方茹辉144-145-b20

《方茹辉144-145-b20》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、CHAPTER4CarrierTransportandExcessCarrIerPhenomenaThedriftvelocityversuselectricfieldcharacteristicofgalliumarsenideismorecomplicatedthanforsiliconorgermanium.Atlowfields,theslopeofthedriftvelocityversus-fieldisconstantandisthclow-fieldelectronmobility,whichbapproximately8500forgalli

2、umarsenide.Thelow-fieldelectronmobilityingalliumarsenideismuchlargerthaninsilicon.Asthefieldincreases,theelectrondriftvelocityingalliumarsenidereachesapeakandthendecreases.Adifferentialmobilityistheslopeoftheversus-fieldataparticularpointonthecurveandthenegativeslopeofthedriftveloci

3、tyversuselectricfieldrepresentsanegativediffer-entialmobility.Thenegativedifferentialmobilityproducesanegativedifferentialresistance;thischaracteristicisusedinthedesignofoscillators.NegativedifferentialThenegativedifferentialmobilitycanbeunderstoodbyconsideringtheEversusMobilitydiag

4、ramforgalliumarsenide,whichisshownagaininFigure4.8.ThedensityofStateseffectivemassoftheelectroninthelowervalleyisThesmalleffectivemassleadstoalargemobility.Asthe-fieldincreases,theenergyoftheelectronincreasesandtheelectroncanbescatteredintotheuppervalley,wheretbedensityofstateseffec

5、tivemassis.Thelargereffectivemassintheuppervalleyyieldsasmallermobility.Thisintervalleytransfermechanismresultslnade-creasingaveragedriftvelocityofelectronswithelectricfield,orthenegativediffer-entialmobilitycharacteristic.Figure4.8

6、Energy-bandstructureforgalliumarsenideshowingtheup

7、pervalleyandlowervalleyintheconductionband.(FromSze[15])TYU4.1Adriftcurrentdensityofisrequiredinasendconductordeviceusingn-typesiliconwithanappliedelectricfieldof=25V/cm.Determinetheimpuritydopingconcentrationthatwillachievethisspecification.(Ans.if,then)TYU4.2SiliconatT=300Kisdoped

8、withimpuritydopingconcentrationsofand.(a)Whataretheelectronandholemobilities?(b)Determinetheresistivityandconductivityofthematerial.[Ans.(a),;(b),]TYU4.3RepeatTYU4.1forGaAs.(Ans.If,then)TYU4.4RepeatTYU4.2fOrGaAs.[Ans.(a),;(b),]4.2ICARRIERDIFFUSIONObjective:Describethemechanismofcarr

9、ierdiffusionanddiff