Whati

sSchot
tkyDi
ode?
Theschot t
kydi odei
sat y
peofmet al–semiconduct orj
uncti
ondi ode,whichisalsoknownas
hot-
car r
ierdiode,lowv ol
tagediodeorschottkybar r
ierdiode.Theschot t
kydiodei sf
ormedby
thejunctionofasemi conductorwithamet al
.Schot tkydiodeoffersfastswit
chingacti
onand
hasal owf orwardvol
tagedr op.Aswear eawaret hatinaPNj uncti
ondiode,p-t
ypeandn-ty
pe
arejoinedt ogethert
of orm aPNj uncti
on.Whereas,inaSchot tkydiodemet al
sli
keplati
num or
aluminum ar eusedinsteadofPt ypesemiconductors

Whati
sSchot
tkyDi
odeSy
mbol
?
Thesymbol
fort
heSchottkybar
ri
erdi
odeisbasedar
oundthebasi
cdi
odeci
rcui
tsy
mbol
.The
ci
rcui
tsy
mboloftheSchott
kydi
odeisshowninthef
igur
e.

Schot
tkyDi
odeSy
mbol

V-
IChar
act
eri
sti
csofSchot
tkyDi
ode
TheV-Icharact
eri
sti
csofSchott
kydiodesareverymuchsimil
artothePNj uncti
ondiode.
Curr
entisthedependentv
ariabl
ewhilevolt
ageisthei
ndependentv
ariabl
eint heSchott
kydi
ode.
Theforwardvolt
agedropoftheSchottkydi
odeislowbetween0.2to0.3volts.
Wor
kingofaSchot
tkyDi
ode

 Theoper
ati
onr
eli
esont
hepr
inci
plet
hatt
heel
ect
ronsi
ndi
ff
erentmat
eri
alshav
e
di
ff
erentpot
ent
ial
ener
gy.

 N-
typesemi
conduct
orshav
ehi
gherpot
ent
ial
ener
gyt
hanel
ect
ronsofmet
als.

 Whent
heset
woar
ebr
oughti
ntocont
act
,ther
eisaf
lowofel
ect
ronsi
nbot
hdi
rect
ions
acr
osst
hemet
al-
semi
conduct
ori
nter
face.

 Av
olt
agei
sappl
i
edt
otheSchot
tkysot
hatt
hemet
ali
sposi
ti
vewhencompar
edt
othe
semi
conduct
or.

 Thev
olt
ageopposest
hebui
l
t-i
npot
ent
ial
andmakest
hecur
rentf
loweasy
.

Appl
i
cat
ionsofSchot
tkyDi
ode
Schott
kydiodeshavebeenusefulf
ortheindust
ryofel
ect
ronicst
hathasspot
tedmany
appl
icati
onsindioderect
if
ier
sbecauseofit
suniquepr
opert
ies.Her
earesomemajorar
eas
whereiti
swi del
yused.

Adv
ant
agesofSchot
tkydi
ode
Fol
l
owi
ngar
etheadv
ant
agesofSchot
tkydi
ode:

 Thecapaci
tanceoft
hedi
odei
slowast
hedepl
eti
onr
egi
onoft
hedi
odei
snegl
i
gibl
e.

 Ther
ever
ser
ecov
eryt
imeoft
hedi
odei
sver
yfast
,thati
sthechangef
rom ONt
oOFF
 st
atei
sfast
.

 Thecur
rentdensi
tyoft
hedi
odei
shi
ghast
hedepl
eti
onr
egi
oni
snegl
i
gibl
e.

 Thet
urn-
onv
olt
ageoft
hedi
odei
s0.
2to0.
3vol
ts,
whi
chi
sver
ylow.

Di
sadv
ant
agesofSchot
tkydi
ode
Theonl
ydi
sadv
ant
ageofSchot
tkydi
odesi
sthatt
her
ever
sesat
urat
ioncur
rentoft
hedi
odei
slar
ge.

