highfieldtransport42

5.4Diffusioncurrent43

Sesison6NonequilibriumExcessCarriersinSemiconductor48

6.1Recombination48

6.2MinorityCarrierLifetime50

6.3AmbipolarTransport52

ReadingMaterials53

Session7ThepnJunction（I）55

7.1Introduction55

7.2BasicStructureofthepnJunction55

7.3EnergybandsforapnJunction56

7.4IdealCurrent-VoltageRelationship57

7.5Characteristicsofapracticaldiode58

Readingmaterials59

Session8ThepnJunction（II）63

8.1Breakdowninpnjunction63

8.2JunctionCapacitance64

8.3Diffusionorstoragecapacitance65

8.4Diodetransients67

8.5Circuitmodelsforjunctiondiodes67

Readingmaterials68

Session9Metal-SemiconductorContacts72

9.1Schottkycontacts72

9.2Ohmiccontacts78

ReadingMaterials81

Session10Heterojunctions84

10.1StrainandStressatHeterointerfaces84

10.2HeterojunctionMaterials85

10.3Energy-BandDiagrams88

ReadingMaterials89

Session11TheBipolarJunctionTransistor（I）93

11.1TheBipolarJunctionTransistorConstruction93

11.2Transistoraction93

11.3Nonidealeffects95

11.4BaseResistance97

Readingmaterials98

Session12TheBipolarJunctionTransistor（II）102

12.1BreakdownVoltage102

12.2FrequencyLimitsofBJT103

12.3TheSchottky-ClampedTransistor104

12.4Small-signalTransistorModel105

Readingmaterials107

Session13BasicsofMOSFETs111

13.1Introduction111

13.2GeneralCharacteristicsofaMOSFET111

13.3MOSsystem112

13.4WorkFunctionDifferences113

13.5Flat-BandVoltage114

13.6Thresholdvoltage115

ReadingMaterials115

Session14NonidealeffectsofMOSFETs119

14.1Introduction119

14.2Effectivemobility119

14.3Velocitysaturation120

14.4Channel-lengthmodulation120

14.5DIBL121

14.6Hot-carriereffect122

14.7GIDL123

Readingmaterials123

Session15AdvancedMOSFETDevices128

15.1Introduction128

15.2ChannelDopingProfile128

15.3GateStack129

15.4Source/DrainDesign129

15.5Schottky-BarrierSource/Drain130

15.6RaisedSource/Drain131

15.7SOI132

15.8ThreeDimensionalstructure133

Readingmaterials133

Session5CarrierTransport

5.1Overviewofcarriertransport

Electronicdevicesrelyontransportofelectrons（holes）inmaterials.Thistransportoccurseitherundertheinfluenceofanelectricfieldorcarrierconcentrationgradients.Thechargesinasolidcanbelooselyclassifiedasfixedandmobile.1Whenanexternalperturbationisapplied（e.g.,anelectricfield）themobilechargescanmovefromonepointinspacetoanother.Inparticulartheycanmovefromonecontactonadevicetoanother.Thefixedcharge,however,canonlybedisturbedslightlyfromitsequilibriumposition,butcannotmoveoverthelengthofadevice.2AsshowninFig.5.1bothfixedchargesandmobilechargesplayanimportantroleinthephysicsofsemiconductors.Essentiallyallelectronicdevicessuchasfieldeffecttransistors,bipolartransistors,diodes,aswellasoptoelectronicdevices,suchaslasersanddetectorsdependuponfreeormobilecharges.3Mobilechargesaretheelectronsintheconductionbandandholesinthevalencebandforsemiconductorsandinsulators.Aswehavediscussedinthepreviouschapter,inmetalsthemobilechargesaretheelectronsintheconductionband.

Fig.5.1Anoverviewoffixedandmobilechargesinsolidsandtheirimpactonphysicalphenomena.Semiconductordevicesaredependentuponmobileelectronsandholes.

Beforediscussingissuesinfreecarrier（ormobilecarrier）transportweremindthereaderofthenatureofelectronicstatesinsolidsinFig.5.2.Inthecaseoftheperfectcrystalweseethatintheconductionandvalencebandstheelectronicstatesare“free”.Therearenoallowedenergylevelsinthebandgap（densityofstatesiszerointhebandgap,asshown）.Inthecaseofacrystalwithdefectswestillhavethefreestatesintheconductionandthevalencebands,butwealsohavedefect-relatedallowedstatesinthebandgapregion,asshowninFig.5.2b.Inthesestates（trapstates）electronsarenotfreetomove.

