Chapter14

SemiconductorElectronics:
Materials,DevicesandSimpleCircuitsIntroduction

Semiconductorsarethebasicmaterialsusedinthepresentsolidstate
electronicdeviceslikediode,transistor,ICs,etc.Latticestructurea
ndthe atomic structure of constituent elements decide whether
a particular material will be insulator, metal or semiconductor.

ClassificationofMetals,ConductorsandSemicond
uctors On the basis of conductivity:
Onthebasisoftherelativevaluesofelectricalconductivity(σ)orresi
stivity (ρ = 1/σ ), the solids are broadly classified as:

(i) Metals:Theypossessverylowresistivity(orhighconduct
ivity). ρ ~ 10–2– 10–8Ω m
σ~102–108Sm–1
(ii) Semiconductors:Theyhaveresistivityorconductivityinter
mediate to metals and insulators.
ρ ~ 10–5– 106Ω
m σ~105–10–
6Sm–1

(iii) Insulators:Theyhavehighresistivity(orlowconductivity).
ρ~1011–1019Ωm
σ~10–11–10–19Sm–1
Semiconductorswhichcouldbe:
(i) Elementalsemiconductors:SiandGe
(ii) Compoundsemiconductors:Examplesare:
• Inorganic:CdS,GaAs,CdSe,InP,etc.
• Organic:anthracene,dopedpthalocyanines,etc.
• Organicpolymers:polypyrrole,polyaniline,polythioph
ene,etc. Most of the currently available semiconductor devices
are based on elemental semiconductors Si or Ge and compound
inorganic semiconductors.

EnergyBandsIn Solids
  Insidethecrystaleachelectronwillhaveadifferentenergylevel.
Thesedifferentenergylevelswithcontinuousenergyvariationf
orm energy bands.
 Theenergybandwhichincludestheenergylevelsoftheva
lence electrons is called the valence band.
 Theenergybandwhichincludestheenergylevelsofcond
uction electrons is called the conduction band.
 Theconductionbandisabovethevalenceband.Normal
lythe conduction band is empty and valence band is
occupied.
 The gap between the top of the valence band and
bottom of the conduction band is called the energy
band gap (Energy gapEg). It is measured in electron
volt.

ClassificationofMetals,Conductorsand Semiconductors
Onthebasisofenergybands
(i) metals

In some metals, the conduction band is partially filled and the

valence band
ispartiallyemptywithsmallenergygapandinsomeothersthecondu
ction and valance bands overlap. When there is overlap electrons from
valence band can easily move into the conduction band.Therefore, the resistance
of such
materialsislowortheconductivityishigh.
(ii) Insulators
 Ininsulatorsalargebandgap,Eg>3eV.Therearenoelectronsinthe
conductionband,andthereforenoelectricalconductionispossible.The
energygapissolargethatelectronscannotbeexcitedfromthevalenceb
and to the conduction band by thermal excitation.

(iii) Semiconductors

In semiconductors a finite but small band gap (Eg< 3 eV) exists.

Because of thesmall band gap,at room temperature some
electrons from valence band
canacquireenoughenergytocrosstheenergygapandenterthecond
uction band. These electrons (though small in numbers) can
move in the
conductionband.Hence,theresistanceofsemiconductorsislowertha
nthat of insulators.

Whentheelectronsfromvalencebandmovetotheconductionbandvaca
nt energy levels will be created in the valence band . This vacancy
of electrons
iscalledhole.Othervalenceelectronscanmovetothisholethereby
producing hole current.

IntrinsicSemiconductor
Puresemiconductorsarecalled‘intrinsicsemiconductors’.
SiandGehavefourvalenceelectrons.InapureSiorGecrystal,eachato
m make covalent bond with four neighbouring atoms and share
the four valence electrons.
As the temperature increases, these electrons get more
thermal energy , break–away the covalent bonds and become
free electrons contributing to
conduction.Thesefreeelectrons(withcharge –
q)leavesavacancywithan effective charge (+q ). This vacancy
with the effective positive electronic charge is called a hole.

Inintrinsicsemiconductors,thenumberoffreeelectrons,neisequal
tothe number of holes, nh.

ne=nh=ni
whereniiscalledintrinsiccarrierconcentration.

Thefreeelectronsmoveasconductionelectronandgivesrisetoanele
ctron current, Ieunder an applied electric field. Under the
action of an electric field, the holes move towards negative
potential giving the hole current, Ih.
Thetotalcurrent,Iisthusthesum
oftheelectroncurrentIeandthehole current Ih:
I=Ie+Ih

Energy-BandDiagramofanIntrinsicSemiconductoratT=0K
AnintrinsicsemiconductorwillbehavelikeaninsulatoratT=0K.
Energy-BandDiagramofanIntrinsicSemiconductoratT>0K
Attemperatures(T>0K),someelectronsareexcitedfromtheval
ence band to the conduction band, leaving equal number of
holes there.