Whati
sthedi
ff
erencebet
weenSchot
tkydi
odeandPNj
unct
ion
di
ode?
Schot
tkydi
ode PNj
unct
iondi
ode

I
nt hi
sdiode,t
hejunct
ioni
sformed Int
hisdiode,
thejunct
ioni
sfor
med
betweenthen-ty
pesemiconduct
orand bet
weent hep-t
ypeandn-t
ype
themetalplat
e semiconduct
ors

Thef
orwar
dvol
tagedr
opf
orpn
Thef
orwar
dvol
tagedr
opi
slow
j
unct
iondi
odei
smore

Reverser
ecov
eryl
ossandrev
erse Reverser
ecov
eryl
ossandr
ever
se
recov
eryti
mearever
yless recov
eryti
mearemore

I
tisauni
pol
ardev
ice I
tisabi
pol
ardev
ice

Theconduct
ionofcur
renthappens
Theconduct
ionofcurr
enthappens
duetothemovementofelect
rons
onl
yduetothemov ementofelect
rons
andhol
es

Whati
saLi
ghtEmi
tt
ingDi
ode?

Theli
ght
ingemit
ti
ngdiodeisap-njunct
iondiode.I
tisaspecial
lydopeddiodeandmadeupofa
speci
alt
ypeofsemiconduct
ors.Whentheli
ghtemitsint
hef orwardbi
ased,theni
tiscal
l
eda
l
ight-
emi
tti
ngdi
ode.
TheLEDsy mbolissimi
lart
oadi odesymbolexceptf
ortwosmal
lar
rowsthatspeci
fythe
emissi
onofli
ght,thusi
tiscal
ledLED( l
i
ght
-emitt
ingdi
ode)
.TheLEDincl
udestwoterminal
s
namelyanode(+)andthecathode(-)
.TheLEDsy mboli ow.
sshownbel

Const
ruct
ionofLED
Theconstructi
onofLEDi sv erysimplebecauseitisdesi gnedthr
oughthedeposi ti
onoft hree
semiconductormat er
iallayersoverasubst rat
e.Theset hreel
ayer
sarearrangedonebyone
wherethetopr egionisaP- t
yperegion,t
hemi ddl
eregioni sacti
veandfi
nal
ly
, thebottom r egion
i
sN-type.Thet hreer egionsofsemi conductormateri
alcanbeobser vedintheconst ructi
on.I n
theconstructi
on,t heP- typeregionincludesthehol es;t heN-t
yperegi
oni ncludesel ections
whereastheact i
ver egionincl
udesbot hholesandelectrons.

Whent hevolt
agei snotappliedtotheLED, t
hent herei
snof lowofel ectr
onsandhol essothey
arestable.Oncet hevolt
agei sappli
edt hentheLEDwi llfor
war dbiased,sotheelectr
onsinthe
N-regionandhol esfrom P-regionwil
lmov et otheacti
v eregion.Thisregionisalsoknownas
thedepl et
ionregion.Becauset hechargecarri
erslikeholesincludeaposi ti
vechargewhereas
electr
onshav eanegat ivechar gesothel i
ghtcanbegener atedthrought herecombinat
ionof
polari
tycharges.

Howdoest
heLi
ghtEmi
tt
ingDi
odeWor
k?
Thel i
ght-emi
tti
ngdi odesimply
,weknowasadi ode.Whent hediodeisforwardbiased,t
hent he
electr
ons& hol esar emov ingfastacr ossthej uncti
onandt heyarecombi nedconst ant
ly,
remov i
ngoneanot herout.Soonaftertheelectronsaremov i
ngf r
om then-typetot hep- t
ype
sil
icon,itcombineswi ththeholes,thenitdisappears.Hencei tmakest hecompl eteatom &
mor estableanditgivestheli
tt
leburstofener
gyi ntheform ofatinypacketorphotonoflight.
Theabov ediagram showshowt helight
-emit
tingdiodewor ksandt hest epbyst eppr ocessof
thediagram.From theabov edi agram,wecanobser vethatt heN- typesi li
conisi nredcol or
i
ncludingtheelect
ronswhi char eindicat
edbyt heblackcir
cles.TheP- ty
pesi l
iconisint heblue
coloranditcontai
nshol es,t
heyar eindi
cat
edbyt hewhitecircles.
Thepowersuppl yacrosst he
p-njuncti
onmakest hediodef orwardbiasedandpushi ngtheel ectronsf rom n-ty
pet op- t
ype.
Pushingtheholesint heoppositedi r
ecti
on.
Elect r
onandholesatt hejunctionar ecombi ned.
The
photonsaregivenoffast heelectronsandholesar erecombined.

