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Unit1:SemiconductorMaterials
andDiodes
Contents • ReviewofSemiconductormaterialsand
proper5es
• ThePNjunc5on• Introduc5ontoSemiconductorDiodeTheory
• DiodeCircuits:DCAnalysisandModels
ACEquivalentCircuits
• TemperatureEffects
• DiodeTypes:SolarCell,Photodiode,LightEmiHngdiode,SchoIkybarrierdiode
• UnderstandingManufacturersSpecifica5on
Reviewof
SemiconductorMaterial
MaterialClassifica5on
• Conductors– Gold,Silver,Copper.– GoodconductorsofElectricity
• Insulators– Porcelain,glass,quartz,Rubber,Backellite,etc.– BadconductorsofElectricity
• Semiconductorsarematerialswhoseelectricalproper5eslie
betweenConductorsandInsulators.
Ex:SiliconandGermanium
Semiconductors
• Neithergoodconductorsnorgoodinsulators.
• Atroomtemp.
• Haveconduc5vityconsiderablylowerthanCONDUCTORSbuthigherthanINSULATORS
• ThereforecalledasSEMICONDUTORS
• Atabsolutezerotemp.
SemiconductorbehavesasanINSULATORS
• IncreaseinTemperature.
• Conductors:• conduc5vitydecreases• resis5vityincreases,• ithasposi5veTemperaturecoefficientofresistance.
• Semiconductors:
• conduc5vityincreases• resis5vitydecreases• ithasnega7vetemperatureofcoefficient.
Semiconductors(Contd.)
Conduc5vityofSC
Semiconductors(Contd.)
• Controlledbycontrollingtheamountof
impurityaddedtoit
• canbechangedtoalargeextentbyaddingasmallamountofimpurity.
EnergyBandDiagram
StructureofAtom
Energyleveldiagram
eV
eV–UnitofEnergy
• Thechargeofsingleelectronis1.6x10-19Coulomb.
• TheenergyismeasuredinJoules.
• OneeVisdefinedastheenergywhichanelectronacquiresinmovingthroughapoten5al
differenceof1V..
EnergyBandDiagram
EnergyBandDiagram
Differen'atebetweentheEnergyLevelsofanInsulator,SemiconductorandConductor
Classifica5onofSemiconductor
IntrinsicSemiconductor
ExtrinsicSemiconductor
Q:Comparetheextrinsicandtheintrinsicsemiconductormaterial
Intrinsic,Extrinsic
• Semiconductorsareeitherintrinsicorextrinsic.
• Inanintrinsicsemiconductor,nandpare
determinedbythethermallygenerated
electronsandholes.
• Inanextrinsic(doped)semiconductor,the
carrierconcentra5on(n,p)arecontrolledby
doping.
Representa5onof
SiliconandGermaniumatom
IntrinsicSemiconductor
• Semiconductorinitspurestform.
Thermalgenera5onof
ElectronHolePairin
IntrinsicSemiconductor
• IntrinsicSCactsasaPERFECTINSULATORatabsolutezero(-2730C).
• Atroomtemperature(300K)IntrinsicSChasSMALLCONDUCTIVITYduetothermalgenerated
ElectronHolepairs.
• Conduc5vityofSiislessthanthatofGeEg(Si)=1.12eV(moreforSi)
Eg(Ge)=0.72eV(lessforGe)
IntrinsicSemiconductor
IntrinsicSemiconductor
• EffectoftemperatureonIntrinsicSC
• Higheristheconcentra5onofchargecarriers.
• Higheristheconduc5vity.
• Resis5vitydecreases.
• Semiconductorhas
NEGATIVETEMPERATURECOEFFICIENTOFRESISTANCE.
CurrentConduc5onin
IntrinsicSemiconductor
ExtrinsicSemiconductor
• DopedSemiconductor
Conduc5vityofIntrinsicSCisverysmall
Semiconductorsaredopedto
improveitsconduc5vity
Doping:Deliberatelyaddingimpurity
TypesofExtrinsicSemiconductor
• NtypeSemiconductor
• PtypeSemiconductor
ExtrinsicSemiconductor
• Thesemiconductoriscontaminatedor"doped".
• "Doping"istheinten5onalintroduc5onofchemicalelements
intothesemiconductor.
ExtrinsicSemiconductor
• Dependingonthetypeofdopantonecanobtainasurplusofeither
posi5vechargecarriers(called)p-conduc'ngsemiconductorornega5vechargecarriers(called)n-conduc'ngsemiconductor
• Bydopingtrivalentelement,wegetp-typesemiconductor.
