LED

9/12/2013 Mahesh J. vadhavaniya 1


Objectives…Objectives…Objectives…Objectives…Objectives…Objectives…Objectives…Objectives…

�� IntroductionIntroduction

�� ExplainExplain withwith helphelp ofof energyenergy bandband structure,structure, thethe processprocessofof opticaloptical emissionemission fromfrom semiconductorsemiconductor..


�� LEDLED StructuresStructures

�� LEDLED CharacteristicsCharacteristics

�� AdvantagesAdvantages && disadvantagesdisadvantages ofof ILDILD overover LEDLED forforlonglong haulhaul opticaloptical fiberfiber cc00mmunicationmmunication

Point of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of Discussion

ExplainExplain withwith thethe helphelp ofof energyenergy bandbandstructure,structure, thethe processprocess ofof opticaloptical emissionemissionstructure,structure, thethe processprocess ofof opticaloptical emissionemissionfromfrom semiconductorssemiconductors..


Process of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical EmissionProcess of Optical Emission

�� ToTo allowallow considerationconsideration ofof semiconductorsemiconductor opticalopticalsourcessources itit isis necessarynecessary toto reviewreview somesome ofof thethe propertiesproperties ofofsemiconductorsemiconductor materials,materials, especiallyespecially withwith regardregard toto pp--nnjunctionjunction..

semiconductorsemiconductor

IntrinsicIntrinsic ExtrinsicExtrinsic



�� AA perfectperfect semiconductorsemiconductor crystalcrystal containingcontaining nonoimpuritiesimpurities andand latticelattice effecteffect..

Intrinsic Semiconductor :Intrinsic Semiconductor :

Extrinsic Semiconductor :Extrinsic Semiconductor :

�� AA semiconductorsemiconductor crystalcrystal whichwhich isis mademade upup byby thetheprocessprocess ofof doping,doping, ii..ee.. addingadding impurityimpurity toto itit..



The energy band structureThe energy band structure

The FermiThe Fermi--Dirac probability distributionDirac probability distribution



�� ForFor aa semiconductorsemiconductor inin thermalthermal equilibriumequilibrium thethe energyenergylevellevel occupationoccupation isis describeddescribed byby fermifermi diracdirac distributiondistributionfunctionfunction..

�� ConsequentlyConsequently thethe probabilityprobability P(E)P(E) thatthat anan electronelectrongainsgains sufficientsufficient thermalthermal energyenergy atat anan absoluteabsolutegainsgains sufficientsufficient thermalthermal energyenergy atat anan absoluteabsolutetemperaturetemperature TT suchsuch thatthat itit willwill bebe foundfound occupyingoccupying aaparticularparticular energyenergy levellevel E,E, isis givengiven byby thethe FermiFermi--DiracDiracdistributiondistribution..

KTEEEP

F )exp(1

1)(

−+=



�� TheThe FermiFermi levellevel isis defineddefined asas thethe highesthighest occupiedoccupied molecularmolecularorbitalorbital inin thethe valencevalence bandband atat 00 K,K, soso thatthat therethere areare manymany statesstatesavailableavailable toto acceptaccept electrons,electrons, ifif thethe casecase werewere aa metalmetal..

�� ThisThis isis notnot thethe casecase inin insulatorsinsulators andand semiconductorssemiconductors sincesince thethevalencevalence andand conductionconduction bandsbands areare separatedseparated..

�� ThereforeTherefore thethe FermiFermi levellevel isis locatedlocated inin thethe bandband gapgap..

�� TheThe probabilityprobability ofof thethe occupationoccupation ofof anan energyenergy levellevel isis basedbased onon thetheFermiFermi functionfunction..



EnergyEnergy

Energy Band DiagramsEnergy Band Diagrams

nn–– type semiconductortype semiconductor pp–– type semiconductortype semiconductor



�� TheThe pp--nn junctionjunction diodediode isis formedformed byby creatingcreating adjoiningadjoiningpp andand nn typetype semiconductorsemiconductor layerslayers inin singlesingle crystalcrystal..

�� AA thinthin depletiondepletion regionregion isis formedformed atat thethe junctionjunctionthroughthrough carriercarrier recombinationrecombination..