Wewillfirstprovideasimpleoverviewofhowelectronsrespondtoappliedelectricfields.InFig.5.3weshowaschematicofhowelectrons（holes）movethroughasamplewhenanelectricfieldisapplied.InFig.5.3aweshowthesituationinagood-qualitycrystallinematerial.Theelectronmovesundertheelectricfieldforce,butsuffersanumberofscatteringprocesses.Thescatteringoccursduetovariousimperfections,suchasdefectsandvibrationsofatoms（duetothermalenergy）.Therelationbetweentheelectronvelocityordistancetraveledandappliedfieldiscomplex.Howeveratlowfieldstherelationcanbedescribedbyasimplerelation.IfweexaminethedistanceversustimetrajectoryofatypicalelectronweobservethattheelectronshowsapathasshowninFig.5.3.Onaveragetheelectrontrajectoryisdescribedby

（5.1）

wheredisthedistancetraveledintimet.Thevelocityvisproportionaltotheelectricfieldappliedthroughμ,themobility.Whentheelectricfieldislargetherelationshipbetweenvelocityandappliedfieldisnotsosimpleandwillbediscussedlater.

（a）

（b）

Fig.5.2Aschematicofthenatureofelectronicstatesinsolids:

（a）foraperfectcrystal,

（b）foracrystalwithdefects.

Fig.5.3Atypicalelectrontrajectoryinasampleandthedistanceversustimeprofile.

5.2Lowfieldtransport

Firstweconsidersmallelectricfields.Inthelow-fieldregimethevelocityisproportionaltotheelectricfield.

5.2.1Mobility

Themobilityisdefined（scalarterms）as

（5.2）

Bydefinition,itisanegativenumberforelectronsandpositiveforholes.4However,thenumericalvalueisusuallygivenasapositivenumberforbothcarriertypes.Inanintrinsicsemiconductorthemobilityisdeterminedbyscatteringwithphonons.Furtherscatteringisintroducedbyimpurities,defectsoralloydisorder.Theconductivityis

（5.3）

Asaunit,usually

isused.WhileCuatroomtemperaturehasamobilityof35

semiconductorscanhavemuchhighervalues.Intwo-dimensionalelectrongases,themobilitycanreachseveral107

atlowtemperature.Inbulksemiconductorswithsmallbandgap,ahighelectronmobilityiscausedbyitssmalleffectivemass.SometypicalvaluesaregiveninTable5.1.

Table5.1.Mobilities（in

）forelectronsandholesatroomtemperatureforvarioussemiconductors

5.2.2MicroscopicScatteringProcesses

Therearetwocollisionorscatteringmechanismsthatdominateinasemiconductorandaffectthecarriermobility:

phononorlatticescattering,andionizedimpurityscattering.

Theatomsinasemiconductorcrystalhaveacertainamountofthermalenergyattemperaturesaboveabsolutezerothatcausestheatomstorandomlyvibrateabouttheirlatticepositionwithinthecrystal.Thelatticevibrationscauseadisruptionintheperfectperiodicpotentialfunction.Aperfectperiodicpotentialinasolidallowselectronstomoveunimpeded,orwithnoscattering,throughthecrystal.Butthethermalvibrationscauseadisruptionofthepotentialfunction,resultinginaninteractionbetweentheelectronsorholesandthevibratinglatticeatoms.Thislatticescatteringisalsoreferredtoasphononscattering.

Latticescatteringisrelatedtothethermalmotionofatoms,therateatwhichthescatteringoccursisafunctionoftemperature.5IfwedenoteμLasthemobilitythatwouldbeobservedifonlylatticescatteringexisted,thenthescatteringtheorystatesthattofirstorder6

（5.4）

Mobilitythatisduetolatticescatteringincreasesasthetemperaturedecreases.Intuitively,weexpectthelatticevibrationstodecreaseasthetemperaturedecreases,whichimpliesthattheprobabilityofascatteringeventalsodecreases,thusincreasingmobility.

Fig.5.4showsthetemperaturedependenceofelectronandholemobilitiesinsilicon.Inlightlydopedsemiconductors,latticescatteringdominatesandthecarriermobilitydecreaseswithtemperatureaswehavediscussed.ThetemperaturedependenceofmobilityisproportionaltoT-n.Theinsertsinthefigureshowthattheparameternisnotequalto3/2asthefirst-orderscatteringtheorypredicted.However,mobilitydoesincreaseasthetemperaturedecreases.

（a）（b）

Fig.5.4（a）Electronand（b）holemobilityinsiliconversustemperatureforvariousdopingconcentration

Thesecondinteractionmechanismaffectingcarriermobilityiscalledionizedimpurityscattering.Wehaveseenthatimpurityatomsareaddedtothesemiconductortocontroloralteritscharacteristics.Theseimpuritiesareionizedatroomtemperaturesothatacoulombinteractionexistsbetweentheelectronsorholesandtheionizedimpurities.Thiscoulombinteractionproducesscatteringorcollisionsandalsoaltersthevelocitycharacteristicsofthechargecarrier.IfwedenoteμIasthemobilitythatwouldbeobservedifonlyionizedimpurityscatteringexisted,thentofirstorderwehave

（5.5）

where

isthetotalionizedimpurityconcentrationinthesemiconductor.Iftemperatur