AE UNIT I.ppt

Students can know about the PN Junction Diode and few types of special diode

1. UNIT I
PN JUNCTION AND SPECIAL DIODE
ANALOG ELECTRONICS

2. Introduction
Types of extrinsic semiconductors
N-type -Electrons are majority charge carriers and
while holes are minority charge carriers
P-type –Holes are majority charge carriers and
while electrons are minority charge carriers
These two types of materials are chemically
combined with a special fabrication technique to
form a p-n junction. Such a p-n junction form an
electronics device called diode.

3. PN JUNCTION DIODE
Join a piece of P-type semiconductor to a piece of N-
type semiconductor such that the crystal remains
contionues at the boundary.
PN junction forms very useful device and is called a
seiconductor diode or PN junction diode as shown
Holes and Electrons –mobile charge carriers
Positive and negative ions –immobile charges

4. Formation of Depletion Layer in a
PN junction
In P-region has holes(Majority carrier) and negatively
charged impurity atoms, called negative ions (acceptor
ions)
In N-region has free electrons(Majority carrier) and
positively charged impurity atoms, called negative ions
(donor ions)
Holes and Electrons are the mobile charge carriers.
Positive and Negative ions are immobile charges and it
do not in conduction.
As soon as the PN junction is formed, some of the holes
in P-region and free electrons in N-region diffuse each
other and disappear due to recombination.
The region containing the uncovered acceptor and
donor ions is called depletion region. The depletion

5. Formation of Depletion Layer in a PN
junction

6. V-I characteristics under forward
biased condition
Cut in voltage (V0) 0.3 v for Germanium
0.7 v for silicon
At cut in voltage, potential barrier is overcome and current through
the junction starts increases rapidly.

7. When positive terminal of the battery is connected to
the P-type and negative terminal to the N-type of the PN
junction diode, the bias is known as forward bias.
Applied positive potential repels the holes in P-type
region so that holes move towards the junction and the
applied negative potential repels the electrons in the N-
type region and the electrons moves towards the
junction.
When applied potential is more then disappear
depletion region.
When VF<V0then forward current almost zero.
When VF>V0then large forward current flows. Here
potential barrier or depletion layer is disappear.

8. V-I characteristics under reverse
biased condition

9. When negative terminal of the battery is connected to the P-
type and positive terminal to the N-type of the PN junction
diode, the bias is known as reverse bias.
Under reverse bias, holes of majority carrier in P-type region
move towards negative terminal of the battery and electrons
of majority carrier in N-type region move towards positive
terminal of the battery.
Which is increases the depletion layer or potential barrier.
Ideally there is no current flows. Practically very small current
of order of few microampere flows.
Minority carrier in P-region and N-region trying to flows
across junction and give rise to small reverse current. This
current known as Reverse saturation current.
When large reverse voltage applied then sufficient energy to
dislodge valance electron. Then conduction takes place and
which voltage called breakdown voltage

10. Ideal Diode
Thefirstelectronicdevicetobeintroducediscalledthe
diode.Itisthesimplestofsemiconductordevicesbut
playsaveryvitalroleinelectronicsystems,having
characteristicsthatcloselymatchthoseofasimple
switch.Itwillappearinarangeofapplications,
extendingfromthesimpletotheverycomplex.
Inadditiontothedetailsofitsconstructionand
characteristics,theveryimportantdataandgraphsto
befoundonspecificationsheetswillalsobecoveredto
ensureanunderstandingoftheterminologyemployed
andtodemonstratethewealthofinformationtypically
availablefrommanufacturers.

11. Thetermidealwillbeusedfrequentlyinthistextas
newdevicesareintroduced.Itreferstoanydeviceor
systemthathasidealcharacteristics—perfectinevery
way.
Itprovidesabasisforcomparison,anditreveals
whereimprovementscanstillbemade.Theideal
diodeisatwo-terminaldevicehavingthesymboland
characteristicsshowninfigures