�� ThisThis establishesestablishes aa potentialpotential barrierbarrier betweenbetween thethe pp andand nn�� ThisThis establishesestablishes aa potentialpotential barrierbarrier betweenbetween thethe pp andand nntypetype regionsregions whichwhich restrictsrestricts thethe interinter diffusiondiffusion ofofmajoritymajority carrierscarriers fromfrom theirtheir respectiverespective regionsregions..

�� AnAn externalexternal appliedapplied voltagevoltage formform currentcurrent flowflow throughthroughthethe devicedevice asas theythey continuouslycontinuously diffusediffuse awayaway fromfrom thetheinterfaceinterface..

�� However,However, thisthis situationsituation inin suitablesuitable semiconductorsemiconductorallowsallows carriercarrier recombinationrecombination withwith thethe emissionemission ofof lightlight..





�� AA PNPN junctionjunction (that(that consistsconsists ofof directdirect bandband gapgapsemiconductorsemiconductor materials)materials) actsacts asas thethe activeactive ororrecombinationrecombination regionregion..

�� WhenWhen thethe PNPN junctionjunction isis forwardforward biased,biased, electronselectrons andand

Semiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light SourcesSemiconductor Light Sources

holesholes recombinerecombine eithereither radiativelyradiatively (emitting(emitting photonsphotons)) orornonnon--radiativelyradiatively (emitting(emitting heat)heat).. ThisThis isis simplesimple LEDLEDoperationoperation..

�� InIn aa LASER,LASER, thethe photonphoton isis furtherfurther processedprocessed inin aaresonanceresonance cavitycavity toto achieveachieve aa coherent,coherent, highlyhighly directionaldirectionalopticaloptical beambeam withwith narrownarrow lineline widthwidth..


The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )The Light Emitting Diode ( LED )

�� LEDsLEDs isis usedused toto convertconvert electricalelectrical energyenergy intointo lightlight

�� AA lightlight emittingemitting diodediode (LED)(LED) isis essentiallyessentially aa PNPNjunctionjunction optoopto--semiconductorsemiconductor thatthat emitsemits aa monochromaticmonochromatic(single(single color)color) lightlight whenwhen operatedoperated inin aa forwardforward biasedbiaseddirectiondirection..

�� LEDsLEDs isis usedused toto convertconvert electricalelectrical energyenergy intointo lightlightenergyenergy byby recombinationrecombination ofof holesholes andand electronselectrons atat thethe PP NNJunctionJunction..


Light EmissionLight EmissionLight EmissionLight EmissionLight EmissionLight EmissionLight EmissionLight Emission

BasicBasic LEDLED operationoperation ::

��WhenWhen anan electronelectron jumpsjumps fromfrom aa higherhigher energyenergy statestate((EcEc)) toto aa lowerlower energyenergy statestate ((EvEv)) thethe differencedifference inin energyenergyEcEc -- EvEv isis releasedreleased eithereither ……EcEc -- EvEv isis releasedreleased eithereither ……

��asas aa photonphoton ofof energyenergy EE == hhνν ((radiativeradiativerecombination)recombination)

��asas heatheat (non(non--radiativeradiative recombination)recombination)



�� ForFor fiberfiber--optics,optics, thethe LEDLED shouldshould havehave aa highhigh radianceradiance(light(light intensity),intensity), fastfast responseresponse timetime andand aa highhigh quantumquantumefficiencyefficiency

�� EmittedEmitted wavelengthwavelength dependsdepends onon bandband gapgap energyenergy

λν /hchEg ==

�� EmittedEmitted wavelengthwavelength dependsdepends onon bandband gapgap energyenergy

(eV)

2399.1m)(

gE=µλ

λλ = wavelength in microns= wavelength in micronsh = Planks constanth = Planks constantc = speed of lightc = speed of lightE = Photon energy in E = Photon energy in eVeV


�� ForFor photonicphotonic communicationscommunications requiringrequiring datadata raterate isis100100--200200 Mb/sMb/s withwith multimodemultimode fiberfiber withwith tenstens ofofmicrowatts,microwatts, LEDsLEDs areare usuallyusually thethe bestbest choicechoice

�� LEDLED configurationsconfigurations beingbeing usedused inin photonicphotoniccommunicationscommunications::


communicationscommunications::

11 -- SurfaceSurface EmittersEmitters (Front(Front Emitters)Emitters)22 -- EdgeEdge EmittersEmitters


Surface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LEDSurface Emitting LED

Schematic of high-radiance surface-emitting LED. The active region is limitted to a circular cross section that

has an area compatible with the fiber-core end face.