Li
ghtEmi
tt
ingDi
odeCi
rcui
tforBi
asi
ng
MostoftheLEDshav
evol
tager
ati
ngsf
rom 1v
olt
-3v
oltwher
easf
orwar
dcur
rentr
ati
ngsr
ange
f
rom 200mA-100mA.

Ifthev oltage( 1Vto3V)i sappliedtotheLED,thenitfuncti

onspr operl
yduet othef l
ow of
currentfort heappliedvol
tagewi l
lbei
nt heoper
ati
ngrange.Similarly,i
ftheappli
edvoltageto
anLEDi shi ghthant heoper
atingvol
tagethenthedepleti
onr egionwi thi
nt hel
ight
-emi t
ti
ng
diodewi l
lbr eakdownduet othehighfl
owofcurrent
.Thisunexpect edhighflowofcurrentwill
damaget hedev ice.

Thiscanbeavoi
dedbyconnect
ingaresist
orinseri
eswi
tht
hevol
tagesour
ce&anLED.The
safevol
tager
ati
ngsofLEDswil
lberangesfrom 1Vto3Vwher
eassafecur
rentr
ati
ngsr
ange
fr
om 200mAt o100mA.

Here,t
her esist
orwhi chisarrangedinbetweenthev ol
tagesour
ceandLEDi sknownasthe
cur
rentli
mi t
ingresistorbecausethisresi
storr
estr
ictsthef l
ow ofcur
rentot
her
wisetheLED
maydestroyit.Sothisresi
storplay
sakeyr ol
einpr
ot ecti
ngtheLED.

Mat
hemat
ical
l
y,t
hef
lowofcur
rentt
hrought
heLEDcanbewr
it
tenas

I
F=Vs–VD/
Rs

Wher
e,

‘
I
F‘i
sfor
war
dcur
rent

‘
Vs’
isav
olt
agesour
ce

‘
VD’
ist
hev
olt
agedr
opacr
osst
hel
i
ght
-emi
tt
ingdi
ode
‘
Rs’
isacur
rentl
i
mit
ingr
esi
stor

Theamountofv ol
tagedroppedtodefeatthebarri
erofthedepleti
onregion.TheLEDv ol
tage
dropwi l
lrangefrom 2Vto3Vwhi leSiorGedi odei
s0. 3other
wise0.7V.Thus,theLEDcanbe
operatedbyusinghighvoltageascomparedwithSiorGediodes.
Li
ght-emitti
ngdiodesconsumemor eenergythansil
iconorgermani
um diodestooperat
e.

Wor
kingPr
inci
pleofLED
Thewor kingpri
ncipl
eoftheLight -
emitti
ngdiodeisbasedonthequantum theor
y.Thequantum
theorysay sthatwhentheelectroncomesdownf rom t
hehigherener
gylevelt
othelowerenergy
l
ev elthen,theener gyemitsfrom t hephoton.Thephot onenergyi
sequalt otheenergygap
betweent hesetwoener gylevels.IfthePN- j
unct
iondiodeisintheforwardbiased,t
hent he
currentflowsthroughthediode.

Thef low ofcurr

entinthesemi
conductorsiscausedbyt hefl
ow ofhol
esintheopposit
e
direct
ionofcurr
entandthefl
owofelect
ronsinthedirect
ionoft
hecurr
ent
.Hencether
ewil
lbe
recombinati
onduetothefl
owofthesechargecarr
ier
s.

Ther ecombi
nati
oni ndicatest hattheelect
ronsintheconducti
onbandj umpdownt ot
he
val
enceband.Whent heel ect
ronsjumpf r
om onebandtoanotherbandtheel
ect
ronswi
l
lemit
theelectr
omagneti
cener gyint hef or
m ofphotonsandthephot onenergyi
sequaltot
he
for
biddenener
gygap.