ExtrinsicSemiconductor
When a small amount of a suitable impurity is added to the
pure
semiconductor,theconductivityofthesemiconductorisincreased.
Such materials are known as extrinsic semiconductors or
impurity semiconductors.
The deliberate addition of a desirable impurity is called doping
and the
impurityatomsarecalleddopants.Suchamaterialisalsocalledadop
ed semiconductor.

TherearetwotypesofdopantsusedindopingSior Ge:
(i)Pentavalent (valency 5)
Eg:Arsenic(As),Antimony(Sb),Phosphorous(P),etc.
(ii)Trivalent (valency 3)
Eg:Indium(In),Boron(B),Aluminium(A𝒍),etc.

Dependingonthetypeofimpuritiesadded,therearetwotypesof
semiconductors –
(i) n-typesemiconductor
(ii) p-typesemiconductor

n-typesemiconductor
n-
typesemiconductorisobtainedbydopingSiorGewithpentavalentat
oms (donors) like As, Sb, P, etc. The four valence electrons of
pendavalent impurity atombond with the four silicon
neighbours ,while the fifth one is free to move in the lattice of
the semiconductor ,at room temperature. Thus, the pentavalent
dopant is donating one extra electron for conduction and hence
is known as donor impurity.
Forn-typesemiconductors,ne>> nh
Hereelectronsbecomethemajoritycarriersandholestheminoritycarri
ers.
Theelectronandholeconcentrationinasemiconductorinthermal
equilibrium is given by
nenh=ni2
Energybandsofn-typesemiconductoratT>0K

Forn-
typeSisemiconductor,thedonorenergylevelED,isslightlybelowthe
bottom ECof the conduction band .The electrons from this level
move into the conduction band with very small supply of
energy.

p-type semiconductor
p-type semiconductor is obtained when Si or Ge is doped with
a trivalent impurity like A𝑙, B, In, etc. The dopant has only 3
valence electrons and can form covalent bonds with
neighbouring three Si atoms but does not have
anyelectrontooffertothefourthSiatom.Thisvacancyofelectroncre
atesa hole. As the pendavalent impurities createsholes ,which
can accept electrons from neighbouring atom, these impurities
are called acceptor impurities.
Forp-typesemiconductors,nh>>ne
Hereholesbecomethemajoritycarriersandelectronstheminorityca
rriers. The electron and hole concentration in a semiconductor
in thermal equilibrium is given by
nenh=ni2
Energybandsofp-typesemiconductoratT>0K

Forp-
typesemiconductor,theacceptorenergylevelEAisslightlyaboveth
e top EVof the valence band . With very small supply of energy
an electron from the valence band can jump to the level EAand
ionise the acceptor . negatively.
p-njunction

A p-n junction can be formed by adding a small quantity

of pendavalentimpuritytoap-
typesemiconductororbyaddingasmall quantity of trivalent
impurity to an n-type semiconductor.

Twoimportantprocessesoccurduringtheformationofap-
njunction: diffusion and drift.

1. Diffusion
Theholesdiffusefromp-sideton-
side(p→n)andelectronsdiffusefromn- side to p-side (n → p).
This motion of charge carriers give rise to Diffusion
currentacrossthejunction.
Duetodiffusion,alayerofpositivecharge(orpositivespace-
chargeregion) is developed on n-side of the junction and a
layer of negative charge (or negative space-charge region) is
developed on the p-side of the junction .

Depletionregion(Depletionlayer)
Thespace-charge
regiononeithersideofthejunctiontogetherisknownas
depletionregion.Thedepletionlayerconsistofimmobileion-
coresandno free electrons or holes. This is responsible for a
junction potential barrier.

2. Drift
Thepositivechargeonn-
sideofthejunctionandnegativechargeonp-side of the junction
develops an electric field.Due to this field, an
electron(minoritycarrier)onp-sideofthejunctionmoveston-sideanda
hole(minoritycarrier)onn-sideofthejunctionmovestop-
side.Themotion of charge carriers due to the electric field is
called drift.
Initially,diffusioncurrentislargeanddriftcurrentissmall.Asthediff
usion process continues,the electric field strength increases
and hence drift
currentalsoincreases.Thisprocesscontinuesuntilthediffusioncurrent
equalsthedriftcurrent..Thusinap-
njunctionunderequilibriumthereisno net current. .
BarrierPotential

Thelossofelectronsfromthen-regionandthegainofelectronbythep-
regioncausesadifferenceofpotentialacrossthejunctionofthetworegio
ns.
Sincethispotentialtendstopreventthemovementofelectronfromt
hen region into the p region, it is often called a barrier
potential.
ThebarrierpotentialofaGediodeis0.2VandthatofaSidiodeis0.7V.

SemiconductorDiode

Asemiconductordiodeisbasicallyap-
njunctionwithmetalliccontacts
providedattheendsfortheapplicationofanexternalvoltage.Itis
atwo terminal device.

