1.Introduction to Semi Conductors [EngineeringDuniya.com]

7/28/2019 1.Introduction to Semi Conductors [EngineeringDuniya.com]

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Department of Electronics and Communication Engineering,Manipal Institute of Technology, Manipal, INDIA BASIC ELECTRONICS

Subject Code : ECE

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BASIC ELECTRONICS

COURSE MATERIALFor

1ST & 2ND Semester B.E.

(Revised Credit System)

DEPARTMENT OF

ELECTRONICS & COMMUNICATION ENGINEERING


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Mr Jagadish NayakB.E(E&C),M.Tech (DEAC),MISTE,MBMESI

Senior Grade Lecturer

Dept of Electronics and

Communication Engineering

MIT, Manipal

BASIC ELECTRONICSBY


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Syllabus

Module 1. SEMI CONDUCTOR THEORY Pg. 1 26

Module 2. PN JUNCTION DIODE AND ITS APPLICATIONS Pg. 27 49

Module 3.

TRANSISTORS AND APPLICATIONS Pg. 50

72

Module 4. COMMUNICATION SYSTEMS Pg. 73 82

Module 5. OPERATIONAL AMPLIFIERS Pg. 83 96

Module 6. DIGITAL ELECTRONICS Pg. 97 131


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Syllabus of module 1

Module 1

What is an A tom

Structure of an A tom

Energy Band Theory

EV-Unit o f Energy

Classif ic ation of Materialsbased on Energy BandTheory

Proper t ies of Semico nduc tor

Mobi l i ty, Current Dens ity,conduct iv i ty

In t r ins ic Semico nduc tor

Electro n and hole in Intr ins icsemiconductor

Conduc t ion by electron andholes

Conduc t iv i ty of asemiconductor

Law of Mass act ion Dono rand acceptor impu r i t ies

Energy b and diagram forextrins ic semico nduc tor

Dif fusion

Drift

PN junc t ion

PN jun ct ion as a diode

VI characterist ics


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Reference for module 1 :

Integrated Electronics Millman Jocobs,Halkies.C.C

Electronics Principle Robert boylsted


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SEMI CONDUCTOR THEORY

Introduction

We know the importance of using the materials like copper,

aluminum etc. in electrical applications. This is because copper,

aluminum etc are good conductors. Similarly, some materials likeglass, wood, paper etc. Also, find wide applications in electrical

and electronic applications. These are called insulators. There is

another category of materials whose ability to carry current, called

conductivity, lies between that of conductor and insulators. Such

materials are known as semi conductors. Germanium and siliconare two well-known semiconductors.


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What are atoms?

Atoms are the basic building blocks of matter

that make up everyday objects. A desk, the air,even you are made up of atoms!

There are 90 naturally occurring kinds of atoms.

Scientists in labs have been able to make about

25 more


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The Atom


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neutrons carry no electrical charge at all



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The protons and neutrons cluster together in the central part of the

atom,called the nucleus, and the electrons 'orbit' the nucleus



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Electrons carry a negative electrical charge= -1.6x10-19 Coulombs


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Atoms and Elements

Ordinary matter is made up of protons, neutrons, and electrons and is

composed of atoms. An atom consists of a tiny nucleus made up of

protons and neutrons, on the order of 20,000 times smaller than the size

of the atom.

The outer part of the atom consists of a number of electrons

equal to the number of protons, making the normal atom

electrically neutral.

A chemical element consists of those atoms with a specific

number of protons in the nucleus; this number is called the

atomic number


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Elements are represented by a chemical symbol, with the atomic number andmass number sometimes affixed as indicated below. The mass number is the

sum of the numbers of neutrons and protons in the nucleus.

The atoms of an element may differ in the number of

neutrons; atoms with different neutron numbers are

said to be different isotopes of the element.



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Constituents of Atoms


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Atomic Structure



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Atomic ShellsThe discrete electron levels are arranged in

shells. Each shell has a maximum occupancy.

The first electronic shell can have at most 2

electrons, the second shell has room for 8

electrons and so on.

The 1st shell has the lowest energy. Thus,

elements, in their lowest energy state fill the 1stlevel first, and then fill the 2nd level next. These

elements are listed in the 1st and 2nd rows of

the periodic table.

Atoms are most stable if their outer shell is

full.

