Optical Instrumentation 10. Light Emitting Diode

This article gives a vivid description of the principle and working procedure of a Light Emitting Diode. It provides a comprehensive understanding of how this very important optical device is useful in our daily applications, its types, structure and other related information.

1. OPTOMETRY –Part X
LIGHT EMITTING DIODE
ER.FARUKBINPOYEN
DEPT.OFAEIE,UIT,BU,BURDWAN,WB,INDIA
FARUK.POYEN@GMAIL .COM

2. Contents:
1.BasicTheory
2.LightGenerationProcess
3.TypesofRecombinationofCarriers
4.DirectRecombination
5.IndirectRecombination
6.LightExtractionProcess
7.LEDStructure
8.LEDDeviceStructure
9.LEDMaterial
10.LEDEfficiency
11.AdvantagesofLED
12.DisadvantagesofLED
13.ApplicationsofLED
2

3. Basic Theory:
ALightemittingdiode(LED)isessentiallyapnjunctiondiode.
Whencarriersareinjectedacrossaforward-biasedjunction,itemitsincoherentlight.
MostofthecommercialLEDsarerealizedusingahighlydopednandapJunction.
3

4. Theory:
Tounderstandtheprinciple,letusconsideranunbiasedpn+junction(Figureinthenext
slideshowsthepn+energybanddiagram).
Thedepletionregionextendsmainlyintothep-side.
Thereisapotentialbarrierfrom�
??????onthen-sidetothe�
??????onthep-side,calledthe
built-involtage,??????
0.
Thispotentialbarrierpreventstheexcessfreeelectronsonthen+sidefromdiffusing
intothepside.
WhenaVoltageVisappliedacrossthejunction,thebuilt-inpotentialisreducedfrom
??????
0to??????
0–V.
Thisallowstheelectronsfromthen+sidetogetinjectedintothep-side.Sinceelectrons
aretheminoritycarriersinthep-side,thisprocessiscalledminoritycarrierinjection.
Buttheholeinjectionfromthepsideton+sideisverylessandsothecurrentis
primarilyduetotheflowofelectronsintothep-side.
4

5. Theory:
Theseelectronsinjectedintothep-siderecombinewiththeholes.
Thisrecombinationresultsinspontaneousemissionofphotons(light).
Thiseffectiscalledinjectionelectroluminescence.
Thesephotonsshouldbeallowedtoescapefromthedevicewithoutbeingreabsorbed.
Recombinationofanelectronholepair(EHP)involveselectronintheconductionband
occupyingaholeinthevalenceband.
Thisresultsintheannihilationoftheelectron-holepair.
5

6. Light Generation Process:
Whenap-njunctionisbiasedintheforwarddirection,theresultingcurrentflow
acrosstheboundarylayerbetweenthepandnregionshastwocomponents:holesare
injectedfromthepregionintothenregionandelectronsareinjectedfromthenregion
intothepregion.
Thisso-calledminority-carrierinjectiondisturbsthecarrierdistributionfromits
equilibriumcondition.
Theinjectedminoritycarriersrecombinewithmajoritycarriersuntilthermal
equilibriumisreestablished.
Aslongasthecurrentcontinuestoflow,minoritycarrierinjectioncontinues.
Onbothsidesofthejunction,anewsteady-statecarrierdistributionisestablishedsuch
thattherecombinationrateequalstheinjectionrate.
6

7. Light Generation Process:
Minority-carrierrecombinationisnotinstantaneous.
Theinjectedminoritycarriershavetofindproperconditionsbeforetherecombination
processcantakeplace.
Bothenergyandmomentumconservationhavetobemet.
Energyconservationcanbereadilymetsinceaphotoncantakeuptheenergyofthe
electron-holepair,butthephotondoesnotcontributemuchtotheconservationof
momentum.
Therefore,anelectroncanonlycombinewithaholeofpracticallyidenticaland
oppositemomentum.
Suchproperconditionsarenotreadilymet,resultinginadelay.Inotherwords,the
injectedminoritycarrierhasafinitelifetimeτrbeforeitcombinesradiativelythrough
theemissionofaphoton.
7

