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Semiconductors
Classification of Metals,Conductors, and Semi-conductors
Metals Possess very low resistivity(or high conductivity)
Semi-conductors Possessresistivity or conductivityintermediate
to metals and insulators
Insulators Possess high resistivity(or low conductivity)
Semi-conductors are of two types:
Elemental semi-conductor Example: Si and Ge
Compound semi-conductor Example: CdS, GaAs, CdSe, InP, etc.
Energy band diagram of metals orconductors
Conduction band is partially filledand the valence band is
partiallyempty or the conduction and valenceband overlap.
Due to overlap, electrons can easilymove into the conduction
band. Thissituation makes a large number ofelectrons available for
electricalconduction.
When the valence band is partiallyempty, electrons from their
lowerlevels can move to higher levelsmaking conduction
possible.
Energy band diagram forinsulators
Large band gap Eg exists. (Eg > 3 eV)
Since there are no electrons in theconduction band, no
electricalconduction is possible.
The electron cannot be excited fromthe valence band to the
conductionband by thermal excitation.
Energy band diagram for semi-conductors
Energy band gap Eg is small. (Eg > nh
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p-type semi-conductor
Doped with trivalent atoms such asaluminium, boron, or indium,
etc.
Dopant has one valence electron lessthan Si or Ge. Therefore,
the atomcan form covalent bonds with threeneighbouring Si atoms,
but does nothave any electron to offer to thefourth Si atom.
Therefore, the bond between thefourth neighbour and the
trivalentatom has a vacancy or hole.
Hole is available for conduction.One acceptor atom gives one
hole.
Holes are the majority carriers andelectrons are the minority
carriers.
For p-type semi-conductor, nh >> ne
Energy band diagram of the semi-conductorsat T > 0 K,
n-type semi-conductor
p-type semi-conductor
Diodes & Rectifiers
p-n Junction Formation
A thin p-type semi-conductor waferis considered. A part of it
isconverted into n-Si by adding asmall quantity of
pentavalentimpurity.
Wafer now contains a p-regionand n-region and a
metallurgicaljunction between p-, and n-region.
n-type semi-conductor has moreconcentration of electrons than
holeand p-type semi-conductor has moreconcentration of holes than
electron.Therefore, the holes diffuse from p-side to n-side and
electrons diffusefrom n-side to p-side.
When an electron diffusesfrom n to p, it leaves behind it
anionised donor on n-side. The ioniseddonor (+ ve charge) is
immobile as itis bounded by the surroundingatoms.
Therefore, a layer of positive chargeis developed on n-side of
thejunction.
Similarly, a layer of negative chargeis developed on the
p-side.
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Semiconductors
This space-charge region on eitherside of the junction together
is calleddepletion region.
The positive space-charge regionon n-side of the junction
andnegative space-charge region on p-side of the junction,
appearing aselectric field, is developed anddirected from + ve
charge to vecharge.
Due to the field, an electron from p-side moves to n-side and a
holefrom n-side of the junction movesto p-side.
The motion of charge carriers due toelectric field is called
drift currentand is opposite in direction to thediffusion
current.
Initially, diffusion current is largeand drift current is small.
Asdiffusion continues, the space chargeregions on either side of
the junctionextends, thereby increasing theelectric field strength
and hence driftcurrent. The process continues untilthe diffusion
current is equal to driftcurrent.
Thus, a p-n junction is formed.Under equilibrium, there is no
netcurrent.
Loss of electrons from the n-regionand gain of electron by the
p-region
causes a difference of potentialacross the junction of two
regions.This potential tends to prevent themovement of electronfrom
n to p region. Therefore, it iscalled a barrier potential.
Semi-conductor Diode
A semi-conductor diode is basicallya p-n junction with metallic
contactsprovided at the ends for theapplication of an external
voltage.
A p-n junction diode is symbolicallyrepresented as shown in the
figurebelow.
p-n junction diode under forward bias
p-side is connected to positiveterminal and n-side to the
negativeterminal.
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Applied voltage drops across thedepletion region.
Direction of applied voltage (V) isopposite to the build in
potential (V0).
As the depletion layer widthdecreases, the barrier height
isreduced.
Effective barrier height underforward bias is (V0 V).
