UNIT-I_Junction_capacitance__Breakdown_mechanisms.ppt

Transition capacitance and diffusion capacitance of pn junction diode is discussed. Breakdown mechanisms such as zener and avalanche is explained

1. ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY &
SCIENCES(A)
Department of Electronics and Communication Engineering
ECE 125 Basic Electronics Engineering
Academicyear :2022-23
Class&Section :1/4ECE-A
NameoftheFaculty:Mr.D.AnilPrasad
ANIL PRASAD DADI/ECE/ANITS

2. UNIT-I(Semiconductor Diodes)
•FermilevelinIntrinsic&Extrinsicsemiconductors.Mass-Actionlaw.
Mobilityandconductivity,Halleffect,Generationandrecombination
ofcharges,Driftanddiffusioncurrent,Bandstructureofopen-circuit
p-njunction,V-Icharacteristics,transitionanddiffusioncapacitance,
reverserecoverytime,Avalancheandzenerbreakdown,zener
diodes,LightEmittingDiodes.
ANIL PRASAD DADI/ECE/ANITS

3. Contents
•Junctioncapacitance
–Transitioncapacitance
–Diffusioncapacitance
•Breakdownmechanisms
–Zenerbreakdown
–Avalanchebreakdown
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4. Transition capacitance
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•Transitioncapacitanceexistswhenthe
pnjunctionisreversebiased.
•ChangeinV
R(eitherincreaseor
decreaseofV
R)changeswidthof
depletionregion
•IncreaseinV
Rincreasesdepletionregi
•ononeitherside
•DecreaseinV
Rdecreasesdepletion
regiononeitherside

5. Transition capacitance
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•ChangeinV
Rchangeschargeon
eithersideofthedepletionregion
•WhenRBacrosspnjunctionis
changedthendepletionwidthand
depletionchargealsogetchanged.
Thischangeindepletionchargew.r.t
changeinappliedvoltageisknownas
acapacitancecalledjunction
capacitanceortransitioncapacitance,
C
T

6. Transition capacitance
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7. Diffusion capacitance
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•Thiscapacitanceexistswhenpnjunctionisforwardbiased.
•Thereisnetstorageofelectronsinp-sideandnetstorageofholes
inn-sideminoritychargecarrierchargeisstored(electronsinp-side
andholesinn-side)
•InaFBPNjunctionholesareinjectedfrompton-sideandelectrons
areinjectedfromn-sidetop-sidecontinuously.Thereforethereis
netstorageofminoritychargecarrieracrossthejunction.This
storageofchargeisagainacapacitancecalleddiffusion
capacitance

8. Diffusion capacitance
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9. Breakdown mechanisms
•Zenerbreakdown:Itoccursinrelativelyheavilydopedabruptpnjunction
atbreakdownvoltagesV
Z<6V
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•IncreaseinV
Rresultsin
Breakdownofdiodes
•V
Ziscalledbreakdown
voltage
•Internalelectricfieldis
veryhighinitiallybecause
dopingishighinaheavily
dopedPNjunction

10. Breakdown mechanisms
•IfV
RisincreasedbeyondV
Zthenthenetelectricfieldinsidedepletion
regionisverylarge.
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•Thishighvalueofelectricfieldis
sufficientenoughtobreakthe
covalentbondsinsidethe
depletionregion.Hencecarrier
concentrationincreasesrapidly
insidedepletionregion.Hence
currentincreasesrapidly.This
mechanismofbreakdownis
calledZener breakdown
mechanism.

11. Breakdown mechanisms
•Zenerbreakdownvoltageisnegativetemperaturecoefficient
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•Iftemperatureisincreasedthen
breakdownoccursatavoltage
lessthanV
Z.SinceZener
breakdownisduetosudden
breakingofcovalentbonds
whichbecomeseasywith
increaseintemperature.

12. Breakdown mechanisms
•Avalanchebreakdown:Itoccursinrelativelylightlydopedabruptpn
junctionatbreakdownvoltagesV
Z>6V
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•IncreaseinV
Rresultsin
Breakdownofdiodes
•V
Ziscalledbreakdown
voltage
•Internalelectricfieldis
verysmallbecausedoping
islessinalightlydoped
PNjunction

13. Breakdown mechanisms
•IfV
RisincreasedbeyondV
Zthenhighenergyelectronscollideswithatomandable
toknockoutouterorbitelectronsofsiliconatoms.Hencesecondaryelectronsare
generated
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•Thegeneratedelectronsinturn
generatesfurtherelectronswhen
collidingwithanotheratoms.This
processcontinues,hugenoof
chargecarriersaregenerated.
Thereforehugecurrentflowsanditis
called the avalanche
breakdown(Avalanche means
multiplication).

14. Breakdown mechanisms
•Sinceenergeticelectronsarecollidingwithatomsandgeneratingsecondary
electrons.HencethisprocessiscalledimpactionizationprocesswhereasZener
breakdownisfieldionizationprocess.
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•Avalanchebreakdownispositivetemperature
coefficient.
•Sinceavalanchebreakdownisduetoimpact
ionizationwhereanenergeticelectroncollides
withatomandknockoutouterorbitelectrons.
Whentemperatureincreasesoscillationofatoms
atlatticepointincreases.Hencecollisionbetween
electronsandatomisnotsoimpactfulthatitcan
generatesecondaryelectrons.Hencetoget
breakdownweneedtoapplyahighervoltage.

15. Distinguish between Zener and Avalanche
Breakdown mechanisms
•Zener breakdown
1.Existsinheavilydopedpnjunction
2.V
Z<6V
3.E
intisverylarge
4.Breakdownisduetobreakingof
covalentbonds
5.Negativetemperaturecoefficient
6.Fieldionizationprocess
•Avalanche breakdown
1.Existsinlightlydopedpnjunction
2.V
Z>6V
3.E
intisverysmall
4.Breakdownisduetocollisionof
electronswithatoms
5.Positivetemperaturecoefficient
6.Impactionizationprocess
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16. References
ANIL PRASAD DADI/ECE/ANITS
•RobertLBoylestad,ElectronicDevicesAndCircuitTheory,Prentice
Hall,seventhedition,2021
•JacobMillmanandChristosHalkias,ElectronicsDevicesand
Circuits,Blackedition,October,2017

17. Thank you
for listening
ANIL PRASAD DADI/ECE/ANITS