(withexcessamountofhole).
• Bydopingpentavalentelement,wegetn-typesemiconductor
(withexcessamountofelectron)
ExtrinsicSemiconductor
NTypeSemiconductor
• Asmallamountofpentavalentimpurity(Phosphorous)isadded.
NTypeSemiconductor
• Donorimpurityhasoneextraelectroninitsvalenceshell.
• At roomtemperature ,each impuritydonatesoneelectronto the
conduc5onband.
• Allthedonatedelectronstakepartintheconduc5onofelectriccurrent.
• Besidestherearethermallygeneratedelectronholepairs.
• Majoritychargecarriers:Electrons• Minoritychargecarriers:Holes
• Totalnumberofholesandimmobileionsareexactlysameasthe
numberoffreeelectronscreated.
ExtrinsicSemiconductor
PtypeSemiconductor
• AsmallamountofTrivalentimpurity(Boron)isadded.
PTypeSemiconductor
• Acceptor impurityhasadeficiencyofelectron(hole) in itsvalence
orbit.
• At roomtemperature , the thermalenergy is sufficient toprovide
energytoneighboringelectron,soastofilluptheincompletebonds
aroundtheBoronatom
• Besidestherearethermallygeneratedelectronholepairs.
• Majoritychargecarriers:Holes• Minoritychargecarriers:Electrons
• Totalnumberof electronsand immobile ionsareexactly sameas
the
numberofholescreated.
EffectofTemperatureon
ExtrinsicSemiconductor
• Smallamountofimpurity(donor/acceptor)createslarge
amountofchargecarriersinExtrinsicSC.
• Atroomtemp.Conduc5vityofExtrinsicSCismany5mesthatof
IntrinsicSC.
• Addi5onalthermalenergyincreasesthethermallygenerated
chargecarriers.
• Asaresulttheno.ofmajorityandminoritychargecarriers
increases.
• HenceConduc5onincreases(RESISTANCEDECREASES).
• SEMICONDUCTORShavenega7vetemperaturecoefficientofresistance.(DecreasingresistancewithincreasingTemperature)
Summary
SEMICONDUCTORDIODE
SEMICONDUCTORDIODE
• If two differently contaminated semiconductor layers arecombined,thenaso-calledp-n-junc'onresultsontheboundaryofthelayers.
• By doping trivalent element, we get p-type semiconductor.
(withexcessamountofhole)
• Bydopingpentavalent element,we get n-type semiconductor
(withexcessamountofelectron)
n-typesemiconductorp-typesemiconductor
p-njunc7onlayer
UNBIASEDPNJUNCTION
BIASING
ApplyingexternalvoltagetothePNjunc5on.
UNBIASEDPNjunc5on
• PNjunc5onwithnoexternalvoltage.
PNjunc5onwithnoexternalvoltage.
Deple5onLayer
Barrier
Voltage
PNjunc5onwithnoexternalvoltage.
Deple7onLayerThecombiningofelectronandholesdepletes
theholesinthePregionandtheelectronsin
theNregionnearthejunc5on.
Barrier
Arestrainingforceisautoma5callysetup
acrossthedeple5onlayercalledBARRIER
Theoppositechargesthatbuilduponeachsidecreatesavoltage
acrossthePNjunc5on.
PNjunc5onwithnoexternalvoltage.
BarrierVoltage
Thebarriervoltageistheamountofelectromo5veforcerequired
tostartconduc5onacrossthePNjunc5on.
ForSiPNjunc5ons:0.7V
ForGePNjunc5ons:0.3V
ThebarriervoltageopposestheflowofmajoritycarriersacrossPN
junc5onandassiststheflowofminoritycarriersacrossthejunc5on.
Unequaldopingdensity
Equaldopingdensity:Deple5onlayerofequal
widthoneitherside.
IfPregionisheavilydoped:
Deple5onlayerpenetratesmorein
thelightlydopedNregion.
IfNregionisheavilydoped:
Deple5onlayerpenetratesmorein
thelightlydopedPregion.
Deple7onregionpenetratesdeepestintothemorelightlydopedside.