Symbolofap-njunctionDiode
p-njunctiondiodeunderforward bias
Ifp-sideofthediodeisconnectedtothepositiveterminalandn-
sidetothe negative terminal of the battery, it is said to be
forward biased.
 Thedirectionoftheappliedvoltage(V)isoppositetobarrierpote
ntial V0. As a result, the depletion layer width decreases
and the barrier height is reduced.
 Theeffectivebarrierheightunderforwardbiasis(V0–V).
 Athighappliedvoltage,electronsfromn-sidecrossthedepletion
regionandreachp-side.Similarly,holesfromp-
sidecrossthejunction and reach the n-side.
 Thismotionofmajoritycarriersoneithersidegivesrisetodiffu
sion current.
 ThemagnitudeofthiscurrentisusuallyinmA.

p-njunctiondiodeunderreversebias

Ifn-sideofthediodeisconnectedtothepositiveterminalandp-
sidetothe negative terminal of the battery, it is said to be
reverse biased.

 Thedirectionoftheappliedvoltage(V)issameasbarrierpotenti
alV0.
Asaresult,thedepletionlayerwidthincreasesandthebarrierhei
ght is incresaed.
 Theeffectivebarrierheightunderreversebiasis(V0+V).
 The flow of electrons from n → p and holesfrom p → nis
suppressed.
Thus,diffusioncurrent,decreasesenormouslycomparedtoth
ediode under forward bias.
 Theelectricfieldofthejunctionissuchthattheminoritycarrie
rsare drifted to majority zone which gives rise todrift
current.
 ThedriftcurrentisoftheorderofafewμA.
 V-Icharacteristicsofasilicondiode.

 In forward bias, the current first increases very slowly, till

the voltage across the diode crosses a certain value. . This
voltage is called the thresholdvoltageorcut-
involtage(0.2Vforgermaniumdiodeand0.7 V for silicon
diode).
 Afterthresholdvoltage,thediodecurrentincreasessignificant
ly,even for a very small increase in the diode bias voltage.
 For the diode in reverse bias, the current is very small
(~μA) and almost remains constant with change in bias. It
is called reverse saturation current. However, at very high
reverse bias called break
downvoltageVbr,thecurrentsuddenlyincreases.Thegeneralp
urpose diode are not used beyond the reverse saturation
current region.

ThresholdVoltage
Theforwardvoltagebeyondwhichthediodecurrentincreasessignific
antly is called threshold voltage or cut-in voltage.

BreakdownVoltage
Thereversevoltageatwhichthereversecurrentincreasessuddenlyis
called break down voltage.

Dynamic Resistance(rd)
DynamicresistanceisdefinedastheratioofsmallchangeinvoltageΔVt
oa small change in current ΔI.
rd=
𝚫𝐕
𝚫𝐈
Example
TheV-
Icharacteristicofasilicondi
odeisshownintheFigure.C
alculatethe resistance of
the diode at (a) ID= 15 mA
and (b) VD= –10 V.

Applicationof JunctionDiodeas aRectifier

Thediodeallowscurrenttopassonlywhenitisforwardbiased.
Ifanalternatingvoltageisappliedacrossadiodethecurrentflowson
lyin that part of the cycle when the diode is forward biased.
This property is used to rectify alternating voltages.
Rectifier
Theprocessofconversionofacvoltagetodcvoltage
iscalledrectification and the circuit used for rectification is
called rectifier.
HalfwaveRectifier
Inthe positive half-cycle of ac there isa currentthrough the load
resistor 𝑅𝐿 and we get an output voltage, whereas there is no
current in the negative
halfcycle.Sincetherectifiedoutputofthiscircuitisonlyforhalfofthei
nput ac wave it is called as half-wave rectifier.
Inputacvoltageandoutputvoltagewaveformsfromtherectifiercircuit.

Fullwave rectifier

 Forafull-
waverectifierthesecondaryofthetransformerisprovided
with a centre tapping and so it is called centre-tap
transformer.
 During this positive half cycle, diode 𝐷1 gets forward
biased and conducts ,while 𝐷2 being reverse biased is not
conducting. Hence we
getanoutputcurrentandaoutputvoltageacrosstheloadresist
or 𝑅𝐿.
 Duringnegativehalfcycle,diode𝐷1wouldnotconductbutdiode 𝐷2
conducts,givinganoutputcurrentandoutputvoltageacross
𝑅𝐿inthe same directionas in positive half.
 Thus,wegetoutputvoltageduringboththepositiveaswella


sthe negative half ofthe cycle.This isa more efficient

circuit for getting rectified voltage or current than the
halfwave rectifier.
 Inputacvoltageandoutputvoltagewaveformsfromthere
ctifiercircuit.

Filters
Togetsteadydcoutputfromthepulsatingvoltageacapac
itoris connected parallel to the output terminals.
Thecicuitsthatfilterouttheacrippleandgiveapure
dcvoltagearecalled filters.