Electron in n-region moves towardsthe p-n junction and holes in
p-region move towards the junction.The width of the depletion
layerdecreases and hence, it offers lessresistance.
Diffusion of majority carriers takesplace across the
junction.
This leads to forward current.
p-n junction diode under reverse bias
Positive terminal of battery isconnected to n-side and
negativeterminal to p-side.
Reverse bias supports the potentialbarrier. Therefore, the
barrier heightincreases and the width of depletionregion also
increases.
Effective barrier height underreverse bias is (V0 + V).
No conduction across the junctiondue to majority carriers;
fewminority carriers cross the junctionafter being accelerated by
highreverse bias voltage
This constitutes a current that flowsin opposite direction
celled reversecurrent.
Rectifier
It is a device used for converting alternatingcurrent/voltage
into direct current/voltage.
Half wave rectifier is based on theprinciple that the resistance
of p-n junction becomes low when it is
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forward biased and becomes highwhen reverse biased.
When voltage at A is positive, itconducts; and when negative, it
doesnot conduct.
Therefore, in the positive half cycleof ac, there is a
currentthrough RL and we obtain O/Pvoltage.
In the negative half cycle, there is nocurrent.
Since the rectified output of thiscircuit is only for half of
i/p ac wave,it is called half wave rectifier.
Full wave rectifier
Two diodes are used to give rectifiedO/P corresponding to both
positiveas well as negative half cycles.
When voltage at A with respect tothe centre tap is positive, and
thevoltage at B is negative. Then, D1 isforward biased and D2 is
reversedbiased. Hence, D1 conducts andD2 does not.
When voltage of A becomesnegative, then B becomes +
ve.Therefore, D1 does not conduct andD2 conducts. Hence, we
obtainoutput voltage during both thepositive as well as negative
half ofcycle.
Special Purpose p-n Junction Diodes
Zener Diode
It can operate in the reversebreakdown voltage
regioncontinuously without beingdamaged.
Symbol
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It is a heavily doped p-n junction.Due to this, depletion region
formedis very thin and the electric field ofthe junction is
extremely high, evenfor a small reverse bias voltage.
The IV characteristics of a zenerdiode are shown in the figure
below.
It is widely used to regulate thevoltage across the circuit.
Zener Diode as Voltage Regulator
After the break down voltage, smallchange in voltage across the
zenerdiode produces a large change incurrent through the
circuit.
If voltage is increased beyond zenervoltage, then the resistance
of thezener diode drops considerably.
Zener diode and a resistor areconnected to a fluctuating dc
supplysuch that the zener diode is reversebiased.
When the voltage across the diodetends to increase, the current
throughthe diode rises out of proportion andcauses a sufficient
increase involtage drop across the resistor.Therefore, the O/P
voltage lowersback to normal.
Photodiode
A junction diode made from lightsensitive semi-conductor is
called aphotodiode.
Current AB that flows when no lightis incident is called dark
current.
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When photons of light havingenergy hfall on the photodiode,more
electrons from valence bandmove to the conduction band,provided hv
is greater than forbiddenenergy gap.
The current in the circuit increases.As the intensity of light
is increased,the current goes on increasing as inpart BC.
When the current does not increasewith the increase in intensity
of light,the photodiode is said to besaturated. Portion CD of the
graphrepresents saturated current.
Light Emitting Diodes (LEDs)
All junction diodes emit some lightwhen forward biased.
Junction diode is made of galliumarsenide (GaAs). The energy
isreleased in infrared region whilethose made of gallium
arsenidephosphide (GaAsP) emit radiation invisible region. They are
called LEDs.
The most important part of a LED isthe p-n junction. The
junction acts asa barrier to the flow of electrons
between the p and n regions. Onlywhen sufficient voltage is
applied tothe LED, the electrons cross thejunction into the
p-region andcurrent flows through it.
Diode is encapsulated with atransparent cover so that
emittedlight can come out.
LEDs are biased such that the lightemitting efficiency is
maximum.
Semi-conductor used to fabricatevisible LEDs must have at least
1.8eV band gap.
LEDs have low operational voltageand less power. They requires
lesswarm-up time.