Edge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LEDEdge Emitting LED

Schematic of an edge-emitting double heterojunction LED. The output beam is lambertian in the plane of

junction and highly directional perpendicular to pn junction. They have high quantum efficiency & fast

response.


Light Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source MaterialLight Source Material

�� MostMost ofof thethe lightlight sourcessources containcontain IIIIII--VV ternaryternary &&quaternaryquaternary compoundscompounds

�� byby varyingvarying xx itit isis possiblepossible toto controlcontrol thethebandband--gapgap energyenergy andand therebythereby thethe emissionemission wavelengthwavelengthoverover thethe rangerange ofof 800800 nmnm toto 900900 nmnm.. TheThe spectralspectral widthwidth

AsAlGa xx1−

isis aroundaround 2020 toto 4040 nmnm

�� byby changingchanging 00<x<<x<00..4747;; yy isis approximatelyapproximately22..22x,x, thethe emissionemission wavelengthwavelength cancan bebe controlledcontrolled overover thetherangerange ofof 920920 nmnm toto 16001600 nmnm.. TheThe spectralspectral widthwidth variesvariesfromfrom 7070 nmnm toto 180180 nmnm whenwhen thethe wavelengthwavelength changeschangesfromfrom 13001300 nmnm toto 16001600 nmnm.. TheseThese materialsmaterials areare latticelatticematchedmatched..

y1yxx1 PAsGaIn −−



Spectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED typesSpectral width of LED types


PLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDPLANAR LEDLight

Output

OhmicContacts

p – type epitaxial layer

n – type substrate Light Output

�� SimplestSimplest ofof thethe structuresstructures availableavailable..

�� PP typetype diffusiondiffusion intointo nn typetype substratesubstrate..

�� LambertianLambertian spontaneousspontaneous emissionemission..

�� LightLight emitsemits fromfrom allall surfacessurfaces..

�� TotalTotal internalinternal reflectionreflection.. �� RadianceRadiance isis lowlow..9/12/2013 Mahesh J. vadhavaniya 24

DOME LEDDOME LEDDOME LEDDOME LEDDOME LEDDOME LEDDOME LEDDOME LED

Semiconductor material is shaped like a hemisphere

�� HemisphereHemisphere ofof nn typetypeGaAsGaAs isis formedformed aroundaround pptypetype regionregion..

�� HigherHigher amountamount ofof internalinternal

n+

Electrodes

p

�� HigherHigher amountamount ofof internalinternalemissionemission reachingreaching thethesurfacesurface withinwithin thethe criticalcriticalangleangle ofof GaAsGaAs airair interfaceinterface..

�� HigherHigher externalexternal efficiencyefficiencythanthan thethe PlanarPlanar LEDLED..


Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?Why do we need the DOME ?

Semiconductor material is shaped like a hemisphere

Plastic Dome

n+

Electrodes

ppn Junction

Electrodes

To reduce TIR …To reduce TIR …9/12/2013 Mahesh J. vadhavaniya 26

How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?How to solve TIR problem ?

�� GaAsGaAs--airair interface,interface, thethe θθ == 1616oo whichwhich meansmeans thatthat�� GaAsGaAs--airair interface,interface, thethe θθcc == 1616oo whichwhich meansmeans thatthatmuchmuch ofof thethe lightlight sufferssuffers TIRTIR..

�� ToTo solvesolve thethe problemproblem wewe couldcould::�� ToTo solvesolve thethe problemproblem wewe couldcould::

1.1. ShapeShape thethe surfacesurface ofof thethe semiconductorsemiconductor intointo aa1.1. ShapeShape thethe surfacesurface ofof thethe semiconductorsemiconductor intointo aadomedome oror hemispherehemisphere soso thatthat lightlight raysrays strikestrike thethedomedome oror hemispherehemisphere soso thatthat lightlight raysrays strikestrike thethesurfacesurface anglesangles << θθcc thereforetherefore doesdoes notnot experienceexperienceTIRTIR.. ButBut expensiveexpensive andand notnot practicalpractical toto shapeshape pp--nnjunctionjunction withwith domedome--likelike structurestructure..

