Lecture 61.ppt

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1、SemiconductorDevicesforIlluminationEngineeringLEDaresemiconductorp-njunctionsthatunderforwardbiasconditionscanemitradiationbyelectroluminescenceintheUV,visibleorinfraredregionsoftheelectromagneticspectrum.Thequantaoflightenergyreleasedisapproximatelypropor

2、tionaltothebandgapofthesemiconductor.WhatisLED?ApplicationsofLEDsYourfancytelephone,i-pod,palmpilotanddigitalcameraLongevity:Thelightemittingelementinadiodeisasmallconductorchipratherthanafilamentwhichgreatlyextendsthediode’slifeincomparisontoanincandescen

3、tbulb(10000hourslifetimecomparedto~1000hoursforincandescencelightbulb)Efficiency:Diodesemitalmostnoheatandrunatverylowamperes.GreaterLightIntensity:SinceeachdiodeemitsitsownlightCost:NottoobadRobustness:Solidstatecomponent,notasfragileasincandescencelightb

4、ulbAdvantagesofLightEmittingDiodes(LEDs)LEDchipisthepartthatweshalldealwithinthiscourseLEDdesignLuminescenceistheprocessbehindlightemissionLuminescenceisatermusedtodescribetheemissionofradiationfromasolidwhenthesolidissuppliedwithsomeformofenergy.Electrolu

5、minescenceexcitationresultsfromtheapplicationofanelectricfieldInap-njunctiondiodeinjectionelectroluminescenceoccursresultinginlightemissionwhenthejunctionisforwardbiasedExcitationElectron(excitedbythebiasedforwardvoltage)isintheconductionbandHoleisinvalan

6、cebandNormallytherecombinationtakesplacebetweentransitionofelectronsbetweenthebottomoftheconductionbandandthetopofthevalanceband(bandexterma).Theemissionoflightistherefore;hc/=Ec-Ev=Eg(onlydirectbandgapallowsradiativetransition)EkHowdoesitwork?P-njunction

7、ElectricalContactsAtypicalLEDneedsap-njunctionJunctionisbiasedtoproduceevenmoree-handtoinjectelectronsfromntopforrecombinationtohappenTherearealotofelectronsandholesatthejunctionduetoexcitationsElectronsfromnneedtobeinjectedtoptopromoterecombinationRecombi

8、nationproduceslight!!RecombinationandEfficiencyeVoEgpn+h=EgEgpn+(a)(b)ElectronsinCBHolesinVBECEVEFIdealLEDwillhaveallinjectionelectronstotakepartintherecombinationprocessInrealdevicenotallelectronwillrecombi