Forexample,l
etusconsi
derthequantum t
heory,
theenergyofthephot
onist
heproductofbot
h
thePlanckconst
antandfrequencyofel
ectromagnet
icradi
ati
on.Themathemat
icalequat
ioni
s
shown

Eq=hf

Wher
ehi sknownasaPl anckconstant
,andthevel
ocityofel
ect
romagneti
cradiat
ionisequalt
o
t
hespeedofl i
ghti
.ec.Thefrequencyr
adiat
ioni
srelatedtot
heveloci
tyofl
ightasanf =c/λ.λ
i
sdenotedasawav el
engthofelectr
omagneticr
adi
ationandtheaboveequati
onwi l
lbecomeas
a

Eq=he/λ

Fr
om t
heabov
eequat
ion,wecansayt
hatt
hewav
elengt
hofel
ect
romagnet
icr
adi
ati
oni
s
i
nv ersel
yproport
ionalt
otheforbiddengap.Ingeneralsi
li
con,ger
mani um semiconductorst hi
s
forbiddenenergygapi sbetweent heconditionandv al
encebandsar esucht hatthet otal
radiati
onofelect
romagneti
cwav eduringrecombinat
ionisintheform ofi
nfrar
edr adi
ation.We
can’tseethewavelengt
hofinfr
aredbecauset heyar
eoutofourv i
sibl
erange.

The inf
rared radiat
ion is saidt o be as heatbecause t he si
li
con and t
he ger manium
semiconductors are not di rect gap semi conductors rat
her these ar
e indir
ect gap
semiconductors.Buti nthedi rectgap semi conductors,themaximum energyleveloft he
val
encebandandmi nimum ener gyleveloftheconduct ionbanddoesnotoccuratt hesame
momentofel ectrons.Therefor
e, dur
ingtherecombi nat
ionofelect
ronsandhol
esaremi gr
ation
ofel
ectronsfrom theconduct i
onbandt othev alencebandt hemomentum oft
heelect
r onband
wil
lbechanged.

Whati
stheDi
ff
erencebet
weenaDi
odeandaLED?
Themai
ndi
ff
erencebet
weenadi
odeandaLEDi
ncl
udest
hef
oll
owi
ng.

Di
ode LED

Thesemiconductordev
icel
i
keadi
odeconduct
sTheLED i
sonet
ypeofdi
ode,usedt
ogener
ate
si
mplyi
nonedi r
ecti
on. li
ght
.

TheLEDisdesignedwit
ht hegall
ium phosphi
de&
gal
li
um arseni
de whose elect
rons can gener
ate
Thedesi gni
ngoft hediodecanbedonewi t
ha l
ightwhi
letr
ansmit
ti
ngthee ner
gy.
semiconductormater
ial&thefl
ow ofelectr
onsin
thi
smat eri
alcangi
vethei
rener
gytheheatform.

Thedi
odechangest
heACi
ntot
heDC TheLEDchangest
hev
olt
agei
ntol
i
ght

I
thasahi
ghr
ever
sebr
eakdownv
olt
age I
thasal
ow-
rev
ersebr
eakdownv
olt
age.

Theon-
statevol
tageoft
hediodei
s0.
7vf
orsi
l
iconTheon-st
atevol
tageofLEDappr
oxi
mat
elyr
anges
wher
eas,forger
manium,i
tis0.
3v fr
om 1.
2to2.0V.

Thedi
odei
susedi nv ol
tagerect
ifi
ers,cl
i
ppi
ng&
cl
ampi
ngci
rcui
ts,
vol
tagemulti
pli
ers.

The appli
cati
ons of LED are traf
fi
c signal
s,
automoti
veheadl
amps,i
nmedicaldevi
ces,
camera
fl
ashes,
etc.
I
-VChar
act
eri
sti
csofLED
Therear edi f
fer
entt ypesofl ight-emitt
ingdiodesar eavail
abl
ei nthemar ketandt hereare
diff
erentLEDchar acteri
sti
cswhi chi ncl
udethecolorli
ght,
orwav el
engt
hr adiati
on,l
i
ghtintensi
ty.
Thei mportantcharacteri
sticoftheLEDi scol
or.Inthestart
inguseofLED, therei
st heonlyred
color.AstheuseofLEDi sincreasedwi t
hthehelpoft hesemiconductorprocessanddoi ngthe
researchont henewmet alsforLED, thedif
fer
entcolorswereformed.

I
-VChar
act
eri
sti
csofLED

Thefol
l
owinggraphshowstheappr
oxi
matecurvesbetweenthef
orwar
dvol
tageandthecur
rent
.
Eachcurv
einthegr aphi
ndicat
esadiff
erentcol
or.Thetableshowsasummar yoftheLED
char
act
eri
sti
cs.

