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PN junction.... Foundation of semi-conductor componentsSemi-conductor:

empty

11 11 11 ILL 33psibe spSi Si Si as

I I l 11 is

empty conduction bandgapto all e areftp.zef µ chemicalpotentialinthevalence band

Otto small eµvalenceband

fraction e 8972kt

of e are in the conductionbandThe same fractionofholes are inthevalenceband

KT is 25mW roomtemperature so thisfractionis verysmall

P type semi-conductorN-type semiconductor

a si MEEEGalliumhasonly3valenceelectrons

and is an electronacceptor

When an electronhops thevacancy hole moves in the

opposite direction in thep typesemi conductor

s PIE'sif I'm'Efer Fam

A phosphorous atomhas 5 valence 71

electrons The extra one can easily M Fbreak loose

P is an electron donor

e can easily move in a n type s c

PN junction: when electrons meets holes

tRandomwalk diffusion of e and holes

I

tI K

depletionregimenegatively changed on The left sidepositive charged on theright side

Depletonregime hasveryfewmobilechange carriers

Whenequilibrium is established Diffusionbalancedbyelectricalforce

Idiffusion Idrift jelectron

energy xfburrier topreventfurtherdiffusion

n0

barriertopreventholediffusion

Forward biased: depletion regime narrowed Reverse bias: depletion regime widened

47ftl K depletion

efIfyq Er µIdiffusion Idrift T

V

Emdemand

EFoLIdiffusion 7drift

Current through a biased diode:

A simplifiedderivation

Jdiffusion Idrift I Iholenee'M T jdriftdm me eM T j drift

Notethat je o when to NeeM II driftje jdriftCe l

I Io e T l

A

whathappened here

I

Avalanche regime

vtdepletion

e

po d

spontaneous e holepaingenerationor photon induced E hole pairin photodiodes