BIASEDPNJUNCTION
BiasedPNJunc5on
• ForwardbiasedPNjunc5on• BaIeryconnectedwithposi5veterminaltoP
sideandNega5veterminaltoNsideofthe
PNjunc5on
• ReversebiasedPNjunc5on• BaIeryconnectedwithposi5veterminaltoN
sideandNega5veterminaltoPsideofthePN
junc5on
Deple5onlayerofUnbiasedPNjuc5on
ForwardBiasedPNJunc5on
WhenexternalvoltageisappliedtounbiasedPNjunc5on
ForwardBiasedPNJunc5on
ForwardBiasedPNJunc5on
• Deple5onwidthisreduced• Barriervoltageisreduced• Barriervoltageeffec5velydisappears• Majoritychargecarriersflowsacrossthejunc5on
Asappliedvoltageisincreasedfrom0
Asappliedvoltageis
increasedfrom0
• Diodedoesnotconductun5l
theexternalvoltageovercomes
thebarriervoltage.
• Whentheappliedvoltagereaches0.7V(Si),largeno.offreeelectronsandholescrossthejunc5on.
• Above0.7V,evenasmallincreaseinvoltagecausesasharprisein
current.
• Thevoltageatwhichthecurrentstartstoincreaserapidlyis
calledCUTINorKNEEVoltage(Vγ).
ForwardCharacteris7cPlotofforwardcurrent(IF)versusforwardvoltage(VF)
ReversebiasPNjunc5on
ReversebiasedPNjunc5on
ReversebiasedPNjunc5on
ReverseBiasedPNJunc5on
• Deple5onwidthisincreased
• Barriervoltageisincreased
• Majoritychargecarrierscannotflowsacrossthejunc5on
Asappliedreversevoltageisincreasedfrom0
• Minoritychargecarrierscanflowsacrossthejunc5on(Small
reversecurrentflows)
• Rateofgenera5onofminoritycarriersdependson
temperature.
• Atconstanttemperaturerateofminoritycarriergenera5onis
constantwhetherappliedvoltageisloworhigh(REVERSESATURATIONCURRENT).
Asappliedreversevoltageistoolarge
• Currentincreasesabruptly.
• Thevoltageatwhichthisphenomenonoccursiscalledas
BREAKDOWNVOLTAGE.
Breakdownin
PNjunc5ondiode
ZenerBreakdown
Avalanche
Breakdown
BREAKDOWN
ZenerBreakdown
• Whenreversebiasisincreased,theelectricfieldatthe
junc5onalsoincreases.
• Thehighelectricfieldcausesthecovalentbondstobreak.• Thevalenceelectronsintheatomsarepulledoutbythe
electrosta5cforceexperiencedatthejunc5on.
• Thusalargenumberofcarriersaregenerated.
• Thiscausesalargecurrenttoflow.• Thismechanismofbreakdowniscalledaszenerbreakdown.
Q:Explainthebreakdownmechanismofzenerdiode
AvalancheBreakdown
• Avalanchebreakdownoccurswhenthereversevoltageacrossthe
diodeisincreasedtoahugevalue
• Chargecarriers(minoritycarriers)inthedeple5onregionare
acceleratedtogainkine7cenergy,whichcollideswiththecarriersofstableatoms.
• Thehighenergycarriersbreakthecovalentbonds.
• Theprocessresultsinanotherfreeelectronhavingvelocity.
• Thegeneratedfreeelectroninturngainkine5cenergyandcollides
withanotheratomtoproducemoreandmoreelectronsfree.
• Whenthenumberoffreeelectronincreasesrapidlydueto
thesubsequentcollisionoffreeelectronswithotheratoms,a
hugecurrentdevelopsatthejunc5onwhichresultsindamagingofthediodeduetooverhea7ng.
• AvalanchebreakdownoccursathighvoltageandinalessdopeddiodeascomparedtoZenerbreakdown.
Q:Differen;atebetweenZenerBreakdownandAvalancheBreakdownZenerBreakdown
1.Thisoccursatjunc5onswhichbeing
heavilydopedhavenarrowdeple5on
layers
2.Thisbreakdownvoltagesetsa
verystrongelectricfieldacrossthe
narrowlayer.
3.Heretheelectricfieldisverystrongto
rupturethecovalentbondsthereby
genera5ngelectronholepairs.Soevena
smallincreaseinreversevoltageis
capableofproducingalargenumberof
currentcarriers.ThisleadstoZener
Breakdown.
AvalancheBreakdown
1.Thisoccursatjunc5onswhich
beinglightlydopedhavewide
deple5onlayers.
2.Heretheelectricfieldisnotstrong
enoughtoproduceZenerbreakdown
3. Here theminority carriers collide
with semiconductor atoms in the
deple5on region which breaks the
covalent bond and electron hole
pa i r s a re genera ted . New ly
generatedcarriersareacceleratedby
the electric field which results in
collisionandgenerates avalancheof
charge carriers. This results in
avalanchebreakdown.