22.. EncapsulationEncapsulation ofof thethe semiconductorsemiconductor junctionjunction22.. EncapsulationEncapsulation ofof thethe semiconductorsemiconductor junctionjunctionwithinwithin aa domedome--shapedshaped transparenttransparent plasticplastic mediummedium(an(an epoxy)epoxy) thatthat hashas higherhigher refractiverefractive indexindex thanthan airair..


Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )


�� TheThe surfacesurface layerlayer isis keptkept asas thinthin asas possiblepossible ((1010--1515 µµm)m)�� TheThe surfacesurface layerlayer isis keptkept asas thinthin asas possiblepossible ((1010--1515 µµm)m)toto minimizeminimize reabsorptionreabsorption..

Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )Surface Emitter LED ( SLED )

�� AA wellwell isis etchedetched inin aa GaAsGaAs substratesubstrate toto preventprevent heavyheavy�� AA wellwell isis etchedetched inin aa GaAsGaAs substratesubstrate toto preventprevent heavyheavyabsorptionabsorption ofof thethe emittedemitted radiationradiation && toto physicallyphysicallyaccommodateaccommodate thethe fibrefibre..

�� PhotonsPhotons areare generatedgenerated inin thethe thinthin pp--GaAsGaAs && regionregion�� PhotonsPhotons areare generatedgenerated inin thethe thinthin pp--GaAsGaAs && regionregionemissionemission fromfrom thethe toptop surfacesurface isis ensuredensured heterostructureheterostructure &&�� PhotonsPhotons areare generatedgenerated inin thethe thinthin pp--GaAsGaAs && regionregionemissionemission fromfrom thethe toptop surfacesurface isis ensuredensured heterostructureheterostructure &&reflectionreflection fromfrom thethe backback crystalcrystal faceface.. ThusThus forwardforward radianceradianceofof thesethese devicedevice isis veryvery highhigh..

�� TheThe toptop nn--GaAsGaAs contactcontact layerlayer ensuresensures lowlow contactcontact�� TheThe toptop nn--GaAsGaAs contactcontact layerlayer ensuresensures lowlow contactcontactresistanceresistance && thermalthermal resistance,resistance, thereforetherefore allowingallowing forfor highhighcurrentcurrent densitiesdensities && highhigh raditionradition intensityintensity..

�� TheThe internalinternal absorptionabsorption inin thisthis devicedevice isis veryvery lowlow duedue toto�� TheThe internalinternal absorptionabsorption inin thisthis devicedevice isis veryvery lowlow duedue totothethe largerlarger bandgapbandgap confiningconfining layerslayers..


Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )Edge Emitter LED ( ELED )


N+- GaAlAs

GaAs(n) substrate

Metal contact


Active layer n- GaAlAs

N GaAlAs

N+- GaAlAs

Metal contact

P GaAlAs

P+ GaAlAs

n- GaAlAsLight emits

from the edge


�� ActiveActive layerlayer isis usuallyusually lightlylightly dopeddoped oror undopedundoped && aa veryvery�� ActiveActive layerlayer isis usuallyusually lightlylightly dopeddoped oror undopedundoped && aa veryverylargelarge populationpopulation forfor recombinationrecombination isis createdcreated inin thisthis regionregion bybyforwardforward biasbias injectioninjection..


�� PhotonsPhotons areare generatedgenerated inin thethe veryvery thinthin activeactive regionregion &&�� PhotonsPhotons areare generatedgenerated inin thethe veryvery thinthin activeactive regionregion &&spreadspread intointo thethe guidingguiding layers,layers, withoutwithout reabsorptionreabsorption becausebecause ofoftheirtheir largelarge bandgapsbandgaps..

�� MostMost ofof thethe propagatingpropagating lightlight isis emittedemitted atat oneone edgeedge ofof�� MostMost ofof thethe propagatingpropagating lightlight isis emittedemitted atat oneone edgeedge ofofstructurestructure byby puttingputting aa reflectivereflective coatingcoating atat thethe nonnon emittingemitting endend &&structurestructure byby puttingputting aa reflectivereflective coatingcoating atat thethe nonnon emittingemitting endend &&puttingputting anan antireflectiveantireflective coatingcoating atat thethe emittingemitting endend..