Whati
saTunnel
Diode?
ATunnel Diodei
saheavil
ydopedp-njunct
iondiode.Thet
unneldiodeshowsnegat
ive
resi
stance.Whenvol
tageval
ueincr
eases,cur
rentfl
owdecreases.Tunneldi
odeworksbasedon
TunnelEffect
.

t
hesy
mbol
ofaTunnel
Diode.

Themat erial
susedfort
hisdiodear
eGer manium,Gall
i
um ar
senideandothersi
l
iconmater
ial
s.
Tunneldiodeshowsanegat i
veresi
stancei
ntheiroper
ati
ngrange.So,
itcanbeusedas
ampli
fi
er, osci
ll
ator
sandinanyswitchi
ngcir
cuis.
t
Wi
dthoft
heDepl
eti
onRegi
oni
nTunnel
Diode
Whenmobi l
echar gecarr
iersbothfreeel ectr
onsandhol esaremissing,ther
egi
oninap- n
j
unct
ionhasar egi
oncall
edDepl et
ionr egion.Tost opthefl
ow ofelectr
onsfr
om then-ty
pe
semi
conductorandholesfrom t
hep-typesemi conductor
,depl
eti
onr
egionactsasabarr
i .
er

Dependingont henumberofi mpurit

iesadded,wi
dthofdepleti
onr egionvari
es.Toincr ease
el
ectr
icalconduct
ivi
tyofthep-
typeandn-typesemi
conductori
mpur i
ti
esar eadded.Awi deand
bi
gdepl et
ionregi
oni sformedwhenasmal l
ernumberofi mpur
iti
esi saddedt op-
nj unction
di
ode.Att hesametime,whenagr eat
ernumberofimpuri
ti
esisadded, narr
owdepleti
onr egion
occur
s.

Thep-ty
peandn- t
ypesemiconduct
ori
sheavil
ydopedinatunneldiodeduet oagr
eaternumber
ofimpuri
ti
es.Heavydopi
ngresult
sinanarr
owdepleti
onregi
on.Whencompar edt
oanor malp-
njunct
iondiode,t
unneldi
odehasanar r
owdepleti
onwidth.Therefore,whensmallamountof
vol
tageisappl
ied,
itpr
oducesenoughel
ectr
iccur
renti
nthetunneldiode.
Tunnel
i
ngEf
fect
Inelectroni
cs,Tunnelingisknownasadi rectflowofelectronsacrossthesmalldepleti
onregion
from n-sideconductionbandi ntothep-sidev alenceband.Inap- njuncti
ondi
ode, bothpositi
ve
andnegat i
veionsf ormt hedepl eti
onr egion.Duet ot heseions,in-bui
ltel
ectr
icpot ent
ialor
electr
icf i
eldispresentint hedepleti
onr egion.Thiselect
ricfi
eldgivesanelectr
icf or
cetot he
oppositedirecti
onofext ernal
lyappli
edvol t
age.

Ast hewidt
hoft
hedeplet
ionlay
erreduces,
chargecar
ri
erscaneasi
lycrosst hej
uncti
on.Charge
carr
iersdonotneedanyform ofkinet
icener
gytomov eacrossthejunction.I
nstead,car
ri
ers
puncht hr
oughj
uncti
on.Thiseff
ectiscall
edTunneli
ngandhencet hedi odeiscal l
edTunnel
Diode.
Tunnel
DiodeWor
kingPhenomenon

Unbi
asedTunnelDi
ode
Inanunbiasedt unneldi
ode,nov ol
tagewi l
lbeappliedtot het unneldiode.Here,duetoheavy
dopingconductionbandofn–t ypesemi conductoroverlapswi thvalencebandofp–t ype
materi
al.Elect
ronsfrom nsideandhol esfrom psideov erlapwi theachot herandtheywill
beat
sameener gylevel.
Someelectronstunnel f
rom theconduct ionbandofn- r
egiontothevalence
bandofp- regi
onwhent emperatureincreases.Simil
arl
y,hol eswillmov efrom val
encebandofp-
regi
ontot heconductionbandofn- r
egion.Final
ly,t
henetcur rentwil
lbezer osinceequal
numbersofel ectr
onsareholesf l
owi noppositedir
ecti
on.