Q:Differen;atebetweenZenerBreakdownandAvalancheBreakdown(Contd.)
ZenerBreakdown
4.Zenerbreakdownoccursatlowvoltages
AvalancheBreakdown
4.Avalanchebreakdownoccursat
highvoltages.
5.Diodesarespeciallyfabricatedwitha
speciallydesignedbreakdownvoltage
inordertooperateinbreakdownregion.
TheseDiodesarecalledasZENERDIODES
5.Avalanchebreakdownisavoidedas
itleadstopermanentdamageofthe
diodeduetooverhea5ng.
VICharacteris5cof
PNjunc5ondiode
• ForwardCharacteris'c
• ReverseCharacteris'c
ForwardCharacteris5cof
PNjunc5on.diode
ReverseBiasCharacteris5cof
PNjn.diode
VICharacteris5cofPNjunc5ondiode
• Diodeconductswellinforwarddirec'on
• Diodeconductspoorlyinreversedirec'on
TemperatureDependence
Q:WhatistheeffectoftemperatureondiodeCharacteris;c
• ForforwardbiasVoltagedecreases2mV/oCforagivencurrent. • Currentincreaseswithtemperatureforagivenvoltage.
TemperatureDependence
• Inforwardbiasregionthecharacteris'cofasilicondiodeshiJtotheleJatarateof2mV/0Cincreaseintemperature
• Thejunc'onforwardvoltagedropisaffectedbytemperature.
ForwardBiasRegion
ReverseBiasRegion
• Thereversecurrentlevelapproximatelydoubleswitheach100Cincreaseintemperature.
TemperatureDependence
DiodeResistance
Q:ExplaintheDCandACresistanceofDiode
DCorSta5cResistance
ACorDynamicResistance
DCorSta5cResistance
ACorDynamicResistance
DiodeCurrentEqua5on
DiodeEqua5on
Forforwardandreversebiasregionthecurrentflowingthrough
thediode:
ID=IS[exp(V/ηVT)-1]-----doideI-Vcharacteris'cs
ID=(VDD-VD)/RKirchhoffloopequa'on
Forforwardandreversebiasregion:
ID=IS[exp(V/nVT)–1] doideI-Vcharacteris'cs
• IDisthediodecurrent.(Positveforforwardandnega5veforreverse)• Isistheconstantreversesatura5oncurrent.
• Vistheappliedvoltage(Posi5veforforwardandnega5veforreverse)
• ηfactordependentuponthenatureofsemiconductor.
(1forgermaniumand2forsilicon)
• VT-voltequivalentoftemperaturewhichisgivenbyT/11600.
(TisTemperatureinKelvin)
• VT(thermalvoltage)=26mVatroomtemperature(300K)
DiodeEqua5on
DCEquivalentCircuitfora
junc5ondiode
EquivalentCircuitofDiode
Significanceofequivalentcircuit
Representsthedevicebehavior.
Madeupofresistorsandvoltagecells.
Adiodecanbereplacedbyitsequivalentcircuitwhen
inves5ga5ngthecircuitcontainingdiode.
DiodeApproxima5ons
• Idealdiodeapproxima5on
• LinearPiecewiseapproxima5on
IdealDiodeApproxima5on
EquivalentCircuit
IdealDiode
• IdealDiodeactsasperfectconductor(zerovoltageacrossit)inforwarddirec'on
• IdealDiodeactsasperfectinsulator(nocurrentthroughit)inreversedirec'on
• IdealDiodeactsasautoma'cswitch.
ApproximateCharacteris5cofDiode
Voltagedrop,Vd=Vγ
EquivalentCircuit
PiecewiseLinearApproxima5on
Takingintoconsidera5onthedynamicforwardresistance
Voltagedrop,Vd=Vγ+Idrd
EquivalentCircuit
rdisthedynamicresistanceinserieswiththevoltagecell.
Idealdiodeisalsoincludedtoshowthatthecurrentflowsonlyin
onedirec5on.
PiecewiselinearVICharacteris5cof
PNdiode
• Ifadiodeisforwardbiasedwithahighvoltageitactslikearesistor(Rf)inserieswitha
voltagesource(Vf).
• Forreversebiasingitactssimplyasaresistor
(Rr).
SummaryTable
ACEquivalentofDiode
Junc5onCapacitances
Dependingonthebiasingcondi5on,twotypes
ofcapaci5veeffectsexistsinthediodes
• DiodeisReverseBiased.• Widthofdeple5onregion
increases.
