2 Semiconductors

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Unit-1Unit-1

SemiconductorsSemiconductorsPhysicsPhysics


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ELECTRONICS

The wordElectronicsis derived fromElectron Mechanicswhich meansthestudy of the behavior of an electron underdifferent conditions of externally appliedfields.

Electronics is a field of Science andEngineering, which deals with electronicdevices and their utilization.

Electronic device is a device in whichconduction takes place by the movementof electrons through vacuum, gas orsemiconductors.


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Applications of Electronics

Communications & Entertainments

Medical Sciences

Defence Applications

Industrial Applications

Instrumentations

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Communications andEntertainments

Line CommunicationsTelegraphyTelephonyTelexTeleprinters

Wireless CommunicationsRadio broadcastingTV broadcastingFacsimile (Radiophoto)

Audio Systems

Public Address (PA) systemsStereo amplifiersRecord playersTape recorders



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Medical Sciences

Electrocardiogram (ECG)

Electrotherapy

Modern X-rays



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Defence Applications

Radar guided missiles

Coded communications



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Industrial Applications

Automatic control systems

Heating and welding systems

Electron Microscope

Computers



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InstrumentationInstrumentation

Precision measuring instruments

Cathode Ray Oscilloscope (CRO)

Vacuum Tube Volt Meter (VTVM)

Digital Volt Meter (DVM)Frequency Counters

Signal generator

pH meterStrain gauge

etc.



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Atomic Structure



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Random Movement of Electronsin Aluminium atoms.



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Atoms



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Permissible Energy Levels(for an isolated hydrogen atom)



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Types of Materials

Conductors

Semiconductors

Insulators



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Conductors Electrical conductivity is excellent. Concentration of free electrons is very

large ~1028 electrons/metre3. Under the influence of an applied electric

field the electrons may acquire additionalenergy and move into higher states.

The conduction and the valence bandoverlaps with each other.

Have positive temperature coefficient of

resistance, i.e., resistance increases withincrease in temperature.



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Insulators

Very poor conductor of electricity.

Concentration of free electrons is verysmall ~ 107 electrons/meter3.

Large forbidden band gap separates filledvalence band from the vacant band.

The energy which can be supplied to anelectron from an applied field is too small

to carry the electrons from the filled tothe vacant band.



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Semiconductors Conductivity is in between that of conductors

and insulators. Concentration of free electrons is moderateand is in between that of conductors andinsulators.

Width of the forbidden gap is relativelysmall~ 1 eV.

At 0 K : Egfor Ge ~ 0.785 eV

Egfor Si ~ 1.21 eV

Band gap energy in a crystal is a function ofinteratomic spacing and hence dependssomewhat on temperature.

Has negativetemperature coefficient ofresistance i.e. resistance decreases withJuly 2, 2013 Introduction 16


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Energy Bands



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Materials for Semiconductors

Germanium and Silicon, which hastetravalent atoms are the two mostimportant semiconductors used in

electronic devices.



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Types of Semiconductors

IntrinsicSemiconductor

The semiconductorformed by pure

tetravalent Siliconor Germanium isknown as theIntrinsic Semi-conductor. A semi-conductor is not

truly intrinsicunless the impuritycontent is less thanone part impurity in100 million parts of

semiconductor.

ExtrinsicSemiconductor

The semiconductorformed by deliberate

doping of trivalent orpentavalent impuritiesin tetravalent Siliconor Germanium isknown as ExtrinsicSemiconductor.



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Intrinsic Semiconductors

Electrons and Holes in IntrinsicSemiconductors

Covalent Bond in IntrinsicSemiconductor Crystal

Generation of Electron-Hole Pair

Effective Mass



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Crystal Structure ofSemiconductors

The valence electrons insemiconductors are not free towander about as in metal, rather

are trapped in a bond betweentwo adjacent atoms.

The crystal structure of

tetravalent Germanium or Siliconcrystal consists of regularrepetition of a unit cell in threedimensions having the form of a

tetrahedron with an atom atJuly 2, 2013 Semiconductors 22


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Crystalline structure(Two dimensional representation)



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Generation of Electron-HolePair

At very low temperature (0 K)the crystal behaves like a perfectinsulator, since no free carriersof electricity are available.