8. Light Generation Process:
Thisaveragetimetorecombineradiativelythroughtheemissionoflightcanbe
visualizedastheaveragetimeittakesaninjectedminoritycarriertofindamajority
carrierwiththerightmomentumtoallowradiativerecombinationwithoutviolating
momentumconservation.
Unfortunately,radiativerecombinationisnottheonlyrecombinationpath.
Therearealsocrystallinedefects,suchasimpurities,dislocations,surfacesetc.thatcan
traptheinjectedminoritycarriers.
Thistypeofrecombinationprocessmayormaynotgeneratelight.
Energyandmomentumconservationaremetthroughthesuccessiveemissionof
phonons.
Again,therecombinationprocessisnotinstantaneousbecausetheminoritycarrierfirst
hastodiffusetoarecombinationsite.
Thisnonradiativerecombinationprocessischaracterizedbyalifetimeτ
n.
8

9. Light Generation Process:
Itisthereforeimportanttodevelopconditionswhereradiativerecombinationoccurs
fairlyrapidlycomparedwithnonradiativerecombination.
Theeffectivenessofthelight–generationprocessisdescribedbythefractionofthe
injectedminoritycarriersthatrecombineradiativelycomparedtothetotalinjection.
Theinternalquantumefficiency,η
icanbecalculatedfromτ
randτ
n.
Thecombinedrecombinationprocessesleadtoatotalminority–carrierlifetimeτgiven
by
1
??????
=
1
??????
??????
+
1
??????
�
η
iissimplycomputedasafractionofcarriersrecombiningradiatively.
�
??????
=
??????
�
??????
??????+??????
�
9

10. Types of Recombination of carriers:
Therecombinationcanbeclassifiedintothefollowingtwokinds
1.•Directrecombination
2.•Indirectrecombination
10

11. Direct Recombination:
Indirectbandgapmaterials,theminimumenergyoftheconductionbandliesdirectly
abovethemaximumenergyofthevalencebandinmomentumspaceenergy(Figure2
showstheE-kplotofadirectbandgapmaterial).
Inthismaterial,freeelectronsatthebottomoftheconductionbandcanrecombine
directlywithfreeholesatthetopofthevalenceband,asthemomentumofthetwo
particlesisthesame.
Thistransitionfromconductionbandtovalencebandinvolvesphotonemission(takes
careoftheprincipleofenergyconservation).
Thisisknownasdirectrecombination.Directrecombinationoccursspontaneously.
GaAsisanexampleofadirectband-gapmaterial.
11

12. Direct Recombination:
Energy-kplot:E-kplotisadifferentwayofdescribingthematerialcharacteristics.
Theeffectsofthecrystallatticeareincludedbydefiningeffectivemassm*.
Fromtheplottheeffectivemasscanbecalculated
�
∗
= Ђ
2
�
2
���
2
whereЂisthePlank’sconstant(h/2π)andd
2
E/dk
2
givesthecurvature.
12

13. Direct Recombination:
Theeffectivemasscanbepositive,negativeorinfinity.
Infinitymeansparticlecannotbeacceleratedbyexternalforces
Negativemeanstheobjectreactstoanattractiveforceasifitwouldexperiencea
repulsiveforce.
13

14. Indirect Recombination:
Intheindirectbandgapmaterials,theminimumenergyintheconductionbandis
shiftedbyak-vectorrelativetothevalenceband.
Thek-vectordifferencerepresentsadifferenceinmomentum.
Duetothisdifferenceinmomentum,theprobabilityofdirectelectronhole
recombinationisless.
Inthesematerials,additionaldopants(impurities)areaddedwhichformveryshallow
donorstates.
Thesedonorstatescapturethefreeelectronslocally;providesthenecessarymomentum
shiftforrecombination.
Thesedonorstatesserveastherecombinationcentres.
ThisiscalledIndirect(non-radiative)Recombination.
14

15. Indirect Recombination:
Figure3showstheE-kplotofanindirectbandgapmaterialandanexampleofhow
NitrogenservesasarecombinationcenterinGaAsP.
Inthiscaseitcreatesadonorstate,whenSiCisdopedwithAl,itrecombinationtakes
placethroughanacceptorlevel.
Theindirectrecombinationshouldsatisfybothconservationenergy,andmomentum.
Thusbesidesaphotonemission,phononemissionorabsorptionhastotakeplace.
15