• PandNregionactsasplatesofcapacitor.
• Deple5onregionactsasdielectric.
Thusthereexistacapacitanceatthepnjunc5oncalled
Transi7oncapacitance,junc7oncapacitance,spacechargecapacitance,barriercapacitance,deple7onregioncapacitance.
• DiodeisForwardBiased.• Widthofdeple5onregiondecreases.
• HolesinPdiffuseinNsideandElectronsinNdiffusetoPside.
• Asappliedvoltageincreases,concentra5onoftheinjectedchargedpar5clesincreases.
Thisrateofchangeofinjectedchargewithapplied
voltageisdefinedasDIFFUSIONCAPACITANCE.
Junc5onCapacitances
ACEquivalentCircuits
• InReverseBiased– VeryhighresistanceRrinparallelwithTransi5onCapacitanceCT.
• InforwardBiased– BaIeryofVγandseriesdynamicresistanceriareinparallelwithdiffusioncapacitanceCD.(CompleterEquivalentCircuit)
• InACequivalentthedcvoltagedropVγisnotincluded.
An equivalent circuit
WhyareSiliconsemiconductor
materialpreferred
Silicondiodeshavea
greatereaseofprocessing
lowercost
greaterpowerhandling
lessleakage
morestabletemperaturecharacterics
→Theflowofcharge(i.e.)currentthrougha
semiconductormaterialareoftwotypes
namelydris&diffusion.
→(i.e.)Thenetcurrentthatflowsthrougha
(PNjunc5ondiode)semiconductormaterial
hastwocomponents
– Driscurrent– Diffusioncurrent
Q:ExplainDriJandDiffusionCurrent
DrisCurrent
DrisCurrent
→Whenanelectricfieldisappliedacrossthesemiconductor
material,thechargecarriersaIainacertaindrisvelocityVd,
whichisequaltotheproductofthemobilityofthecharge
carriersandtheappliedElectricFieldintensityE.
→Holesmovetowardsthenega5veterminalofthebaIery
andelectronsmovetowardstheposi5veterminalofthe
baIery.Thiscombinedeffectofmovementofthecharge
carrierscons5tutesacurrentknownas“thedriscurrent“.
→Thusthedriscurrentisdefinedastheflowofelectriccurrent
duetothemo5onofthechargecarriersundertheinfluenceof
anexternalelectricfield
Diffusioncurrent
DiffusionCurrent
→ It is possible for an electric current to flow in a semiconductor even in the absence of the applied voltage provided a concentration gradient exists in the material.
→ A concentration gradient exists if the number of either electrons or holes is greater in one region of a semiconductor as compared to the rest of the Region.
→ In a semiconductor material the change carriers have the tendency to move from the region of higher concentration to that of lower concentration of the same type of charge carriers. Thus the movement of charge carriers takes place resulting in a current called diffusion current.
Differen5atebetween
DriscurrentandDiffusioncurrent
• Diffusioncurrentisdueto
concentra5ongradient.
• Diffusioncurrentisholesand
electronsmovingfromareasof
highconcentra5ontotheareaof
lowerconcentra5on.Thisoccurs
un5ltheyareuniformly
distributed.
• Diffusioncurrentcanbethere
evenifwedon'tapplyvoltage
• Driscurrentisduetopoten5al
gradient
• Driscurrentistheresponseof
electronsandholestotheelectric
field.Thisoccursun5lthecarriers
available.Holesmovesinthe
direc5onofelectricfieldwhile
electronsmovesintheopposite
direc5onoffield.
• Driscurrentisdependson
electricfield.Onlyappearswhen
weapplyvoltage.
DiffusionCurrentDrisCurrent
DiodeParameters
• VF–Forwardvoltagedrop
• IF–ReverseSatura5oncurrent• VBR–Reversebreakdownvoltage
• rd–dynamicresistance
• IF(max)-maximumforwardcurrent
DiodeTypes
• SolarCell• PhotoDiode• LightEmiHngDiode
• SchoIkyBarrierDiode• ZenerDiode
SolarCellQ:Explaintheworkingofsolarcellwithapplica;on?
Solar cell: Solar cell is a photovoltaic device that convertsthelightenergyintoelectricalenergybasedon theprinciplesofphotovoltaiceffect
Photovoltaiceffect
Defini'on:Thegenera7onof
voltage across the PNj u n c 7 o n i n asemiconductor due tothe absorp7on of lightr ad i a7on i s c a l l edphotovoltaic effect. TheDevices based on thise ff e c t i s c a l l e dphotovoltaicdevice.
