方茹辉144-145-b20

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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