12. Ideally,adiodewillconductcurrentinthedirection
definedbythearrowinthesymbolandactlikeanopen
circuittoanyattempttoestablishcurrentinthe
oppositedirection.
Thecharacteristicsofanidealdiodearethoseofa
switchthatcanconductcurrentinonlyonedirection.
Oneoftheimportantparametersforthediodeisthe
resistanceatthepointorregionofoperation.
Ifweconsidertheconductionregiondefinedbythe
directionofI
DandpolarityofV
Dinfigure.(upper-right
quadrantoffigure.),wewillfindthatthevalueofthe
forwardresistance,R
F,asdefinedbyOhm’slawis

13. Consider the region of negatively applied potential (third
quadrant) of figure.

14. DIODE CURRENT EQUATION
The diode current equation relating the voltage V and
current I is given by
Where
 I = diode current
Io = diode reverse saturation current at room temperature
 V = external voltage applied to the diode
 ղ = a constant, 1 for germanium and 2 for silicon
V
T = kT/q = T/11600, volt-equivalent of temperature, i.e
thermal voltage
 Where
 k = Boltzmann’s constant (1.38066 * 10
-23
) J/ K
 q = charge of electron (1.60219 * 10
-19
C)
 T = temperature of the diode junction (K) = (C+ 273 )

15. At room temperature, (T= 300K ), V
T= 26mV.
Substituting this value in the current equation, we get
If the value of applied voltage is greater than unity, then
the equation of diode current for germanium,
and for silicon
•when the diode is reverse biased, its current may be
obtained by changing the sign of the applied voltage V.
thus, the diode current with reverse bias is
•If V>>V
T, then the term, therefore I ≈ I
O,
termed as reverse saturation current, which is valid as
long as external voltage is below the breakdown voltage.

16. Drift Currents
Whenanelectricfieldisappliedacrossthe
semiconductormaterial,thechargecarriersattaina
certaindriftvelocityv
d,whichisequaltotheproductof
themobilityofthechargecarriersandtheapplied
electricfieldintensity,E.
Theholesmovetowardsthenegativeterminalofthe
batteryandelectronmovetowardsthepositiveterminal.
Thiscombinedeffectofmovementofthechargecarriers
constitutesacurrentknownasthedriftcurrent.
Thusthedriftcurrentisdefinedastheflowofelectric
currentduetothemotionofthechargecarriersunder
theinfluenceofanexternalelectricfield.

17. The equation for the drift current density, Jn, due to free
electron is given by
J
n= qnμ
nE A/cm
2
and the drift current density, Jp due to holes is
given by
Jp = qpμ
pE A/cm
2
Where
n= number of free electrons per cubic centimeter
p= number of holes per cubic centimeter
μ
n= mobility of electrons in cm
2
/V-s
μ
p = mobility of holes in cm
2
/V-s
E= applied electric field intensity in V/cm
Q= charge of an electron = 1.6 * 10
-19
C

18. Diffusion Currents
Itispossibleforanelectriccurrenttoflowina
semiconductorevenintheabsenceoftheapplied
voltageprovidedaconcentrationgradientexistsinthe
material.
Aconcentrationgradientexistsifthenumberofeither
electronsorholesisgreaterinoneregionofa
semiconductorascomparedtotherestoftheregion.
Inasemiconductormaterial,thechargecarriershave
thetendencytomovefromtheregionofhigher
concentrationtothatoflowerconcentrationofthesame
typeofchargecarriers.
Thus,themovementofchargecarrierstakesplace
resultinginacurrentcalleddiffusioncurrent.

19. Asindicatedinfigure,theholesconcentrationp(x)ina
semiconductorbarvariesfromahighvaluetoalow
valuealongthex-axisandisconstantinthey-andz-
directions.
Diffusioncurrentdensityduetoholes,Jpisgivenby

20. Sincetheholedensityp(x)decreaseswithincreasingx
asshowninfigure,dp/dxisnegativeandtheminus
signintheaboveequationisneededinorderthatJp
hasapositivesigninthepositivex-direction.
Diffusioncurrentdensityduetothefreeelectrons,Jnis
givenby

21. Wheredn/dxanddp/dxaretheconcentrationgradients
forelectronsandholesrespectively,inthex-direction
andD
nandD
parethediffusioncoefficientsexpressed
incm
2
/sforelectronsandholes,respectively.
Total Current:
Thetotalcurrentinasemiconductoristhesumofdrift
currentanddiffusioncurrent.Therefore,foraP-type
semiconductor,thetotalcurrentperunitarea,i.ethetotal
currentdensityisgivenby
Jp = qpμ
pE -
Similarly, the total current density for an N-type
semiconductor is given by
Jn = qpμ
nE +