Lightenergy
n-typesemiconductor
p-typesemiconductor
Electrical
Power
p-njunc5on
Solarpanel(or)solararray(or)Solarmodule
Thesolarpanel(or)solararrayistheinterconnec;onofnumberofsolarmoduletogetefficientpower.
• Asolarmoduleconsistsofnumberofinterconnected solarcells.
• Theseinterconnectedcellsembeddedbetweentwo glassplate
toprotectfromthebadwhether.
• Sinceabsorp5onareaofmoduleishigh,moreenergy canbe
produced.
MaterialsforSolarCell
Solarcellsarecomposedofvarioussemiconduc7ngmaterials
1. Crystallinesilicon
2. Cadmiumtelluride
3. Copperindiumdiselenide
4. Galliumarsenide
5. Indiumphosphide
6. Zincsulphide
Note: Semiconductors are materials, which become electrically
conduc5vewhensuppliedwithlightorheat,butwhichoperateas
insulatorsatlowtemperatures
Construc5onofSolarCell
• Solarcell(crystallineSilicon)consistsofan-typesemiconductor(emiZer)layerandp-typesemiconductorlayer(base).Thetwolayersaresandwichedandhencethereis
forma5onofp-njunc'on.
• Thesurfaceiscoatedwithan'-refec'oncoa'ngtoavoidthe loss
ofincidentlightenergyduetoreflec5on.
• Apropermetalcontactsaremadeonthen-typeandp-typeside
ofthesemiconductorforelectricalconnec5on
• Whenasolarpanelexposedtosunlight,thelightenergies
areabsorbedbyasemiconductormaterials.
• Duetothisabsorbedenergy,theelectronsareliberated and
producetheexternalDCcurrent.
• TheDCcurrentisconvertedinto240-voltACcurrentusing an
inverterfordifferentapplica5ons.
Workingofsolarcell
Workingofsolarcell
• Itisapnjunc5ondevicewithnovoltagedirectlyappliedacrossthejunc5on.
• Whenlighthitsthespacechargeregion,
electronsandholesaregenerated.
• Theyarethenquicklyseperatedandsweptoutofthespacechargeregionbytheelectricfield,thuscrea5ng
PHOTONCURRENT.
Applica5on
Soarpumpsareusedforwatersupply.
Domes7c power supply for appliances include
refrigera5on,washingmachine,televisionandligh5ng
Ocean naviga7on aids: Number of lighthouses and
most buoysarepoweredbysolarcells
Telecommunica7on systems: radio transceivers on mountain tops, or telephone boxes in the country can
osenbesolarpowered
Electric power genera7on in space: To providingelectricalpowertosatellitesinanorbitaroundtheEarth
LED
• LightEmiHngDiode
Q:DrawandExplainLEDalongwithoneareaofapplica;on
• AlightemiHngdiode(LED)isessen5allyaPNjunc5onopto
semiconductorthatemitsmonochroma5c(singlecolor)lightwhen
operatedinaforwardbiaseddirec5on.
• ALEDconvertselectricalenergyintolightenergy
• ALEDchiphastworegionsseparatedbyajunc5on.
• Thejunc5onactsasabarriertotheflowofelectronsbetweenthepandthenregions.
• Withsufficientvoltageisappliedtothechipacrosstheleadsofthe
LED,electronscanmoveeasilyinonlyonedirec5onacrossthepandn
regions.
• Whenavoltageisappliedandthecurrentstartstoflow,electronsin
thenregionhavesufficientenergytomoveacrossthejunc5onintothe
pregion.
LEDQ:DrawandExplainLEDalongwithoneareaofapplica;on
• Each5meanelectronrecombineswithaposi5vecharge,electric
poten5alenergyisconvertedintoelectromagne5cenergy.
• Foreachrecombina5onofanega5veandaposi5vecharge,aquantum
ofelectromagne5cenergyisemiIedintheformofphotonoflightwith
afrequencycharacteris5cofthesemiconductormaterial.
LEDQ:DrawandExplainLEDalongwithoneareaofapplica;on
HowmuchenergydoesanLEDemit?
• Theenergy(E)ofthelightemiIedbyanLEDis
relatedtotheelectriccharge(q)ofanelectronandthevoltage(V)requiredtolighttheLEDbytheexpressionE=qVJoules.
• Theexpressionsaysthatthevoltageispropor5onaltotheelectricenergy.
• Theconstantqistheelectricchargeofasingleelectron,-1.6x10-19Coulomb.
