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HowLightEmittingDiodesWorkbyTomHarrisLightemittingdiodes,commonlycalledLEDs,arerealunsungheroesintheelectronicsworld.Theydodozensofdifferentjobsandarefoundinallkindsofdevices.Amongotherthings,theyformthenumbersondigitalclocks,transmitinformationfromremotecontrols,lightupwatchesandtellyouwhenyourappliancesareturnedon.Collectedtogether,theycanformimagesonajumbotelevisionscreenorilluminateatrafficlight.Basically,LEDsarejusttinylightbulbsthatfiteasilyintoanelectricalcircuit.Butunlikeordinaryincandescentbulbs,theydon'thaveafilamentthatwillburnout,andtheydon'tgetespeciallyhot.Theyareilluminatedsolelybythemovementofelectronsinasemiconductormaterial,andtheylastjustaslongasastandardtransistor.Inthisarticle,we'llexaminethesimpleprinciplesbehindtheseubiquitousblinkers,illuminatingsomecoolprinciplesofelectricityandlightintheprocess.WhatisaDiode?Adiodeisthesimplestsortofsemiconductordevice.Broadlyspeaking,asemiconductorisamaterialwithavaryingabilitytoconductelectricalcurrent.Mostsemiconductorsaremadeofapoorconductorthathashadimpurities(atomsofanothermaterial)addedtoit.Theprocessofaddingimpuritiesiscalleddoping.InthecaseofLEDs,theconductormaterialistypicallyaluminum-gallium-arsenide(AlGaAs).Inpurealuminum-gallium-arsenide,alloftheatomsbondperfectlytotheirneighbors,leavingnofreeelectrons(negatively-chargedparticles)toconductelectriccurrent.Indopedmaterial,additionalatomschangethebalance,eitheraddingfreeelectronsorcreatingholeswhereelectronscango.Eitheroftheseadditionsmakethematerialmoreconductive.AsemiconductorwithextraelectronsiscalledN-typematerial,sinceithasextranegatively-chargedparticles.InN-typematerial,freeelectronsmovefromanegatively-chargedareatoapositivelychargedarea.AsemiconductorwithextraholesiscalledP-typematerial,sinceiteffectivelyhasextrapositively-chargedparticles.Electronscanjumpfromholetohole,movingfromanegatively-chargedareatoapositively-chargedarea.Asaresult,theholesthemselvesappeartomovefromapositively-chargedareatoanegatively-chargedarea.AdiodecomprisesasectionofN-typematerialbondedtoasectionofP-typematerial,withelectrodesoneachend.Thisarrangementconductselectricityinonlyonedirection.Whennovoltageisappliedtothediode,electronsfromtheN-typematerialfillholesfromtheP-typematerialalongthejunctionbetweenthelayers,formingadepletionzone.Inadepletionzone,thesemiconductormaterialisreturnedtoitsoriginalinsulatingstate--alloftheholesarefilled,sotherearenofreeelectronsoremptyspacesforelectrons,andchargecan'tflow.Atthejunction,freeelectronsfromtheN-typematerialfillholesfromtheP-typematerial.Thiscreatesaninsulatinglayerinthemiddleofthediodecalledthedepletionzone.Togetridofthedepletionzone,youhavetogetelectronsmovingfromtheN-typeareatotheP-typeareaandholesmovinginthereversedirection.Todothis,youconnecttheN-typesideofthediodetothenegativeendofacircuitandtheP-typesidetothepositiveend.ThefreeelectronsintheN-typematerialarerepelledbythenegativeelectrodeanddrawntothepositiveelectrode.TheholesintheP-typematerialmovetheotherway.Whenthevoltagedifferencebetweentheelectrodesishighenough,theelectronsinthedepletionzoneareboostedoutoftheirholesandbeginmovingfreelyagain.Thedepletionzonedisappears,andchargemovesacrossthediode.WhenthenegativeendofthecircuitishookeduptotheN-typelayerandthepositiveendishookeduptoP-typelayer,electronsandholesstartmovingandthedepletionzonedisappears.Ifyoutrytoruncurrenttheotherway,withtheP-typesideconnectedtothenegativeendofthecircuitandtheN-typesideconnectedtothepositiveend,currentwillnotflow.ThenegativeelectronsintheN-typematerialareattractedtothepositiveelectrode.ThepositiveholesintheP-typematerialareattractedtothenegativeelectrode.Nocurrentflowsacrossthejunctionbecausetheholesandtheelectronsareeachmovinginthewrongdirection.Thedepletionzoneincreases.(SeeHowSemiconductorsWorkformoreinformationontheentireprocess.)WhenthepositiveendofthecircuitishookeduptotheN-typelayerandthenegativeendishookeduptotheP-typelayer,freeelectronscollectononeendofthediodeandholescollectontheother.Thedepletionzonegetsbigger.Theinteractionbetweenelectronsandholesinthissetuphasaninterestingsideeffect--itgenerateslight!Inthenextsection,we'llfindoutexactlywhythisis.HowCanaDiodeProduceLight?Lightisaformofenergythatcanbereleasedbyanatom.Itismadeupofmanysmallparticle-likepacketsthathaveenergyandmomentumbutnomass.Theseparticles,calledphotons,arethemostbasicunitsoflight.Photonsarereleasedasaresultofmovingelectrons.Inanatom,electronsmoveinorbitalsaroundthenucleus.Electronsindifferentorbitalshavedifferentamountsofenergy.Generallyspeaking,electronswithgreaterenergymoveinorbitalsfartherawayfromthenucleus.Foranelectrontojumpfromalowerorbitaltoahigherorbital,somethinghastoboostitsenergylevel.Conversely,anelectronreleasesenergywhenitdropsfromahigherorbitaltoalowerone.Thisenergyisreleasedintheformofaphoton.Agreaterenergydropreleasesahigher-energyphoton,whichischaracterizedbyahigherfrequency.(CheckoutHowLightWorksforafullexplanation.)Aswesawinthelastsection,freeelectronsmovingacrossadiodecanfallintoemptyholesfromtheP-typelayer.Thisinvolvesadr