At room temperature some ofthe covalent bonds will bebroken because of the thermalenergy supplied to the crystal.

An electron is dislodged and isfree to wander in a randomfashion throughout the crystal,

just like electrons in a metal.



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The energy required to breaksuch a covalent bond is about

0.72 eV for Ge and 1.12 eV for Siat room temperature (300 K).

When an electron breaks away,only 3 electrons are left around acore with +4 charge.

This vacancy is called ahole.

Hole is a vacancy in a covalent

bond, which has +1 unit chargeassociated with it.



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In an intrinsic semiconductor thenumber of holes is equal to the

number of free electrons, i.e.,n =p = ni

where n andp are the electron

and hole concentrationrespectively and ni is called the

intrinsic carrier concentration.



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Since n1=

n

2, there is no net flow of

electrons across the planeXY.

There is no current due to randommovement of free electrons.



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Contribution of holesto conductivity

The importance of holes is thatthey serve as the carriers ofelectricity, comparable in

effectiveness with free electron.When a bond is incomplete (a

hole exists), it is relatively easyfor a valence electron in aneighboring atom to leave itscovalent bond to fill this hole.

This leaves a hole in its initial

position.July 2, 2013 Semiconductors 32


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The hole in its new position, maynow be filled up by an electron

of another bond and the hole willcorrespondingly move one morestep in the direction opposite to

the motion of electron.Hence the hole effectively moves

in the direction opposite to that

of the electrons.



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Hole as a Particle

Hole is just a vacancy, having apositive charge.

For convenience, we treat hole as a

particle.Both the electron and hole have

same amount of charge (e = 1.6

10

-19

C).We associate with a hole, a mass

called effective mass, though it ismeaningless.



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In fact, quantum mechanics is used tospecify the motion of a conduction

electron or a hole under the influenceof an external force.

It is possible to treat the hole andelectron as imaginary charged

particle with effective positivemasses mp and mn, respectively.

Effective mass approximation

removes quantum features of theproblem and allows us to useNewtons laws to determine theeffect of external forces on free

electrons and holes in a crystal.July 2, 2013 Semiconductors 35


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The effective mass of a hole ismore than that of an electron.

That is,mp> mn

Why ?

Ans. The hole moves slower thanan electron, when same externalfield is applied.



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Recombination ofElectrons and Holes

Recombination is a process in whichthe free electrons in the conductionband jumps into the valence band tocombine with holes.

In the process of recombination theelectron-hole pair is destroyed.The rate of recombination is

approximately proportional to the

product of electron concentration andhole concentration.In the recombination process the

minimum energy released in the formof electromagnetic radiation is equal

to the band gap.July 2, 2013 Semiconductors 37


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While some electron hole pair is lostby recombination, new pairs are

generated due to thermal excitation.For pure semiconductors at constant

temperature the rate ofrecombination and the rate of

generation of the electron-hole pairsare equal so that the electron andhole concentrations remain constantat their thermal equilibrium value.

r = g

If the temperature increases, thethermal equilibrium value of the

electron and the hole concentrationJuly 2, 2013 Semiconductors 38

Fermi Energy Level


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Fermi Energy Level(In a Conductor)



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The energy distribution ofelectrons in a conductor is as

shown. Fermi energy level is the

maximum energy that a free

electron can have at 0 K, in aconductor.

On raising the temperature, the

total number of electrons remainthe same; but some electronshave energy higher than EF.



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Fermi Energy Level(In a Semiconductor)

It is defined as the energy thatcorresponds to the centre of gravity ofthe conduction electrons (in CB) and

holes (in VB), weighted according totheir energies.

Thus, in an intrinsic semiconductor it lies

in the middle of the forbidden energygap.



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Review

Types of Semiconductors. Intrinsic Semiconductors.

Crystalline structure.

Generation of Electron-Hole Pair. Random Movement of Carriers.

Hole as a Particle.

Recombination of Electrons andHoles.

Fermi Energy level.

2 Semiconductors

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