Solar Cell
It is a semi-conductor device used toconvert photons of solar
light intoelectricity.
It generates emf when solarradiations fall on the p-n
junction.
A p- Si wafer of about 300 m istaken, over which a thin layer n
Siis grown on one side by diffusionprocess.
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Semiconductors
The generation of emf by a solar cellwhen light falls on it is
due tofollowing three processes:
1. Generation of e-h pairs due to lightclose to the junction
2. Separation of electrons and holesdue to electric field of the
depletionregion
3. The electrons reaching the n-side arecollected by the front
contact andholes reaching p-side are collectedby the back contact.
Thus, p-sidebecomes positive and n-sidebecomes negative giving rise
tophotovoltage.
Semi-conductors with band gapclose to 1.5 eV are ideal
materialsfor solar cell fabrication.
Junction Transistor
There are two types of transistors:
n-p-n transistor
p-n-p transistor
n-p-n transistor
Two segments of n-type semi-conductor are separated by asegment
of p-type semi-conductor.
Schematic representation:
Symbol
p-n-p transistor
Two segments of p-type semi-conductor separated by a segmentof
n-type semi-conductor
Schematic representation
Symbol
Three segments of transistor:
Emitter Segment is on one side ofthe transistor. It is of a
moderate sizeand heavily doped. It supplies a huge
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number of majority carriers for thecurrent flow through the
transistor.
Base It is the central segment. It isvery thin and lightly
doped.
Collector It collects major portionof the majority carrier
supplied bythe emitter. It is moderately dopedand large in size
compared toemitter.
Transistor works as an amplifier withits emitter-base junction
forwardbiased and base collector junctionreverse biased.
VCC and VEE create the biasing. Thebiased transistor is said to
be inactive state.
VCB Collector base voltage
VEB Base emitter voltage
Base is the common terminal for thetwo power supplies whose
otherterminals are connected to emitterand collector
respectively.
Heavily doped emitter has a highconcentration of majority
carriers,which will be holes in p-n-p transistor and electron in an
n-p-n transistor.
These majority carriers enter thebase region in large numbers.
Thebase is lightly doped. Therefore, ithas few majority
carriers.
In p-n-p, the base has the majoritycarriers as electrons. The
largenumber of holes entering the basefrom the emitter swamps the
smallnumber of electrons there.
Since the base collector junction isreverse biased, the holes
whichappear as minority carriers at thejunction can easily cross
the junctionand enter the collector.
Base is made thin so that the holescross the junction instead of
movingto the base terminal.
Ih Hole current
Ie Electron current
Total current in a forward biased diodeis Ih + Ie.
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Emitter current
IE = Ih + Ie
Base current, IB
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Semiconductors
IB Base current
VCE Constant collector-emitter voltage
Output resistance (r0)
VCE Collector-emitter voltage
IC Collector current
IB Base current
Current amplification factor ()
IC Collector current
IB Base current
VCE Constant collector-emitter voltage
Transistor as a device
Transistor as a Switch
When the transistor is used in thecut-off or saturation region,
it acts asa switch.
Applying Kirchhoffs voltage rule to abovefigure,
VBB = IBRB + VBE
VCE = VCC ICRC
Where,
VBB dc input voltage (Vi)
VCE dc output voltage (Vo)
Vi = IBRB + VBE and
Vo = VCC ICRC
When Vi < 0.6, the transistor is incut-off.
Therefore, IC = O
Vo = VCC
When Vo > 0.6 V, then IC increases.Therefore, Vo decreases as
theterm ICRC increases.
With increase in Vi, IC increasesalmost linearly and as aresult,
Vo decreases linearly till itsvalue becomes less than about 1.0
V.
Change becomes non-linear and thetransistor goes to saturation.
If Vi isincreased further, then Vo becomesalmost zero.
When Vi is low, Vo is high and if Vi ishigh, then Vo is low.
When thetransistor is not conducting, it is saidto be switched off
and when it is
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driven into saturation, it is said to beswitched on.
Transistor as an Amplifier
When the transistor operates in theactive region, it acts as an
amplifier.
If Vo and Vi are small changes ino/p and i/p voltage, then Vo/Vi
iscalled small signal voltage gain, AV.