22. Effect of temperature on PN junction
diodes
Theriseintemperatureincreasesthegenerationof
electro-holepairsinsemiconductorsandincreases
theirconductivity.
Asaresult,thecurrentthroughthePNjunctiondiode
increaseswithtemperatureasgivenbythediode
currentequation,
Atroomtemperature,i.eat300K,thevalueofbarrier
voltageorcut-involtageisabout0.3Vforgermanium
and0.7Vforsilicon.
Thebarriervoltageistemperaturedependantandit
decreasesby2mV/forbothgermaniumandsilicon.
Thisfactmaybeexpressedinmathematicalform,
whichisgivenby

23. Where I
01= saturation current of diode at temperature
(T
1) and I
02= saturation current of diode at temperature
(T
2).
Thefigureshowstheeffectofincreasedtemperatureon
thecharacteristicscurveofaPNjunctiondiode.A
germaniumdiodecanbeuseduptoamaximumof75
degandsilicondiodetoamaximumof175deg.

24. TRANSITION AND DIFFUSION
CAPACITANCE
Electronicdevicesareinherentlysensitivetoveryhigh
frequencies.Mostshuntcapacitiveeffectsthatcanbe
ignoredatlowerfrequenciesbecausethereactance
X
C=½pifCisverylarge(open-circuitequivalent).
This,however,cannotbeignoredatveryhigh
frequencies.X
Cwillbecomesufficientlysmallduetothe
highvalueofftointroducealow-reactance“shorting”
path.
Inthep-nsemiconductordiode,therearetwocapacitive
effectstobeconsidered.
Bothtypesofcapacitancearepresentintheforward-
andreverse-biasregions,butonesooutweighsthe
otherineachregionthatweconsidertheeffectsofonly
oneineachregion.

25. Inthereverse-biasregionwehavethetransition-or
depletion-regioncapacitance(C
T),whileintheforward-
biasregionwehavethediffusion(C
D)orstorage
capacitance.
Recallthatthebasicequationforthecapacitanceofa
parallel-platecapacitorisdefinedbyC=εA/d,whereε
isthepermittivityofthedielectric(insulator)between
theplatesofareaAseparatedbyadistanced.
Inthereverse-biasregionthereisadepletionregion
(freeofcarriers)thatbehavesessentiallylikean
insulatorbetweenthelayersofoppositecharge.Since
thedepletionwidth(d)willincreasewithincreased
reverse-biaspotential,theresultingtransition
capacitancewilldecrease,asshowninfigure.

26. Transition and diffusion capacitance versus
applied bias for a silicon diode

27. Including the effect of the transition or diffusion
capacitance on the semiconductor diode
Thecapacitiveeffectsdescribedabovearerepresented
byacapacitorinparallelwiththeidealdiode,asshown
infigure.
Forlowormid-frequencyapplications(exceptinthe
powerarea),however,thecapacitorisnormallynot
includedinthediodesymbol.

28. JUNCTION DIODE
CHARACTERISTICS
Diodesareoftenusedinaswitchingmode.Whenthe
appliedbiasvoltagetothePNjunctiondiodeissuddenly
reversedintheoppositedirection,thedioderesponse
reachesasteadystateafteranintervaloftime,called
therecoverytime.
Theforwardrecoverytime,t
fisdefinedasthetime
requiredforforwardvoltageorcurrenttoreacha
specifiedvalue(timeintervalbetweentheinstantof10%
diodevoltagetotheinstantthisvoltagereacheswithin
10%ofitsfinalvalue)afterswitchingdiodefromits
reversetoforwardbiasedstate.
Fortunately,theforwardrecoverytimepossesnoseries
problem.Therefore,onlythereverserecoverytime,t
rr
hastobeconsideredinpracticalapplications.