Pαe(
-A*
E*b*
W)

P–Pr
obabi
l
ityt
hatt
hepar
ti
cl
ecr
ossest
hebar
ri
er

W –Wi
dthoft
hebar
ri
er
E–Ener
gyoft
hebar
ri
er

Smal lVol
tageAppliedtotheTunnelDi ode
Whenasmal lvoltage,thathasl esserv al
uet hant hebui l
t-i
nvoltageoft hedepl eti
onlayer,i
s
applied t
ot he tunneldi ode,therei s no flow off orward currentthrough t he j
uncti
on.
Nev ert
hel
ess,ami nimalnumberofel ectr
onsf rom theconduct ionbandofnr egionwi l
lstar
t
tunneli
ngtov al
encebandi npregion.Therefore,thi
smov ementcr eatesasmal lforwardbiased
tunnelcurr
ent.Whenasmal lvol
tageisappl ied,tunnelcurrentst
artstoflow.

Incr
easedVoltageAppli
edtotheTunnelDi ode
Whent heamountofv ol
tageappliedisincr
eased,thenumberoff r
eeelectr
onsgeneratedatn
sideandhol
esatpsi deisalsoincreased.Duetov ol
tagei
ncrease,ov
erlappi
ngbetweent he
bandsarealsoincr
eased.Maximum t unnelcur
rentfl
owswhent heenergylevel
ofn-side
conducti
onbandandt heenergylevel ofap-si
devalencebandbecomesequal .

Fur
therIncr
easedVol tageAppli
edtot heTunnelDi ode
Afurt
herincreaseintheappli
edv ol
tagewillcauseasl ightmi
sal
ignmentoft
heconducti
onband
andvalenceband.Stil
ltherewil
lbeanov erl
apbet weenconductionbandandval
enceband.The
el
ectr
onsmov efrom conduct
ionbandt ovalencebandofpr egi
on.Ther
efor
e,t
hiscausessmal
l
cur
renttofl
ow.Hence, t
unnelcur
rentstar
tsdecr easing.
LargelyIncr
easedVoltageAppli
edt
otheTunnelDiode
Thet unneli
ngcurr
entwillbezer
owhenappliedv
oltageisincr
easedmoretothemaximum.At
thi
sv olt
agelevel
s,theval
encebandandtheconductionbanddoesnotover
lap.Thi
smakes
tunneldiodetooperatesameasaPNj unct
iondi
ode.

Whenappl
i
edvolt
ageismor ethant hebuil
t-i
n
potent
ialofthedepleti
onlayertheforwardcurr
entstar
tsf
lowi
ngthroughthet unneldi
ode.In
thi
sconditi
on,curr
entporti
onint hecurv
edecreaseswhenthev
olt
ageincreasesandt hi
sisthe
negati
veresi
stanceoftunneldiode.Suchdi
odesoperati
ngi
nnegat
iveresi
stanceregionisused
asampl i
fi
eroroscil
lat
or.

 V-
IChar
acter
ist
icsofTunnelDiode
 Duetof
orwardbiasi
ng,becauseofheav
ydopi
ngconduct
ionhappensi
nthedi
ode.The
maximum cur r
entt
hatadioder
eachesisI
pandvolt
ageappli
edisVp.Thecur
rentv
alue
decreases,whenmoreamountofv ol
tagei
sappl
ied.Cur
rentkeepsdecr
easi
nguntili
t
reachesami ni
malval
ue.
Thesmal lmini
malval
ueofcur
rentisIv
.From theabovegraph,itisseenthatf
rom pointAtoB
curr
entreduceswhenvol
tagei
ncreases.Thati
st henegat
iveresistancer
egionofdiode.I
nthi
s
regi
on,t
unnel di
odepr
oducespowerinsteadofabsorbi
ngit.

Appl
icat
ionsofTunnelDiode
 Tunne ldi
odecanbeusedasaswi tch,ampli
fi
er,andoscill
ator
.
 Sinceitshowsaf astr
esponse,i
tisusedashi ghfrequencycomponent
.
 Tunne ldi
odeactsaslogicmemor ystoragedevi
ce.
 The yareusedi
nosci l
l
atorci
rcui
ts,andinFM receiver
s.Sinceiti
sal
owcur
rentdev
ice,
it
i
snotusedmor e.