Vo = VCC ICRC
Vo = 0 RCIC
Similarly, Vi =IBRB + VBE
Vi = RBIB + VBE
AV = RCIC/RBIB
= ac (RC/RB)
Where, ac is equal to IC / IB
Transistor as an Amplifier (CEConfiguration)
Operating point is fixed in themiddle of its active region.
An ac i/p signal vi is superimposedon bias VBB (dc). The o/p is
takenbetween collector and ground.
Applying Kirchhoff law to theoutput loop,
VCC = VCE + ICRC
VBB = VBE + IBRB
vi O
Then, VBB + vi = VBE + IBRB + IB (RB + ri)
It is current gain denoted by Ai.
Change IC due to change in IB causesa change in VCE and the
voltage dropacross resistor RC, because VCC isfixed.
VCC = VCE + RCIC = 0
VCE = RCIC
Change in VCE is the o/p voltage Vo.
Vo = VCE = ac RCIB
Voltage gain of amplifier
Negative sign represents that the o/p voltageis in opposite
phase to i/p voltage.
Power gain (AP) is the product ofcurrent gain and voltage
gain.
AP = ac Av
Feed back amplifier and transistor oscillator
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In oscillator, the ac o/p is producedwithout any external i/p
signal.
An oscillator is a device in which theo/p power is returned back
to the i/p,in phase with the starting power (i.e.,as a positive
feedback).
Feedback can be achieved byinductive coupling or LC or
RCnetwork.
Feedback is accomplished byinductive coupling from one
coilwinding ( T1) to another coil winding( T2).
Current flows through T2.
The current increases from X to Y.
Coupling between T2 and T1 causes acurrent to flow in the
emitter current.It is a feedback from i/p to o/p.
As a result, this feedback currentalso increases from to .
Current in T2 connected in thecollector circuit acquires value Y
asthe transistor becomes saturated.
There is no further increase incollector current to the
magneticfield amount. T2 stops growing. Dueto static field, there
is no feedbackfrom T2 to T1. Collector currentdecreases fromY to
Z.
Decrease in IC causes the magneticfield to decay around the coil
T2.
T1 also decays. Emitter currentreaches when transistor is
cut-off.Both IE and IC stop flowing.
Therefore, the transistor comes backto its original state.
Resonant frequency of this tunedcircuit determines the frequency
atwhich the oscillator will oscillate.
Digital Electronics and LogicGates
In digital electronics, we use onlytwo levels of voltage 0 and
1.
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Logic Gates
A gate is a digital circuit thatfollows a certain logical
relationshipbetween the input and outputvoltage.
NOT, AND, OR, NAND, NOR arethe five common logic gates.
NOT gate
The output is the inverted version of theinput.
Truth table
Input Output
A Y
0 1
1 0
OR gate
The output is 1 when either of the input orboth the inputs are
1.
Truth table
I O O/P
A B Y
0 0 0
0 1 1
1 0 1
1 1 1
AND gate
The output is 1 only when both the inputsare 1.
Truth table
I O O/P
A B Y
0 0 0
0 1 0
1 0 0
1 1 1
NAND
Output is the inverted version of the outputof AND gate.
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Truth table
I O O/P
A B Y
0 0 1
0 1 1
1 0 1
1 1 0
NOR gate
Output is the inverted version of the outputof OR gate.
Truth table
I O O/P
A B Y
0 0 1
0 1 0
1 0 0
1 1 0
Integrated Circuits (IC)
It is the concept of fabricating anentire circuit on a small
single chipof a semi-conductor.
Monolithic integrated circuit is awidely used technique in which
asingle chip is used. Its dimension is 1mm 1 mm or even
smaller.
Depending on the nature of inputsignals, ICs can be of two
types.
Linear or analogue ICs: The linearICs process analogue signals,
whichchange smoothly and continuouslyover a range of values between
amaximum and a minimum. Theoutput is directly proportional to
theinput. Example: operationalamplifier
Digital ICs These process signalsthat have only two values.
Thesecontain logic gates.
(Logic gate 10) Small scale integration
(Logic gate 100) Medium scaleintegration
(Logic gate 1000) Large scaleintegration
(Logic gate > 1000) Very large scaleintegration
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