29. WhenthePNjunctionisforwardbiased,theminority
electronconcentrationintheP-regionisapproximately
linear.Ifthejunctionissuddenlyreversebiased,att
1,
thenbecauseofthisstoredelectroniccharges,the
reversecurrent(I
R)isinitiallyofthesamemagnitudeas
theforwardcurrent(I
F).
Thediodewillcontinuetoconductuntiltheinjectedor
excesscarrierminoritydensity(p=-p
o)or(n-n
o)has
droppedtozero.
However,asthestoredelectronsareremovedintothe
N-regionandthecontact,theavailablechargequickly
dropstoanequilibriumlevelandasteadycurrent
eventuallyflowscorrespondingtothereversebias
voltageasshowninfigure(c).
As shown in figure (b), the applied voltage V
i=V
Ffor the
time up to t
1is in the direction to forward bias the diode.
The resistance Ris large so that the drop across Ris

30. ThenthecurrentisI= .Thenattimet=t
1,theinput
voltageissuddenlyreversedtothevalueof–V
R.
Duetothereason,thecurrentdoesnotbecomezero
andhasthevalueI= untilthetimet=t
2.
Att=t
2,whentheexcessminoritycarriershavereached
theequilibriumstate,themagnitudeofthediodecurrent
startstodecreasesasshowninfigure(d).
Duringthetimeintervalfromt
1tot
2,theinjectedminority
carriershaveremainedstoredandhencethisintervalis
calledthestoragetime(t
s).
Aftertheinstantt=t
2thediodegraduallyrecoversand
ultimatelyreachesthesteadystate.

31. Thetimeintervalbetweent
2andtheinstantt
3whenthe
diodehasrecoverednominallyiscalledthetransition
time,t
r.
Therecoveryissaidtohavecompleted
(i)wheneventheminoritycarriersremotefromthe
junctionhavediffusedtothejunctionandcrossedit,and
(ii)whenthejunctiontransitioncapacitance,C
Tacrossthe
reversebiasedjunctionhasgotchargedthroughthe
externalresistorR
Ltothevoltage-V
R.
Thereverserecoverytime(orturnofftime)ofadiode,
t
rristheintervalfromthecurrentreversalatt=t1until
thediodehasrecoveredtoaspecifiedextentinterms
eitherofthediodecurrentorofthedioderesistancei.e
t
rr=t
s+t
r
For a commercial switching type diodes the reverse
recovery time t
rr, ranges from 1 ns up to as high as 1μs.
This switching time obviously limits the maximum

32. IfthetimeperiodoftheinputsignalissuchthatT=2.t
rr,
thenthediodeconductsasmuchinreverseasinthe
forwarddirection.Henceitdoesnotbehaveasaone
waydevice.
Inordertominimizetheeffectofthereversecurrent,
thetimeperiodoftheoperatingfrequencyshouldbea
minimumofapproximately10timest
rr.Forexample,ifa
diodehast
rrof2ns,itsmaximumoperatingfrequencyis
Thet
rr,canbereducedbyshorteningthelengthofthe
P-regioninaPNjunctiondiode.
Thestoredchargeandconsequentlytheswitchingtime
canalsobereducedbyintroductionofgoldimpurities
intothejunctiondiodebydiffusion.

34. ZENER DIODE
Zener diode is a reverse biased heavily doped PN
junction diode which operates in breakdown region.
The reverse breakdown of a PN junction may occurs
either due to zener effect or avalanche effect.
Zener effect dominates at reverse voltage less than 6V
and avalanche effect dominates above 6V
For zener diodes, Silicon is preferred to Ge because of
its higher temperature and current capability.
Symbol of zener diode as shown

35. Forward biasing zener diode
Anode connected to positive terminal of battery and
cathode connected to negative terminal of battery.
Its behavior identical to F.B diode
General zener diode not used in F.B condition

36. Reverse biasing zener diode
Cathode connected to positive terminal of battery and
Anode connected to negative terminal of battery.
Its operation is differ from that of diode.
Zener diode in reverse biased condition is used as a
voltage regulator.

37. V-I characteristics of zener diode
V-I characteristics of zenerdiode can be divided into
two parts
Forward characteristics
Reverse characteristics
Forward characteristics
The characteristics as shown
It is almost identical to the as a PN junction diode

38. Reverse characteristics
Reverse voltage increases, initially small reverse saturation current I0, in order
of μA will flow. This current due to thermally generated minority carriers.
At particular reverse voltage, reverse current increase sharp and suddenly.
This indication that breakdown occurs.
This breakdown voltage is called as zener breakdown voltage or zener voltage
and it is denoted by Vz
After breakdown Vz remains constant and further increase only reverse zener
current.
For controlling zener current put R and which avoid excess heat.