�� TheThe waveguidewaveguide reducesreduces thethe divergencedivergence ofof thethe emittedemitted�� TheThe waveguidewaveguide reducesreduces thethe divergencedivergence ofof thethe emittedemittedradiationradiation..

�� InIn directiondirection perpendicularperpendicular toto thethe planeplane ofof layerslayers ,the,the halfhalf�� InIn directiondirection perpendicularperpendicular toto thethe planeplane ofof layerslayers ,the,the halfhalfpowerpower widthwidth isis approxapprox.. 3030 degdeg..

�� InIn thethe planeplane ofof thethe layerlayer thethe outputoutput isis stillstill lambertianlambertian && halfhalf�� InIn thethe planeplane ofof thethe layerlayer thethe outputoutput isis stillstill lambertianlambertian && halfhalfpowerpower widthwidth isis approxapprox.. 120120 degdeg..

�� TheThe waveguidewaveguide andand reducedreduced beambeam divergencedivergence allowsallows moremore�� TheThe waveguidewaveguide andand reducedreduced beambeam divergencedivergence allowsallows moremoreefficientefficient couplingcoupling ofof thethe radiatedradiated beambeam intointo fibrefibre..9/12/2013 Mahesh J. vadhavaniya 32

Advantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LED

AdvantagesAdvantages ::

�� EfficientEfficient�� EfficientEfficient�� ColorColor�� SizeSize�� OnOn // OffOff timetime�� CyclingCycling�� CyclingCycling�� DimmingDimming�� CoolCool LightLight�� SlowSlow FailureFailure�� LifeLife timetime�� ShockShock resistanceresistance�� FocusFocus�� RobustRobust�� LinearLinear


�� LargeLarge lineline widthwidth ((3030--4040 nm)nm)�� LargeLarge lineline widthwidth ((3030--4040 nm)nm)�� LargeLarge beambeam widthwidth (Low(Low couplingcoupling toto thethe fiber)fiber)�� LowLow outputoutput powerpower�� LowLow E/OE/O conversionconversion efficiencyefficiency

Advantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LEDAdvantage & Disadvantages of LED

DrawbacksDrawbacks ::

�� LowLow E/OE/O conversionconversion efficiencyefficiency�� TemperatureTemperature dependancedependance�� VoltageVoltage sensitivesensitive�� HighHigh initialinitial priceprice


Point of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of DiscussionPoint of Discussion

AdvantagesAdvantages && disadvantagesdisadvantages ofof ILDILDoverover LEDLED forfor longlong haulhaul OpticalOptical fiberfiberoverover LEDLED forfor longlong haulhaul OpticalOptical fiberfibercommunicationcommunication..


ILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LED

AdvantagesAdvantages ::

1.1. Modulation CapabilitiesModulation Capabilities2.2. High radianceHigh radiance3.3. Narrow line widthNarrow line width4.4. Relative temporal coherenceRelative temporal coherence5.5. Good spatial coherence Good spatial coherence 6.6. More focused radiation pattern, small fiberMore focused radiation pattern, small fiber6.6. More focused radiation pattern, small fiberMore focused radiation pattern, small fiber7.7. Much higher radiant power, longer spanMuch higher radiant power, longer span8.8. Faster ON, OFF time; higher bit rates possibleFaster ON, OFF time; higher bit rates possible9.9. Monochromatic light, reduces dispersionMonochromatic light, reduces dispersion

DisadvantagesDisadvantages ::

1. Much more expensive

2. Higher temperature, shorter lifespan


LEDLEDLEDLED ILDILDILDILD

• Lower Efficiency •Higher Efficiency

• Slow response rate •High response rate

• Lower data transmission rate

•Higher data transmission rate

ILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LEDILD Versus LED

• Simple construction •Construction is complicated

• Higher distortion level at output

•Lower distortion level at

output

• Higher dispersion •Lower dispersion

• In coherent beam •Coherent beam


9/12/2013 Mahesh J. Vadhavaniya 39

LED

Education

extrinsic semiconductor

intrinsic semiconductor

nn junctionjunction

fermithe fermi

ktee ep f exp1