chapter 23 electronics.pdf

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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Semiconductors

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.


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.


Semiconductors

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


Semiconductors

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


Semiconductors

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.


Semiconductors

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.


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


Semiconductors

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.


Semiconductors

Emitter current

IE = Ih + Ie

Base current, IB

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


Semiconductors

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


Semiconductors

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.


Semiconductors

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.


Semiconductors

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


chapter 23 electronics.pdf

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