39. Application
Zener diode is used as a voltage regulator
Zener diode is used as a peak clipper in wave
shaping circuits
Zener diode is used as a fixed reference voltage
in transistor biasing circuits.
Zener diode is used for meter protection against
damage from accidental application of excessive
voltage.

40. Breakdown mechanism
If reverse bias voltage applied to a PN junction is
increased, a point will reach when the junction
breakdown and reverse current rises sharply to a value
limited only by the external resistance connected in
series.
This specific value of reverse bias voltage is called
breakdown voltage
The breakdown voltage depends on width of depletion
layer. This width of depletion layer depends on doping
level.
Process of causes junction breakdown due to increase
in reverse bias voltage as
Zener breakdown
Avalanche breakdown

41. Zener breakdown
It observed when Vz<6V. If apply Vz then strong electric
field appear across narrow depletion region.
Value of electric field as 3*10^5v/cm.
Due to this electric field pull valance electron into
conduction band to breaking covalent bond.
So large no of free electron causes to reverse current
through zener diode and breakdown occurs due to
zener effect.

42. Avalanche Breakdown
It observed when Vz>6V.
Reverse bias condition conduction due to only in
minority carrier.
Reverse voltage increase, then accelerates minority
carrier and causes to increase K.E
Accelerates minority carrier collide with stationary atom
and K.E causes valance electron present in covalent
bond.
Now valance electron breakdown covalent bond and
become free for conduction.
Now increase more no free electrons collide. This
phenomenon is called as avalanche multiplication.

43. In short time large no of free minority electrons and
holes available for conduction and which causes self
sustained multiplication process called ‘Avalanche
effect’
Large reverse current starts flowing through zener diode
and occur avalanche breakdown.

44. ZENER REGULATOR
Fig shows circuit of zener diode shunt regulator.
Load connected parallel to zener diode and so called
shunt regulator.
Rs limit the current and V0taken across RL
For proper operation Vin>Vz

46. Operation:
The output voltage is mainly varied due to following two
reason
Regulation with varying input voltage
Regulation with varying load current
Regulation with varying input voltage

47. Assume RLis fixed and Vinvaries
If Vin↑, I↑. But IL=const. as Vz=constant. Hence Iz↑, to
keep IL=const.
If Vin↓, I↓. But keep IL=const. Iz↓. As long as Iz is
between Izmax and Izmin, V0 remains const.

48. Regulation with varying load current
Assume Vin=const, vary ILand RL
Vary RL, then current flows through it vary
Iinand voltage across Rs const.
When RL↓, then IL↑, causes Iz↓
VL=const due rise in current equal to drop in resistance
(V=IR)

49. The current limiting resistor (Rs) must be properly
selected to fulfill the following requirements:
1.When the input voltage is minimum and the load
current is maximum, sufficient current must be
supplied to keep the zener diode within its breakdown
region

50. 2. When the input voltage is maximum and the
load current is minimum, the zenercurrent must
not exceed the maximum rated value.

51. Value of load Resistance RL

52. LIGHT EMITTING DIODE
Theincreasinguseofdigitaldisplaysincalculators,
watches,andallformsofinstrumentationhas
contributedtothecurrentextensiveinterestin
structuresthatwillemitlightwhenproperlybiased.
Thetwotypesincommonusetodaytoperformthis
functionarethelight-emittingdiode(LED)andthe
liquid-crystaldisplay(LCD).
Asthenameimplies,thelight-emittingdiode(LED)isa
diodethatwillgiveoffvisiblelightwhenitisenergized.
Inanyforward-biasedp-njunctionthereis,withinthe
structureandprimarilyclosetothejunction,a
recombinationofholesandelectrons.
Thisrecombinationrequiresthattheenergypossessed
bytheunboundfreeelectronbetransferredtoanother

53. Inallsemiconductorp-njunctionssomeofthisenergy
willbegivenoffasheatandsomeintheformofphotons.
Insiliconandgermaniumthegreaterpercentageisgiven
upintheformofheatandtheemittedlightis
insignificant.
Inothermaterials,suchasgalliumarsenidephosphide
(GaAsP)orgalliumphosphide(GaP),thenumberof
photonsoflightenergyemittedissufficienttocreatea
veryvisiblelightsource.
Theprocessofgivingofflightbyapplyinganelectrical
sourceofenergyiscalledelectroluminescence.