16. Indirect Recombination:
Phonons:Phononsareaquantummechanicalversionofaspecialtypeofvibrational
motion,knownasnormalmodesinclassicalmechanics,inwhicheachpartofalattice
oscillateswiththesamefrequency.
Thesenormalmodesareimportantbecauseanyarbitraryvibrationalmotionofalattice
canbeconsideredasasuperpositionofnormalmodeswithvariousfrequencies
accordingtoclassicalmechanics.
Inthissense,thenormalmodesaretheelementaryvibrationsofthelattice.
Althoughnormalmodesarewave-likephenomenainclassicalmechanics,theyacquire
certainparticlelikepropertieswhenthelatticeisanalysedusingquantummechanics.
Theyarethenknownasphonons.
Thepropertiesoflong-wavelengthphononsgiverisetosoundinsolids--hencethe
namephononfromtheGreekφωνή(phonē)=voice
GaPisanexampleofanindirectband-gapmaterial.
16

17. Indirect Recombination:
Thewavelengthofthelightemitted,andhencethecolour,dependsonthebandgap
energyofthematerialsformingthep-njunction.
Theemittedphotonenergyisapproximatelyequaltothebandgapenergyofthe
semiconductor.
Thefollowingequationrelatesthewavelengthandtheenergybandgap.
ℎν=�
??????
ℎ�
λ
=�
??????
λ=
ℎ�
�
??????
WherehisPlank’sconstant,cisthespeedofthelightand�
??????istheenergybandgap
Thus,asemiconductorwitha2eVband-gapemitslightatabout620nm,inthered.A3
eVband-gapmaterialwouldemitat414nm,intheviolet.
17

18. Light Extraction Process:
Generatinglightefficientlywithinasemiconductormaterialisonlyonepartofthe
problemtobuildanefficientlightsource.
ThenextchallengeistheextractionoflightfromwithintheLEDchiptotheoutside.
Thedesignermustconsidertotalinternalreflection.
AccordingtoSnell’slaw,lightcanescapefromamediumofhighindexofrefractionn
1
intoamediumoflowindexrefractionn
0onlyifitintersectsthesurfacebetweenthetwo
mediaatananglefromnormallessthanthecriticalangleθ
cwithθ
cbeingdefinedby
�
�=�??????�sin
�
0
�
1
18

19. Light Extraction Process:
MostsemiconductorLEDshaveanisotropicemissionpatternasseenfromwithinthe
light-generatingmaterial.
AssumingacubicshapefortheLEDchip,becauseofinternalreflections,onlyasmall
fractionoftheisotropicallyemittedlightcanescapeanyofthesixsurfaces.
Asacaseinpoint,letuscalculatetheemissionthroughthetopsurface.Fortypical
light-emittingsemiconductors,n
1isintherangeof2.9to3.6.Ifn
1=3.3andn
0=1.0
(air),wefindθ
c=17.68ᵒ.
Theemissionfromanisotropicsourceintoaconewithahalfangleofθ
cisgivenby(1-
cosθ
c)/2.
AftercorrectingforFresnelreflections,only1.6%ofthelightgeneratedescapes
throughtheLEDtopsurfaceintoair.
Dependingonchipandp-njunctiongeometry,virtuallyalloftheremaininglight(98.4
%)isreflectedandabsorbedwithintheLEDchip.
19

20. Light Extraction Process:
Thefractionoflightcoupledfromchiptoairisafunctionofthenumberofsurfaces
throughwhichthechipcantransmitlighteffectively.MostLEDchipsarecalled
‘‘absorbingsubstrate’’(AS)chips.
Insuchachip,thestartingsubstratematerialhasanarrowbandgapandabsorbsallthe
lightwithenergygreaterthanthebandgapofthesubstrate.
ConsiderthecaseofaGaAsPLEDgrownonaGaAssubstrate.Theemittedlight(Eg>
1.9eV)isabsorbedbytheGaAssubstrate(Eg=1.4eV).
Thus,aGaAsP-emittinglayeronaGaAssubstratecantransmitonlythroughitstop
surface.
Lighttransmittedtowardthesidesurfacesordownwardisabsorbed.
20