Var
act
orDi
ode

Definit
ion:
Varact
ordiodeistheonewhi chworksont hepri
nci
pleofvar
iat
ionincapaci
tanceby
changingthewidthofthedeplet
ionr
egionofP- Njuncti
on.TheP-NJuncti
ondiode
createscapaci
toref
fect.Thecapaci
tanceiscontr
olledbyappli
edvol
tage.Itwor
ksonrev er
se
biasedmode.

Varact
erwor disformedf r
om wor dsVar
iabl
ereactanceorv ar
iabl
er esistor
.Thus, i
tprov
ides
var
iabl
eresistanceorreactanceorcapacit
ancethusitisnamedasav aractordiode.The
symbolofthev aract
ordiodeissameasconv enti
onal di
odeexceptt hesy mbol ofthecapaci
tor
i
smer gedwi t
ht hesymbol ofthediodet
oshowt hecapacitanceeffect.

I
tisal
socal
l
edv
olt
age-
var
iabl
ecapaci
tor(
VVC)orv
ari
capdi
ode.

Var
act
orDi
ode

Definit
ion:
Varact
ordiodeistheonewhichworksonthepr
inci
pleofvari
ati
onincapaci
tanceby
changing the widt
h oft he depl
eti
on regi
on ofP-N junct
ion.The P-N Juncti
on diode
createscapaci
toref
fect.Thecapaci
tanceiscont
rol
l
edbyappliedvol
tage.Itwor
ksonr ever
se
biasedmode.

Varact
erwor
di sfor
medfrom wordsVari
ablereactanceorv
ari
abl
eresi
stor.Thus,i
tprovi
des
var
iabl
eresi
stanceorr
eactanceorcapacit
ancet husitisnamedasav aract
ordiode.The
sy
mbolofthevar
act
ordiodei
ssameasconv
ent
ionaldi
odeexceptthesy
mboloft
hecapaci
tor
i
smergedwit
hthesymboloft
hedi
odet
oshowthecapacit
anceeff
ect.

I
tisal
socal
l
edv
olt
age-
var
iabl
ecapaci
tor(
VVC)orv
ari
capdi
ode.

Const
ruct
ionofVar
act
orDi
ode
Iti
sf ormedofP- t
ypeandN- ty
pesemi conduct
orandr eversebi
asi
ngi sappliedt oi
t.The
majori
tycarr
ier
sinanN-typesemiconductorareelect
ronsandthemajori
tycarr
iersinaP-t
y pe
semiconduct
orareholes.Atthejuncti
on,t heel
ectronsandholesrecombine.Duet owhi ch
i
mmobi lei
onsaccumulateatt
hejuncti
on.Andnomor ecurr
entcanf
lowduet omaj or
it
ycarr
iers.

Thus,t
hedepl
eti
onregi
onisfor
med.Thedepl et
ionregi
oniscal
ledsobecausei
tisdepl
etedof
char
gecarr
ier
si.
e.t
hemajori
tycarr
iersar
eabsentindepl
eti
onregi
on.Thi
sworksasadiel
ectr
ic
l
ayerandPandN-ty
pesemiconductorworksasplat
esofacapacit
or.

Wor
kingofVar
act
orDi
ode

Whent
her
ever
sebi
asi
sappl
i
edt
oP-
Njunct
ion,
thewi
dthofdepl
eti
onl
ayeri
ncr
eases.Andwi
th
thei
ncreaseofrev
er sevol
tagegradual
l
ythedepl
etionl
ayeri
ncr
easesev
enmor
e.Thus,
the
depl
eti
onr egi
oncreatesTr
ansit
ioncapaci
tanceCT.
CT = ɛA/
W

Here,CT i
sTransi
ti
oncapaci
tance,
ɛisdi
elect
ri
cconst
ant
,Ai
sthear
eaofpl
atesoft
he
capacit
orandW isthewidt
hofthedepl
eti
onlayer
.

I
ti sevidentf r
om theabov erelati
onthattransi
ti
oncapaci
tanceisi
nver
sely
propor
ti
onalt owidthofthedeplet
ionlay
er.Thus,i
fwewantthehighmagni
tude
ofcapacitancethewi dt
hshouldbesmal l
.Andt hewidt
hwil
lbesmalli
fwewi l
l
applyl
owr eversevolt
age.