54. Process of electroluminescence in
the LED and its Graphic symbol

55. Theconductingsurfaceconnectedtothep-materialis
muchsmaller,topermittheemergenceofthemaximum
numberofphotonsoflightenergy.
Noteinthefigurethattherecombinationoftheinjected
carriersduetotheforward-biasedjunctionresultsin
emittedlightatthesiteofrecombination.
Theremay,ofcourse,besomeabsorptionofthe
packagesofphotonenergyinthestructureitself,buta
verylargepercentageareabletoleave,asshowninthe
figure.

56. LIQUID-CRYSTAL DISPLAYS
Theliquid-crystaldisplay(LCD)hasthedistinct
advantageofhavingalowerpowerrequirementthan
theLED.
Itistypicallyintheorderofmicrowattsforthedisplay,
ascomparedtothesameorderofmilliwattsforLEDs.
Itdoes,however,requireanexternalorinternallight
sourceandislimitedtoatemperaturerangeofabout
0°to60°C.Lifetimeisanareaofconcernbecause
LCDscanchemicallydegrade.
Thetypesreceivingthemajorinteresttodayarethe
field-effectanddynamic-scatteringunits.
Aliquidcrystalisamaterial(normallyorganicforLCDs)
thatwillflowlikealiquidbutwhosemolecularstructure
hassomepropertiesnormallyassociatedwithsolids.

57. For the light-scattering units, the greatest interest is in
the nematic liquid crystal, having the crystal structure
shown in figure
Theindividualmoleculeshavearodlikeappearanceas
showninthefigure.
Theindiumoxideconductingsurfaceistransparent,and
undertheconditionshowninthefigure,theincident
lightwillsimplypassthroughandtheliquid-crystal
structurewillappearclear.

58. Ifavoltage(forcommercialunitsthethresholdlevelis
usuallybetween6and20V)isappliedacrossthe
conductingsurfaces,asshowninfigure.
Themoleculararrangementisdisturbed,withtheresult
thatregionswillbeestablishedwithdifferentindicesof
refraction.
Theincidentlightisthereforereflectedindifferent
directionsattheinterfacebetweenregionsofdifferent
indicesofrefractionwiththeresultthatthescattered
lighthasafrosted-glassappearance.

59. A digit on an LCD display may have the segment
appearance shown in figure
Ifthenumber2wererequired,theterminals8,7,3,4,
and5wouldbeenergized,andonlythoseregions
wouldbefrostedwhiletheotherareaswouldremain
clear.

60. Thefield-effectortwistednematicLCDhasthesame
segmentappearanceandthinlayerofencapsulated
liquidcrystal,butitsmodeofoperationisverydifferent.
Similartothedynamic-scatteringLCD,thefield-effect
LCDcanbeoperatedinthereflectiveortransmissive
modewithaninternalsource.Thetransmissivedisplay
appearsinfigure.

61. The reflective-type field-effect LCD is shown in figure.
Inthiscase,thehorizontallypolarizedlightatthefarleft
encountersahorizontallypolarizedfilterandpasses
throughtothereflector,whereitisreflectedbackinto
theliquidcrystal,bentbacktotheothervertical
polarization,andreturnedtotheobserver.Ifthereisno
appliedvoltage,thereisauniformlylitdisplay.The
applicationofavoltageresultsinaverticallyincident
lightencounteringahorizontallypolarizedfilteratthe
left,whichitnotbeabletopassthroughandwillbe