The electrons in outer shells are shielded by

the inner shells from the full attraction of the

nucleus. These electrons participate most

readily in chemical reactions.


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Maximum Electron Capacities of the First FourPrinciple Energy Levels (Shells)

58 electronsThe Seventh Level is the Highest Occup ied

Ground -State Electrons in any Element n ow K now n

1) The Principle Energy Level (cont)

n = 4 2n2 = 2 x 42 = 32 electrons

n = 3 2n2 = 2 x 32 = 18 electrons

n = 2 2n2 = 2 x 22 = 8 electrons

n = 1 2n2 = 2 x 11 = 2 electrons

The Quantum Numbers


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Silicon and Germanium

Solid state electronics arises from the unique properties of silicon and germanium,

each of which has four valence electrons and which form crystal lattices in which

substituted atoms can dramatically change the electrical properties.


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Silicon

In solid-state electronics, either pure silicon or germanium may be used as

the intrinsic semiconductor, which forms the starting point for fabrication.

Each has four valence electrons, but germanium will at a given temperature

have more free electrons and a higher conductivity. Silicon is by far the

more widely used semiconductor for electronics, partly because it can be

used at much higher temperatures than germanium.


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Germanium

In solid-state electronics, either pure silicon or germanium may be used as the

intrinsic semiconductor, which forms the starting point for fabrication. Each has

four valence electrons, but germanium will at a given temperature have more

free electrons and a higher conductivity. Silicon is by far the more widely used

semiconductor for electronics, partly because it can be used at much higher

temperatures than germanium.


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Silicon Lattice

The main point here is that a silicon atom has four electrons which it can

share in covalent bonds with its neighbors. These simplified diagrams donot do justice to the nature of that sharing since any one silicon atom will

be influenced by more than four other silicon atoms, as may be appreciated

by looking at the silicon unit cell.


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Valence Electrons

The electrons in the outermost shell of an atom are called valence electrons;

they dictate the nature of the chemical reactions of the atom and largelydetermine the electrical nature of solid matter. The electrical properties

of matter are pictured in the band theory of solids in terms of how much

energy it takes to free a valence electron.


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Electron-volt

The electron-volt (symbol eV, or, rarely and

incorrectly, ev) is a unit of energy. One electron-

volt is a very small amount of energy:1 eV = 1.60217653(14)1019 J. where one

electron volt is the energy required to move an

electron across a potential difference of one volt.


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The electronvolt (symbol eV, or, rarely andincorrectly, ev) is a unit of energy. It is the amount of

kinetic energy gained by a single unbound electron

when it passes through an electrostatic potential

difference of one volt, in vacuum. In other words, it's

equal to one volt times the magnitude of charge of a

single electron. The one-word spelling is the modern

recommendation although the use of the earlier

electron volt still exists.

One electronvolt is a very small amount of energy:1 eV = 1.602 176 531019 J. (Source: CODATA

2002 recommended values)
http://en.wikipedia.org/wiki/Joulehttp://en.wikipedia.org/wiki/Joule


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Band theory

Electron energy levels in an insulator


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Energy levels of an atoms electrons

A ball bouncing down a flight

of stairs provides an analogyfor energy levels of electrons,

because the ball can only rest

on each step, not between

steps.Third energy level (shell)

(a)

Second energy level (shell)

First energy level (shell)

Energyabsorbed

Energy

lost

An electron can move from one level to another only if the energy

it gains or loses is exactly equal to the difference in energy between

the two levels. Arrows indicate some of the step-wise changes in

potential energy that are possible.

(b)

Atomic

nucleus


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18 electrons


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The Valence BandThe valence band is the band made up of the occupied

molecular orbital and is lower in energy than the

so-called conduction band. It is generally completely

full in semi-conductors. When heated, electrons fromthis band jump out of the band across the band gap and

into the conduction band, making the material conductive.

The valance band can be seen in the diagram.


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Conduction Band

The conduction band is the band of orbital that are high

in energy and are generally empty. In reference to

conductivity in semiconductors, it is the band that accepts

the electrons from the valence band. The conduction

band can be seen in the diagram.


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Semiconductor Energy BandsFor intrinsic semiconductors like silicon and germanium,

the Fermi level is essentially halfway between the valence

and conduction bands. Although no conduction occurs at

0 K, at higher temperatures a finite number of electronscan reach the conduction band and provide some current.

In doped semiconductors, extra energy levels are added.