Findingenergyfromvoltage
• LetussaythatyouhavearedLED,andthevoltagemeasuredbetweentheleadsis1.71
Volts.SotheenergyrequiredtolighttheLEDis:
E=qVJoules
E=-1.6x10-19(1.71)=2.74x10-19Joules
Applica5ons
• SensorApplica5ons
– MedicalInstrumenta5on,BarCodereaders,Op5calSwitches,FiberOp5c
Communica5on,Etc.
• MobileApplica5ons
– MobilePhone,PDA’s,DigitalCameras,Laptops,etc.
• SignApplica5ons
– Monocroma5cMessageBoards,Trafific,Transporta5on-Passenger
Informa5on
• LEDsignals
– Traffic,Rail,Avia5on,TowerLight,RunwayLights,
Emergency/PoliceVehicleLigh5ng,etc.
• Indicators
– HouseholdAppliances,DVD/VCR/Stereoandotheraudioandvideodevices,
Toys/Games,Instrumenta5on,SecurityEquipments,Switches
• Illumina5ons
SometypesofLEDs
LEDQ:ListthematerialsusedfordifferentcolorsofLED.
• LEDsareavailableinred,orange,yellow,green,blueandwhite.
• LEDsaremadefromgallium-basedcrystalsthatcontainoneormoreaddi5onalmaterialssuchasphosphorous,Arsenide,Phosphidetoproduceadis5nctcolor.
• ThecolorofanLEDisafunc5onofthematerial
usedtomakethejunc5on.Therearetwomain
flavorsusedinvisiblelightLEDjunc5ons:
• Indiumgalliumnitride(InGaN)isusedtomake
uptheblue,white,truegreen,andUVtypes.
• Aluminumgalliumindiumphosphide(AlGaInP
orAlInGaP)isusedtomakethered,yellow,and
orangetypes.
LEDQ:ListthematerialsusedfordifferentcolorsofLED.
WhatvalueofSeriesResistorisrequiredtolimitthe
currentthroughtheLEDto20mAwithaforward
voltagedropof1.6Vwhenconnectedtoa10Vsupply?
Vf=1.6V
Vs=10V
i=20mA
Problem:
Whatcurrentlimi5ngresistorvalueshouldyouuseifyouhaveoneLEDandwanttopowerit
withasupplyof3.8V(Vf=3.1V,if=30mA)
Vf=3.1V
Vs=3.8V
i=30mA
Problem:
R=23.3ohms
• Photodiode(pnjunc5on)operatedinreversebias.
Photodiodes
• In a reverse biased p-n junc5on a Reverse Satura5on
Current flows due to minority carriers which are thermally
generated.
• Increasingthereversebiasdoesnotincreasesthereverse
currentsignificantly.
• TEMPERATURE and ILLUMINATION increases number of
minoritycarriers(reverse)current.
• Thephotonsimpac5ngthejunc5oncausecovalentbonds
tobreak.
• Theelectricfield inthedeple5onlayersweepsMinority
electronsinpsidetonsideandminorityholesinnsideto
thep-side.
• Reverse current across junc5on- photocurrent is
propor5onaltotheintensityoftheincidentlight.
• Response5meisfast(nanoseconds)
Materialsusedtoproducephotodiodes
• Silicon• Germanium
• IndiumGalliumArsenide
• LeadSulphide
PhotodiodeCircuit
• Ifphotonintensityiszero.
• Onlycurrentflowingisthereversesatura5oncurrent.
(Darkcurrent,normallysmall).
• Photonintensityofsufficientenergystrikesthediode.
• Photoelectriceffect(electronholepairin
spacechargeregion)
• Photocurrentflowsinreversedirec5on.
• Photocurrentissumofdarkcurrentandthelightcurrent.
• PhotocurrentcreatesvoltagedropacrossR.
SchoIkyDiode
• SchoIkydiodeisaspecialpurposedevicewithnodeple5onlayerellimina5ngthestored
chargesatthejunc5on.
Q:Explaintheconstruc;on,workingandcharacteris;cofSchoYkydiode.
• Construc5onisdifferentfromthenormalp-n
junc5on.
Construc5onofSchoIkyDiode
• Itisametalsemiconductorjunc5on.
• Ononesideofthejunc5onametal(gold,
silver,molybdenum,chromeortungsten)is
usedandonothersideofthejunc5onntype
dopedSiisused.
WorkingofSchoIkydiode
• Whendiodeisunbiased.
