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时间:2018-08-01
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1、CHAPTER4CarrierTransportandExcessCarrIerPhenomenaThedriftvelocityversuselectricfieldcharacteristicofgalliumarsenideismorecomplicatedthanforsiliconorgermanium.Atlowfields,theslopeofthedriftvelocityversus-fieldisconstantandisthclow-fieldelectronmobility,whichbapproximately8500forgalliumarseni
2、de.Thelow-fieldelectronmobilityingalliumarsenideismuchlargerthaninsilicon.Asthefieldincreases,theelectrondriftvelocityingalliumarsenidereachesapeakandthendecreases.Adifferentialmobilityistheslopeoftheversus-fieldataparticularpointonthecurveandthenegativeslopeofthedriftvelocityversuselectric
3、fieldrepresentsanegativediffer-entialmobility.Thenegativedifferentialmobilityproducesanegativedifferentialresistance;thischaracteristicisusedinthedesignofoscillators.NegativedifferentialThenegativedifferentialmobilitycanbeunderstoodbyconsideringtheEversusMobilitydiagramforgalliumarsenide,wh
4、ichisshownagaininFigure4.8.ThedensityofStateseffectivemassoftheelectroninthelowervalleyisThesmalleffectivemassleadstoalargemobility.Asthe-fieldincreases,theenergyoftheelectronincreasesandtheelectroncanbescatteredintotheuppervalley,wheretbedensityofstateseffectivemassis.Thelargereffectivemas
5、sintheuppervalleyyieldsasmallermobility.Thisintervalleytransfermechanismresultslnade-creasingaveragedriftvelocityofelectronswithelectricfield,orthenegativediffer-entialmobilitycharacteristic.Figure4.8
6、Energy-bandstructureforgalliumarsenideshowingtheuppervalleyandlowervalleyintheconductionba
7、nd.(FromSze[15])TYU4.1Adriftcurrentdensityofisrequiredinasendconductordeviceusingn-typesiliconwithanappliedelectricfieldof=25V/cm.Determinetheimpuritydopingconcentrationthatwillachievethisspecification.(Ans.if,then)TYU4.2SiliconatT=300Kisdopedwithimpuritydopingconcentrationsofand.(a)Whatare
8、theelectronandholemobilities?(b)Determinetheresistivityandconductivityofthematerial.[Ans.(a),;(b),]TYU4.3RepeatTYU4.1forGaAs.(Ans.If,then)TYU4.4RepeatTYU4.2fOrGaAs.[Ans.(a),;(b),]4.2ICARRIERDIFFUSIONObjective:Describethemechanismofcarrierdiffusionanddiff
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