The increase in conductivity with temperature can be

modeled in terms of the Fermi function, which allows one

to calculate the population of the conduction band.


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Conductor Energy Bands

In terms of the band theory of solids, metals areunique as good conductors of electricity. Thiscan be seen to be a result of their valence

electrons being essentially free. In the bandtheory, this is depicted as an overlap of thevalence band and the conduction band so that atleast a fraction of the valence electrons can

move through the material


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Resistance of a conductorAs long as the current density is totally uniform in the

conductor, the uniform resistance R of a conductor ofregular cross section can be computed as

WhereL is the length of the conductor, measured in meters

A is the cross-sectional area, measured in square meters

is the electrical resistivity (also called specific electrical

resistance) of the material, measured in ohm meter.Resistivity is a measure of the material's ability to oppose

the flow of electric current.


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Properties of Semiconductors

A semiconductor has the following prominent properties:

The resistivity of a semiconductor is less than that of an insulator but

more than that of a conductor

A semiconductor has negative temperature coefficient of resistance,i.e., the resistance of a semiconductor decreases with the increase in

temperature and vice-versa. For example, Germanium is actually an

insulator at low temperature , but it becomes a good conductor at high

temperatures.

When some suitable impurity (e.g. Arsenic, Gallium etc.,) is added to a

semiconductor, its conducting properties change appreciably


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Band structure of a semiconductor


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Silicon Energy Bands

At finite temperatures, the number of electrons, whichreach the conduction band and contribute to current, can

be modeled by the Fermi function. That current is small

compared to that in doped semiconductors under the

same conditions.


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Germanium Energy Bands

At finite temperatures, the number of electrons, whichreach the conduction band and contribute to current, can

be modeled by the Fermi function. That current is small

compared to that in doped semiconductors under the

same conditions


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Electrical conductivity

Electrical conductivity is a measure of a material's abilityto conduct an electric current. When an electrical

potential difference is placed across a conductor, itsmovable charges flow, giving rise to an electric current.The conductivity is defined as the ratio of the current

density to the electric field strength : . .Conductivity is the reciprocal (inverse) of electricalresistivity, and has the SI units of siemens per metre(Sm-1). It is commonly represented by the Greek letter, but or are also occasionally used.

.


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Mobility, Conductivity and Current density of a semi conductor

In a semi conductor , there are two charged particles. One isnegatively charged free electrons while the other is positivelycharged hole. These particles move in opposite direction, under theinfluence of an electric field but as both are of opposite sign, theyconstitute current in the same direction.

If E is the applied electric field and V is the velocity with

which these particles move then,

V=E (1)

Where = mobility of charged particle

The mobility of free electron is denoted as n while the mobility of holes with p.In a pure semi conductor the number of holes and free electrons are same in number

Let n= concentration of free electrons P= concentration of holes


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Then the current density J which is current per unit area of theconducting medium is given by,

J=(nn + pp)eE (2)

Where e= magnitude of charge on one electron

= conductivity measured in (- m)-1

then the current density J can be expressed interms of conductivity as ,J= E (3)

Hence from the above equations (2) and (3) , we can write

=(nn + pp)e (4)

for an intrinsic conductor , n = p = ni = intrinsic concentration,substituting in equation (4) we get ,

i =ni (n +p)e (5)the equation (5) gives the conductivity of an intrinsic semi conductor

denoted as i

2

mA


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Law of mass action:

An important relation related to the charge

densities in a semiconductor is called the law of

mass action

np=ni2

..(1) States that in any semi conductor, regardless of

the donor or acceptor concentrations or

magnitudes of n and p, the product np is always

constant (=ni2) , at a fixed temperature, wherethe subscript i is added for intrinsic material.