• Electronsonthensidehavelowenergylevelsthentheelectronsinthe
metalandsocannotcrossthejunc5onbarriercalledSchoIkybarrier.
• WhendiodeisForwardbiased.
• Electronsonthensidegainenoughenergytocrossthejunc5onandenter
metal.
• Electronsplungeintothemetalwithverylargeenergy,theyarecalledhot
carriersandhencethenameHOTCARRIERDIODE.
• SchoIkydiodeisaunipolardeviceandthereisnodele5onregion.
Characteris5cofSchoIkydiode
• Ithaslowbarrierpoten5al(0.2–0.25V)whereasnormaldiode
has0.7V(Si)
• Ithashigherleakagecurrentsandlowerreversebreakdownvoltage.
• Itismoreefficientfor
highpowerapplica5ons.
Applica5onsofSchoIkydiode
TheSchoIkybarrierdiodesarewidelyusedintheelectronicsindustry
findingmanyusesasdioderec5fier.Itsuniqueproper5esenableittobe
usedinanumberofapplica5onswhereotherdiodeswouldnotbeableto
providethesamelevelofperformance.Inpar5cularitisusedinareas
including:
RFmixeranddetectordiode
Powerrec;fier
Solarcellapplica;ons
Clampdiode
Advantages:
Highspeed,highfrequency,lowforwardvoltage
drop,lowheatdissipa5on,lowloss.
Disadvantages:
Sizeandcost.
SchoIkydiode
ZENERDIODES
Zenerdiodes
(Zener)diodesaredesignedtostabilizevoltage.
Itisaspecialkindofdiodewhichpermitscurrenttoflowintheforwarddirec5onasnormal,butwillalsoallowittoflowinthereversedirec5onwhenthevoltageisaboveacertainvalue-thebreakdownvoltageknownastheZenervoltage.
AZenerdiodeisaspecialdiodethatisop7mizedforopera7oninthebreakdownregion
ZenerdiodeTheV-Icharacteris5cs
VZ–breakdownvoltage
VF–forwardvoltage
IR–reversecurrent
134
ZENERDIODECHARACTERISTICS
• Intheforwardregion,theZenerdiodeactslike
aregularsilicondiode,
witha0.7voltdrop
whenitconducts.
135
ZENERDIODECHARACTERISTICS
• Inthereversebiasregion,areverse
leakagecurrentflows
un5lthebreakdown
voltageisreached.
• Atthispoint,thereversecurrent,called
ZenercurrentIz,
increasessharply.
136
ZENERDIODECHARACTERISTICS
• Voltagea]erbreakdownisalsocalledZenervoltageVz.
• Vzremainsnearlyaconstant,eventhoughcurrentIzvariesconsiderably.
137
ZENERDIODERATINGS
• AZenerdatasheettypicallyprovides-themaximumpowerra5ngPzM
-thenominalzenervoltageVzattestcurrentIzT
-themaximumDCzenercurrentIzM
Example:-1N752hasapowerra5ngof500mW,a
nominalZenervoltageof5.6Vatatest
currentof20mA,amaximumZener
currentof80mA.
138
ZENERDIODEMODEL
EquivalentCircuitofZenerDiode
Prac5calZenerdiode
ZenerdiodeRegulator
ZenerBreakdown
• ZenerbreakdownoccursindiodesspeciallydesignedtowithstandthedamagingcausedbytheAvalanchebreakdown.
• Zenerdiodesareheavilydopedtoreducethedeple5onregionwidth.
• Onapplyingreversepoten5alacrossaZenerdiode,duetothereducedwidthofthedeple5onregion,thevalence
electronsintheatomsarepulledoutbytheelectrosta5c
forceexperiencedatthejunc5on.
• Whichinturnresultsinthebreakdownofthejunc5on,since
thediodeisspeciallydesignedtohaveabreakdownatlowervoltagesascomparedtonormalAvalanchebreakdown.
• Therefore,aZenerdiodehasacontrolledbreakdowninreversebiasedcondi5onoverthezenerregion.
Q:Explainthebreakdownmechanismofzenerdiode
Q:ComparetheextrinsicandtheintrinsicsemiconductormaterialQ:Explainenergybanddiagramforsemiconductorandconductor
Q:Explainbreakdownmechanismofzenerdiode
Q:Differen'atebetweenenergylevelsofinsulator,conductorandsemiconducor
Q:Explainthedcandacresistanceofdiode.
Q:Differen'atebetweenavalancheandzenerbreakdown
Q:Explainthepiecewiselinearequivalentcircuitfordiosewithcharacteris'cs.