62. SCHOTTKY BARRIER (HOT -
CARRIER) DIODES
Inrecentyears,therehasbeenincreasinginterestina
two-terminaldevicereferredtoasaSchottky-barrier,
surface-barrier,orhot-carrierdiode.
Itsareasofapplicationwerefirstlimitedtotheveryhigh
frequencyrangeduetoitsquickresponsetime
(especiallyimportantathighfrequencies)andalower
noisefigure(aquantityofrealimportanceinhigh-
frequencyapplications).
Inrecentyears,however,itisappearingmoreandmore
inlow-voltage/high-currentpowersuppliesandac-to-dc
converters.Otherareasofapplicationofthedevice
includeradarsystems,
SchottkyTTLlogicforcomputers,mixersanddetectors
incommunicationequipment,instrumentation,and

63. Itsconstructionisquitedifferentfromtheconventionalp-
njunctioninthatametalsemiconductorjunctionis
createdsuchasshowninfigure.
Thesemiconductorisnormallyn-typesilicon(although
p-typesiliconissometimesused),whileahostof
differentmetals,suchasmolybdenum,platinum,
chrome,ortungsten,areused.

64. Differentconstructiontechniqueswillresultinadifferent
setofcharacteristicsforthedevice,suchasincreased
frequencyrange,lowerforwardbias,andsoon.
Prioritiesdonotpermitanexaminationofeach
techniquehere,butinformationwillusuallybeprovided
bythemanufacturer.
Ingeneral,however,Schottkydiodeconstructionresults
inamoreuniformjunctionregionandahighlevelof
ruggedness.
Inbothmaterials,theelectronisthemajoritycarrier.In
themetal,thelevelofminoritycarriers(holes)is
insignificant.
Whenthematerialsarejoined,theelectronsinthen-
typesiliconsemiconductormaterialimmediatelyflowinto
theadjoiningmetal,establishingaheavyflowofmajority
carriers.

65. Sincetheinjectedcarriershaveaveryhighkinetic
energylevelcomparedtotheelectronsofthemetal,they
arecommonlycalled“hotcarriers.”
Intheconventionalp-njunction,therewastheinjection
ofminoritycarriersintotheadjoiningregion.
Heretheelectronsareinjectedintoaregionofthesame
electronplurality.
Schottkydiodesarethereforeuniqueinthatconduction
isentirelybymajoritycarriers.
Theheavyflowofelectronsintothemetalcreatesa
regionnearthejunctionsurfacedepletedofcarriersin
thesiliconmaterial—muchlikethedepletionregionin
thep-njunctiondiode.

66. Theadditionalcarriersinthemetalestablisha“negative
wall”inthemetalattheboundarybetweenthetwo
materials.
Thenetresultisa“surfacebarrier”betweenthetwo
materials,preventinganyfurthercurrent.
Thatis,anyelectrons(negativelycharged)inthesilicon
materialfaceacarrier-freeregionanda“negativewall”
atthesurfaceofthemetal.
Theapplicationofaforwardbiasasshowninthefirst
quadrantoffigure.willreducethestrengthofthe
negativebarrierthroughtheattractionoftheapplied
positivepotentialforelectronsfromthisregion.
Comparisonofcharacteristicsofhot-carrierandp-n
junctiondiodesasshown

67. Theresultisareturntotheheavyflowofelectrons
acrosstheboundary,themagnitudeofwhichis
controlledbytheleveloftheappliedbiaspotential.
ThebarrieratthejunctionforaSchottkydiodeisless
thanthatofthep-njunctiondeviceinboththeforward-
andreverse-biasregions.

68. The equivalent circuit for the device (with typical values)
and a commonly used symbol appear in figure.
Anumberofmanufacturersprefertousethestandard
diodesymbolforthedevicesinceitsfunctionis
essentiallythesame.
TheinductanceL
PandcapacitanceC
Parepackage
values,andr
Bistheseriesresistance,whichincludes
thecontactandbulkresistance.