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These concentrations(or densities) are also interrelatedby the law of electrical neutrality which we shall nowstate. Let ND be the concentration of donor atoms, whichare practically all ionized(because of electrical neutralitymentioned above);consequently, ND positive chargesper cubic metre are contributed by the donor ions. Hence

the total positive charge density is ND+p. In a similarmanner, if NA is the concentration of the acceptor ions,these contribute NA negative charges per cubic metre;the total negative-charge density is NA+n. as thesemiconductor is electrically neutral, the positive and

negative charge densities(or Concentrations)must beequal in magnitute ,

or ND+p=NA+n (2)


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Let us take an N-type material having NA=0. Now, in anN-Type semiconductor, the number of electrons is much

greater than the number of holes (i.e., np) thenequation (2) becomes

nND ..(3)

In an n-Type semiconductor, the freeelectronconcentration is approximately equal to the density of the

donor atoms. In order to distinguish between the concentration of

donor and acceptor materials, let us add the subscript nor p for an N-Type or a P-Type material respectivelyhence equation (3) is rewritten as nnND . The

concentration pn of holes in the N-type semiconductor isobtained from equation(1) is now written as nnp

n=ni2.

thus pn

=D

i

N

n2


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Similarly for pType semiconductor,nppp=ni

2

ppNA

np=A

i

N

n

2


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Intrinsic Semiconductor

A silicon crystal is different from an insulator because atany temperature above absolute zero temperature, thereis a finite probability that an electron in the lattice will beknocked loose from its position, leaving behind anelectron deficiency called a "hole. If a voltage is applied,then both the electron and the hole can contribute to asmall current flow.

The conductivity of a semiconductor can be modeled interms of the band theory of solids. The band model of a

semiconductor suggests that at ordinary temperaturesthere is a finite possibility that electrons can reach theconduction band and contribute to electrical conduction.


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The term intrinsic here distinguishes between the properties of pure "intrinsic"

silicon and the dramatically different properties of doped n-type or p-type

semiconductors


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Electrons and Holes in Intrinsic semiconductor

In an intrinsic semiconductor like silicon at temperatures aboveabsolute zero, there will be some electrons which are excited

across the band gap into the conduction band and which can

produce current. When the electron in pure silicon crosses the

gap, it leaves behind an electron vacancy or "hole" in the regular

silicon lattice. Under the influence of an external voltage, both

the electron and the hole can move across the material.

In an n-type semiconductor, the dopant contributes extra electrons, dramatically

increasing the conductivity. In a p-type semiconductor, the dopant producesextra vacancies or holes, which likewise increase the conductivity. It is however

the behavior of the p-n junction which is the key to the enormous variety of

solid-state electronic devices.


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Mechanism of Hole Current in Intrinsic semiconductor


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Mechanism of Hole Current in Intrinsic semiconductor

Intrinsic Semiconductor Current


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Intrinsic Semiconductor Current

Both electrons and holes contribute to current flow in an intrinsic semiconductor


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The current, which will flow in an intrinsic semiconductor, consists

of both electron and hole current. That is, the electrons, which have

been freed from their lattice positions into the conduction band,

can move through the material. In addition, other electrons can hop

between lattice positions to fill the vacancies left by the freed

electrons. This additional mechanism is called hole conduction

because it is as if the holes are migrating across the material in

the direction opposite to the free electron movement.


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The current flow in an intrinsic semiconductor is influenced by the density of energy

states, which in turn influences the electron density in the conduction band. This

current is highly temperature dependent.

Introducing Dopants:


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When a semiconductor is doped, energy states are introduced in the

band gap. If it is doped with donors, the energy states are called

donor states. Because it takes very little energy, much less than the

band gap energy, to free the electron that inhabits the donor state,

the states are shown close to the conduction band. Adding donors,

therefore, adds more electrons to the conduction band (without

adding holes to the valence band) making the semiconductor more

conductive.

Acceptor states are introduced into the forbidden gap if the semiconductor is

doped with acceptors. These initially empty states readily accept an electron

to complete its bonds with the four nearest neighbors in the crystal. When an

electron from the valence band transitions to an acceptor state, it leaves behind

a hole. The energy required for an electron to move to an acceptor state is muchless than the band gap energy so it is shown close to the valence band. Holes

are created without creating electrons


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Doping


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p gn-type: replace few Si atoms by: e.g. As

Si has 4 valence electrons needed for covalentbond As has 5 valence electrons 1 excess electron excess electron needs fractions of eV to

reach the conduction band excess electron state is called donor level Fermi energy is raised towards the conduction

band

p-type: same principle, but one electron too little e.g. replacement of Si by Ga excess vacancy, excess hole electron from the valence band can easily

reach the so called acceptor levels

F

donor levels

acceptor levels

F


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Band Diagram: Donor Dopant in Semiconductor


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For group IV Si, add a group V element to

donate an electron and make n-type Si

(more negative electrons!)