21. Light Extraction Process:
Toincreaselightextraction,thesubstrateorpartoftheepitaxiallayersnearthetopof
thechiphastobemadeofamaterialtransparenttotheemittedlight.
The‘‘transparentsubstrate’’(TS)chipisdesignedsuchthatlighttransmittedtowards
thesidesurfaceswithinθ
chalf-angleconescanescape.
Assumingthatthereisnegligibleabsorptionbetweenthepointoflightgenerationand
thesidewalls,thisincreasestheextractionefficiencybyafactoroffive(fiveinsteadof
oneescapecones).
AcommonapproachistouseahybridchipwithpropertiesbetweenASandTSchips.
Thesechipsutilizeathick,transparentwindowlayerabovethelight-emittinglayer.
Ifthislayerissufficientlythick,thenmostofthelightinthetophalfofthecones
transmittedtowardsthesidesurfaceswillreachthesideofthechipbeforehittingthe
substrate.
Inthiscaseofhybridchips,theefficiency(4.5%,no.ofcones3)isbetweenthatofAS
(1.5%,no.ofcone1)andTS(7.5%,no.ofcones5)chips.
21

22. LED Structure:
TheLEDstructureplaysacrucialroleinemittinglightfromtheLEDsurface.
TheLEDsarestructuredtoensuremostoftherecombinationstakeplaceonthesurface
bythefollowingtwoways.
1.Byincreasingthedopingconcentrationofthesubstrate,sothatadditionalfree
minoritychargecarrierelectronsmovetothetop,recombineandemitlightatthe
surface.
2.Byincreasingthediffusionlength??????=�??????,whereDisthediffusioncoefficient
andτisthecarrierlifetime.Butwhenincreasedbeyondacriticallengththereisa
chanceofre-absorptionofthephotonsintothedevice.
22

23. LED Structure:
TheLEDhastobestructuredsothatthephotonsgeneratedfromthedeviceareemitted
withoutbeingreabsorbed.
Onesolutionistomaketheplayeronthetopthin,enoughtocreateadepletionlayer.
Therearedifferentwaystostructurethedomeforefficientemitting.
Followingpictureshowsthelayeredstructure.
23

24. LED Structure:
LEDsareusuallybuiltonann-typesubstrate,withanelectrodeattachedtothep-type
layerdepositedonitssurface.
P-typesubstrates,whilelesscommon,occuraswell.
ManycommercialLEDs,especiallyGaN/InGaN,alsousesapphiresubstrate.
LEDdomeshapes:TheLEDdomesareconstructedsuchmostofthelightgetsemitted
efficiently.
Followingpictureshowsthetwodifferentkindsofdomes.
24

25. LED Device Structure:
LEDdevicescomeinabroadrangeofstructures.
Eachmaterialsystemrequiresadifferentoptimization.
TheonlycommonfeatureforallLEDstructuresistheplacementofthep-njunction
wherethelightisgenerated.
Thep-njunctionispracticallyneverplacedinthebulk-grownsubstratematerialfor
thefollowingreasons:
Thebulk-grownmaterialssuchasGaAs,GaP,andInPusuallydonothavetheright
energygapforthedesiredwavelengthoftheemittedlight.
Thelight-generatingregionrequiresmoderatelylowdopingthatisinconsistentwith
theneedforalowseriesresistance.
Bulk-grownmaterialoftenhasarelativelyhighdefectdensity,makingitdifficultto
achievehighefficiency.
25

26. LED Device Structure:
Forthesereasons,practicallyallcommerciallyimportantLEDstructuresutilizea
secondarygrowthstepontopofasingle-crystalbulk-grownsubstratematerial.
Thesecondarygrowthstepconsistsofasingle-crystallayerlatticematchedtothe
substrate.
Thisgrowthprocessisknownasepitaxialgrowth.
The commonly used epitaxial structures can be classified into the following categories:
Homojunctions
1.grown
2.diffused
Heterojunctions
1.singleconfinement
2.doubleconfinement
26

27. LED Material:
AnimportantclassofcommercialLEDsthatcoverthevisiblespectrumaretheIII-V
ternaryalloysbasedonalloyingGaAsandGaPwhicharedenotedbyGaAs
1-yP
y.
InGaAlPisanexampleofaquarternary(fourelement)III-Valloywithadirectband
gap.
TheLEDsrealizedusingtwodifferentlydopedsemiconductorsthatarethesame
materialiscalledahomojunction.
Whentheyarerealizedusingdifferentbandgapmaterialstheyarecalledahetero
structuredevice.
AheterostructureLEDisbrighterthanahomojunctionLED.
27

