OPTICAL TRANSDUCER-Photodiode-biosensors and transducers

biosensors and transducers

1. PHOTODIODE
-
Mukesh Sundararajan

2. Photodiodes
▶Photodiodesaresemiconductorlightsensorsthat
generateacurrentorvoltagewhentheP-Njunctionin
thesemiconductorisilluminatedbylight.Itisatypeof
photodetectorcapableofconvertinglightenergyinto
electricalenergyineithercurrentorvoltage,depending
uponthemodeofoperation.Solarcellisanexampleof
it.

3. ▶Whenaphotonofsufficientenergystrikesthediode,it
excitesanelectron,therebycreatingafreeelectron(anda
positivelychargedelectronhole).Thismechanismisalso
knownastheinnerphotoelectriceffect.
▶Thisdevicecanbeusedinthreemodes:photovoltaicasa
solarcell,reversed–biasedasaphotodetector,andforward–
biasedasanLED.

4. Structure

6. Operation
▶Aphotodiodeisdesignedtooperateinreversebias.
▶Photoelectric effect
The photoelectric effectistheobservationthat many
metalsemit electronswhenlightshinesupon them.
Electronsemittedin thismannercanbecalled
photoelectrons.
▶Movementofthoseelectronsgiverisetophotocurrent.

7. Characteristics
▶Photodiodesaresimilartoregularsemiconductordiode
exceptthattheymaybeeitherexposed(todetectUVor
X-rays)orpackagedwithawindoworopticalfiberto
allowlighttoreachthesensitivepartofthedevice.
Manydiodesdesignedforusespecificallyasa
photodiodeuseaPINjunctionratherthanap-n
junction,toincreasethespeedofresponse.

8. Characteristics

9. Typesofphotodiode
▶Pinphotodiode
▶P-Nphotodiode
▶Avalanchephotodiode

10. FeaturesofPhotodiode
❑Excellent linearity with respect to
incidentlight
❑Lownoise
❑Widespectralresponse
❑Mechanicallyrugged
❑Compactandlightweight
❑Longlife

11. ▶Materialscommonly used toproducephotodiodesinclude:
MATERIALS ELCTROMAGNETIC
SPECTRUMWAVELENGTH
RANGE(NM)
SILICON 190-1110
GERMANIUM 400-1700
INDIUMGALLIUMARSENIDE800-2600
LEADSUFIDE 100-3500

12. PHOTODIODECONSTRUCTION
CROSSSECTIONOFTHESILICONPHOTODIODE

13. ▶Ntypesiliconisthestartingmaterial.Athin"p"layeris
formedonthefrontsurfaceofthedevicebythermal
diffusionorionimplantationoftheappropriatedoping
material(usuallyboron).
▶Theinterfacebetweenthe"p"layerandthe"n"siliconis
knownasap-njunction.Smallmetalcontactsareapplied
tothefrontsurfaceofthedeviceandtheentirebackis
coatedwithacontactmetal.
▶Thebackcontactisthecathode,thefrontcontactisthe
anode.Theactiveareaiscoatedwitheithersilicon
nitride,siliconmonoxideorsilicondioxideforprotection
andtoserveasananti-reflectioncoating.
▶Thethicknessofthiscoatingisoptimizedforparticular
irradiationwavelengths.Asanexample,aCentroVision
Series5-Tphotodiodehasacoatingwhichenhancesits
responsetothebluepartofthespectrum.

14. ▶AtthePNjunctiontherewillaconcentration
gradientthatcauseselectronstodiffuseinto
thep-layerandholestodiffuseintothen-
layer.Thisdiffusionresultsinanopposing
electricalpotential,oftenreferredtoasan
internalbias(depletionregion).
▶Inagenericp-nphotodiode,lightentersthe
devicethroughthethinp-typelayer.Absorption
causeslightintensitytodropexponentiallywith
penetrationdepth.
▶Anyphotonsabsorbedinthedepletionregion
producechargecarriersthatareimmediately
separatedandsweptacrossthejunctionbythe
naturalinternalbias.Eventuallytherewillbethe
movementofthechargecarriers.

15. ▶Thismovementofchargecarriersacrossthe
junctionupsetstheelectricalbalanceand
producesasmallphotocurrent,whichcanbe
detectedattheelectrodes.
▶Inmanyapplicationsitisdesirabletomaximize
thethicknessofthedepletionregion.For
example,deviceresponseisfasterwhenmostof
thechargecarriersarecreatedinthedepletion
region.
▶Thisalsoincreasesthequantumefficiencyofthe
device,sincemostchargecarrierswillnothave
theopportunitytorecombine.Thequantum
efficiencyisdefinedastheratioofthe
photocurrentinelectronstoincidentlight
intensityinphotons.

16. APPLICATIONS
▶Photodiodesareusedinconsumerelectronicsdevicessuch
asCDplayers,smokedetectors,andthereceiversforinfraredremotecontrol
devicesusedto controlequipmentfromtelevisionsto airconditioners.
▶Photodiodesareusedasa lightsensors.
▶Photodiodesareoftenusedforaccuratemeasurementoflightintensityin
scienceand industry.
▶Theyarealsowidelyusedinvariousmedicalapplications,suchasdetectorsfor
computertomography,instrumentstoanalyzesamples,andpulseoximeters.