69. VARACTOR (VARICAP)
DIODES
Varactor[alsocalledvaricap,VVC(voltage-variable
capacitance),ortuning]diodesaresemiconductor,
voltage-dependent,variablecapacitors.
Theirmodeofoperationdependsonthecapacitance
thatexistsatthep-njunctionwhentheelementis
reverse-biased.
Underreverse-biasconditions,itwasestablishedthat
thereisaregionofuncoveredchargeoneithersideof
thejunctionthattogethertheregionsmakeupthe
depletionregionanddefinethedepletionwidthW
d.
Thetransitioncapacitance(C
T)establishedbythe
isolateduncoveredchargesisdeterminedby

70. Asthereverse-biaspotentialincreases,thewidthofthe
depletionregionincreases,whichinturnreducesthe
transitioncapacitance.
Thecharacteristicsofatypicalcommerciallyavailable
varicapdiodeappearinfigure.
NotetheinitialsharpdeclineinC
Twithincreasein
reversebias.ThenormalrangeofV
RforVVCdiodesis
limitedtoabout20V.

71. Intermsoftheappliedreversebias,thetransition
capacitanceisgivenapproximatelyby
K = constant determined by the semiconductor material
and construction technique
V
T= knee potential
V
R = magnitude of the applied reverse-bias potential
n =1/ 2 for alloy junctions and 1/ 3 for diffused junctions
In terms of the capacitance at the zero-bias condition
C(0), the capacitance as a function of V
Ris given by

72. Thesymbolsmostcommonlyusedforthevaricap
diodeandafirstapproximationforitsequivalentcircuit
inthereverse-biasregionareshowninFig.
Sinceweareinthereverse-biasregion,theresistance
intheequivalentcircuitisverylargeinmagnitude—
typically1MΩorlarger—whileR
S,thegeometric
resistanceofthediode,is,asindicatedinfigurevery
small.
ThemagnitudeofC
Twillvaryfromabout2to100pF
dependingonthevaricapconsidered.

73. In figure, the varactor diode is employed in a tuning
network.
That is, the resonant frequency of the parallel L-C
combination is determined by
The selected frequencies of the tuned network are then
passed on to the high input amplifier for further
amplification.

74. PHOTODIODES
Theinterestinlight-sensitivedeviceshasbeen
increasingatanalmostexponentialrateinrecentyears.
Theresultingfieldofoptoelectronicswillbereceivinga
greatdealofresearchinterestaseffortsaremadeto
improveefficiencylevels.
Throughtheadvertisingmedia,thelaypersonhas
becomequiteawarethatlightsourcesofferaunique
sourceofenergy.
Thisenergy,transmittedasdiscretepackagescalled
photons,hasaleveldirectlyrelatedtothefrequencyof
thetravelinglightwaveasdeterminedbythefollowing
equation:
W = hf joules
where h is called Planck’s constant and is equal to 6.624

75. Thefrequencyis,inturn,directlyrelatedtothe
wavelength(distancebetweensuccessivepeaks)ofthe
travelingwavebythefollowingequation
where =wavelength, meters
v= velocity of light, 3 * 10^8 m/s
f= frequency of the traveling wave, hertz
The wavelength is usually measured in angstrom units
(Å) or micrometers (μm),
Where 1 Å= 10
-10
m and 1 µm = 10
-6
m
Thenumberoffreeelectronsgeneratedineachmaterial
isproportionaltotheintensityoftheincidentlight.
Lightintensityisameasureoftheamountofluminous
fluxfallinginaparticularsurfacearea.

76. Luminous flux is normally measured in lumens (lm) or
watts. The two units are related by
1 lm = 1.496 * 10
-10
W
The light intensity is normally measured in lm/ft
2
,footcandles (fc), or W/m
2
, where
1 lm/ft
2
= 1 fc = 1.609* 10
-9
W / m
2
The photodiode is a semiconductor p-n junction device
whose region of operation is limited to the reverse-bias
region.
The basic biasing arrangement, construction, and symbol
for the device appear in Fig.

77. Thereversesaturationcurrentisnormallylimitedtoa
fewmicroamperes.
Itisduesolelytothethermallygeneratedminority
carriersinthenandptypematerials.

78. Theapplicationoflighttothejunctionwillresultina
transferofenergyfromtheincidenttravelinglightwaves
(intheformofphotons)totheatomicstructure,resulting
inanincreasednumberofminoritycarriersandan
increasedlevelofreversecurrent.
Thisisclearlyshowninfigure.fordifferentintensity
levels.
Thedarkcurrentisthatcurrentthatwillexistwithno
appliedillumination.Notethatthecurrentwillonlyreturn
tozerowithapositiveappliedbiasequaltoV
T.