28. LED Material:
Semiconductorsintheperiodictable:
AnexampleofIII-VcomponentsisGaPorGaAs.
Thefollowingtableshowsthesemiconductorsintheperiodictable.
28
II III IV V VI
B C
Al Si P S
Zn Ga Ge As Se
Cd In Sn Sb Te

29. LED Material:
Followingisalistofsemiconductormaterialsandthecorrespondingcolours:
AluminiumGalliumArsenide(AlGaAs)—redandinfrared
AluminiumGalliumPhosphide(AlGaP)—green
AluminiumGalliumIndiumPhosphide(AlGaInP)—high-brightnessorange-red,orange,yellow,andgreen
GalliumArsenidePhosphide(GaAsP)—red,orange-red,orange,andyellow
GalliumPhosphide(GaP)—red,yellowandgreen
GalliumNitride(GaN)—green,puregreen(oremeraldgreen),andbluealsowhite(ifithasanAlGaNQuantum
Barrier)
IndiumGalliumNitride(InGaN)—450nm-470nm—nearultraviolet,bluishgreenandblue
SiliconCarbide(SiC)assubstrate—blue
Silicon(Si)assubstrate—blue(underdevelopment)
Sapphire(Al2O3)assubstrate—blue
ZincSelenide(ZnSe)—blue
Diamond(C)—ultraviolet
AluminiumNitride(AlN),AluminiumGalliumNitride(AlGaN),AluminiumGalliumIndiumNitride(AlGaInN)
—neartofarultraviolet(downto210nm)
29

30. LED Efficiency:
AveryimportantmetricofanLEDistheexternalquantumefficiencyη
ext.
Itquantifiestheefficiencyoftheconversionofelectricalenergyintoemittedoptical
energy.
Itisdefinedasthelightoutputdividedbytheelectricalinputpower.
ItisalsodefinedastheproductofInternalradiativeefficiencyandExtractionefficiency.
η
�??????�
= ??????
������??????���??????∗??????
Forindirectbandgapsemiconductorsη
extisgenerallylessthan1%,whereasforadirect
bandgapmaterialitcouldbesubstantial.
η
??????��
= ??????�����??????��??????��??????��??????�����??????���??????��??????����??????�����??????���??????��
Theinternalefficiencyisafunctionofthequalityofthematerialandthestructureand
compositionofthelayer.
30

31. Advantages of using LED:
LEDsproducemorelightperwattthanincandescentbulbs;thisisusefulinbatterypowered
orenergy-savingdevices.
LEDscanemitlightofanintendedcolourwithouttheuseofcolourfilters.
ThesolidpackageoftheLEDcanbedesignedtofocusitslight.
Whenusedinapplicationswheredimmingisrequired,LEDsdonotchangetheircolourtint
asthecurrentpassingthroughthemislowered.
LEDsareidealforuseinapplicationsthataresubjecttofrequenton-offcycling,
LEDs,beingsolidstatecomponents,aredifficulttodamagewithexternalshock.
LEDscanhavearelativelylongusefullife.
LEDsmostlyfailbydimmingovertime,ratherthantheabruptburn-outofincandescent
bulbs.
LEDslightupveryquickly.AtypicalredindicatorLEDwillachievefullbrightnessin
microseconds.
LEDscanbeverysmallandareeasilypopulatedontoprintedcircuitboards.
LEDsdonotcontainmercury,unlikecompactfluorescentlamps.
31

32. Disadvantages of LED:
LEDsarecurrentlymoreexpensive,priceperlumen.
LEDperformancelargelydependsontheambienttemperatureoftheoperating
environment.Over-drivingtheLEDinhighambienttemperaturesmayresultin
overheatingoftheLEDpackage,eventuallyleadingtodevicefailure.
LEDsmustbesuppliedwiththecorrectcurrent.
LEDsdonotapproximatea"pointsource"oflight,sotheycannotbeusedin
applicationsneedingahighlycollimatedbeam.
ThereisincreasingconcernthatblueLEDsandwhiteLEDsarenowcapableof
exceedingsafelimitsoftheso-calledblue-lighthazardasdefinedintheeyesafety
specifications.
32

33. Applications of LED:
Devices,medicalapplications,clothing,toys
RemoteControls(TVs,VCRs)
Lighting
Indicatorsandsigns
Opto-isolatorsandopto-couplers
Sensors:Transmissive,Reflective,Scattering
FibreOpticSource
AlphaNumeric/NumericDisplay
33