unit 1 : semiconductor materials diodes

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


•  Semiconductorinitspurestform.

Thermalgenera5onof

ElectronHolePairin


•  IntrinsicSCactsasaPERFECTINSULATORatabsolutezero(-2730C).

•  Atroomtemperature(300K)IntrinsicSChasSMALLCONDUCTIVITYduetothermalgenerated

ElectronHolepairs.

•  Conduc5vityofSiislessthanthatofGeEg(Si)=1.12eV(moreforSi)

Eg(Ge)=0.72eV(lessforGe)



•  EffectoftemperatureonIntrinsicSC

•  Higheristheconcentra5onofchargecarriers.

•  Higheristheconduc5vity.

•  Resis5vitydecreases.

•  Semiconductorhas

NEGATIVETEMPERATURECOEFFICIENTOFRESISTANCE.

CurrentConduc5onin



•  DopedSemiconductor

Conduc5vityofIntrinsicSCisverysmall

Semiconductorsaredopedto

improveitsconduc5vity

Doping:Deliberatelyaddingimpurity

TypesofExtrinsicSemiconductor

•  NtypeSemiconductor

•  PtypeSemiconductor


• Thesemiconductoriscontaminatedor"doped".

•  "Doping"istheinten5onalintroduc5onofchemicalelements

intothesemiconductor.


• Dependingonthetypeofdopantonecanobtainasurplusofeither

posi5vechargecarriers(called)p-conduc'ngsemiconductorornega5vechargecarriers(called)n-conduc'ngsemiconductor

•  Bydopingtrivalentelement,wegetp-typesemiconductor.

(withexcessamountofhole).

•  Bydopingpentavalentelement,wegetn-typesemiconductor

(withexcessamountofelectron)


NTypeSemiconductor

• Asmallamountofpentavalentimpurity(Phosphorous)isadded.

NTypeSemiconductor

• Donorimpurityhasoneextraelectroninitsvalenceshell.

• At roomtemperature ,each impuritydonatesoneelectronto the

conduc5onband.

• Allthedonatedelectronstakepartintheconduc5onofelectriccurrent.

• Besidestherearethermallygeneratedelectronholepairs.

• Majoritychargecarriers:Electrons• Minoritychargecarriers:Holes

• Totalnumberofholesandimmobileionsareexactlysameasthe

numberoffreeelectronscreated.


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


•  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


Deple7onLayerThecombiningofelectronandholesdepletes

theholesinthePregionandtheelectronsin

theNregionnearthejunc5on.

Barrier

Arestrainingforceisautoma5callysetup

acrossthedeple5onlayercalledBARRIER

Theoppositechargesthatbuilduponeachsidecreatesavoltage

acrossthePNjunc5on.


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


•  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

•  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.


•  Inforwardbiasregionthecharacteris'cofasilicondiodeshiJtotheleJatarateof2mV/0Cincreaseintemperature

•  Thejunc'onforwardvoltagedropisaffectedbytemperature.

ForwardBiasRegion

ReverseBiasRegion

•  Thereversecurrentlevelapproximatelydoubleswitheach100Cincreaseintemperature.


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


•  Inthereversebiasregion,areverse

leakagecurrentflows

un5lthebreakdown

voltageisreached.

•  Atthispoint,thereversecurrent,called

ZenercurrentIz,

increasessharply.

136


•  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.

unit 1 : semiconductor materials diodes

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