Simi
larl
y,i
fwerequi
relowcapacit
ancethewidt
hshouldbelargeandtoincr
ease
thewidt
htherever
sev ol
tageappl
i
edshouldbehigh.
Thus,
thiswidthcanbe
cont
roll
edwit
happliedrever
sevol
tage.

Vol
t-
amper
eChar
act
eri
sti
csofVar
act
orDi
ode

I
tcanbeseenint
hechar act
eri
sti
ccurvethatasreversevol
tagei
ncr
easef
rom 0
Vthet
ransi
ti
oncapaci
tanceisdecreasi
ngex ponent
iall
y.

Ther
elat
ionbet
weent
het
ransi
ti
oncapaci
tance,
VR andVk.

CT =K/(
Vk+VR)
n

Here,CTi
sjuncti
oncapacit
ance,Vkisthekneev ol
tageandVRi stherever
se
volt
ageandt hevalueofnforall
oyedj
unctionsi
s1/ 2anddiffusedjuncti
onsi
tis
1/3.Thus,t
hev ol
tageisi
nversel
yproport
ionalt
ocapacitance.
Thetuni
ngrangeofthecapaci
torvar
ieswiththedopinglevel
ofthediode.For
abr
uptdopingj
uncti
onthedopingwil
l beuni
form,butforthehyperabrupt
j
uncti
on,t
hedopingprof
il
ewi l
lbenon-uni
form.

Adv
ant
agesofVar
act
orDi
ode

1.LowNoise:I
tgener
ateslessnoi
seascomparedtotheotherP-Njunct
ion
di
ode.Thus,
thepowerlossduetonoi
seisl
owi nvaract
ordiodes.

2.Por
tabi
l
ity
:Iti
spor
tabl
eduet
othesmal
lsi
zeandl
i
ght
wei
ght
.

3.Rel
i
abi
l
ity
:Iti
smor
erel
i
abl
ethanot
herP-
Njunct
iondi
odes.

4.Economical:
Iti
sal
ow-
costdi
odet
hus,
iti
seconomi
cal
tousei
nvar
ious
appl
icat
ions.
Di
sadv
ant
agesofVar
act
orDi
ode

Thesearespeci
all
ydesi
gnedtoworki
ntherev
ersebiasedmode,
itpossesses
thel
eastsi
gnifi
cancewhenoper
atedi
nfor
wardbiasi
ng.

Appl
i
cat
ionsofVar
act
orDi
ode

1.Televisi
onrecei
vers:
Varactordi
odesareusedastunedcapacit
orsandhave
replacedmechanical
lyt
unedcapacit
orsinvar
iousappl
icat
ions.I
tisusedi
n
tel
ev i
sioni
ntheresonanttankci
rcui
t.
 2.Radi
orecei
ver
s:Radi
orecei
ver
sal
souset
hisdi
odef
ort
uni
ngpur
poses.

3.Fr
equencyMult
ipl
ier
:Iti
sal
sousedasaf
requencymul
ti
pli
eri
nvar
ious
el
ect
roni
ccir
cuit
s.

4.PhaseLockedLoops:
ItisusedinPhaselockedloopforf
requency
modulat
ion.Var
act
ordiodeshelpinachi
evi
ngfrequencymodulati
on.Thus,
i
ncommuni cati
ondevi
cesvaractordi
odesaresi
gnifi
cant
.

5.Voltagecont r
oll
edoscill
ator
s: Volt
agecont
rolosci
ll
ator
sareused
extensivel
yintransmissi
onandr ecei
vi
ngci
rcuit
sincommuni cat
ion.And
varactordiodeplaysasignif
icantrolei
nconstr
ucti
onofv ol
tagecontr
oll
ed
oscill
ator.

6.Par
ametri
cAmpl
i
fier
s:I
tisusedi
npar
amet
ri
campl
i
fierasasi
gni
fi
cant
component
.
Varact
ordiodeproducescapaci
tanceeff
ect,t
hisisbecauseitcathodeand
anodeter
mi nal
sactsastheplat
eoft hecapaci
torandregionbetweenthem act
s
asdiel
ect
ri
cmedi um.