Extra electrons donated from donor energylevel ED just below EC.

Resultant electrons in conduction

band increase conductivity by

increasing free carrier density n.

Fermi level EF moves up because there are

more carriers.

Increase the conductivity of a semiconductor by

adding a small amount of another material called a

dopant (instead of heating it!)

EF ED

n-type Si

Fermi Function & Doping: http://jas.eng.buffalo.edu/education/semicon/fermi/bandAndLevel/fermi.html

EC

EV

Band Diagram: Acceptor Dopant in Semiconductor
http://jas.eng.buffalo.edu/education/semicon/fermi/bandAndLevel/fermi.htmlhttp://jas.eng.buffalo.edu/education/semicon/fermi/bandAndLevel/fermi.htmlhttp://jas.eng.buffalo.edu/education/semicon/fermi/bandAndLevel/fermi.htmlhttp://jas.eng.buffalo.edu/education/semicon/fermi/bandAndLevel/fermi.html


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For Si, add a group IIIelement to accept an

electron and makep-type Si (more positive

holes!)

Missing electrons trapped in acceptor

energy level EA just above EV.

Resultant holes in valence bandincrease conductivity.

Fermi level EF moves down because there are

fewer carriers.

EA

EC

EVEF

p-type Si

The Doping of Semiconductors


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The addition of a small percentage of foreign atoms in the regular

crystal lattice of silicon or germanium produces dramatic changes

in their electrical properties, producing n-type and p-type semiconductors.

Pentavalent impurities Impurity atoms with 5 valence electrons produce n-type

semiconductors by contributing extra electrons. Trivalent impurities Impurity

atoms with 3 valence electrons produce p-type semiconductors by producing

a "hole" or electron deficiency


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P- and N- Type Semiconductors


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The addition of pentavalent impurities such as antimony, arsenic

or phosphorous contributes free electrons, greatly increasing theconductivity of the intrinsic semiconductor. Phosphorous may be

added by diffusion of phosphine gas (PH3).


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P-Type Semiconductor

The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic

semiconductor creates deficiencies of valence electrons, called "holes". It is typicalto use B2H6 diborane gas to diffuse boron into the silicon material.

Charge densities in an Extrinsic Semiconductor


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In intrinsic semi conductor, the electron density and hole

density are equal (ie ni=pi). In extrinsic semiconductor

the product of electron density n and hole density p is

equal to the square of the intrinsic concentration ni.

i.e., np =ni2

The above equation is called law of mass action. Thedensities of free electrons and holes are related by the

law of electrically neutrality



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Let ND be concentration of donor atoms. Sinceall donor atoms get ionized at room temperature,ND immobile positive charges per volume arecontributed by the donor ions. Thus, the totalpositive charge density is p+N

D. Similarly, let N

A

be concentration of acceptor atoms and let itcontribute NA immobile negative charges pervolume. Thus the total negative charge densityis n+NA but the semi conductor is electrically

neutral. Hence the magnitude of positive chargedensity must be equal to the magnitude ofnegative charge density.



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i.e., p+ND=n+NA

let us consider an n-type semiconductor with no acceptor

doping(I.e.,NA=0). In such material the concentration of

electron n is much greater than the concentration of

holes p(i.e.,n>>p). then, above equation reduces to

nNDThus we conclude that in an n type material, the free

electron concentration is approximately equal to the

density of donor atoms



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Similarly in a P-type semiconductor with no

donor doping (i.e., ND=0), the concentration ofholes is very much greater than concentration offree electrons(i.e., p>>n) then the aboveequation reduces to pNA

Thus we conclude that in a p-type material thehole concentration is approximately equal to thedensity of acceptor atoms.

If a semiconductor is doped with equal donorand acceptor densities, then it remains intrinsic.

In this case, the holes produced by the acceptorcombines with the electron produced by thedonors, thus resulting in no free charge carriers

Diffusion:


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Diffusion, being the spontaneous

spreading of matter (particles), heat, ormomentum, is one type of transport

phenomena. Diffusion is the movement of

particles from higher chemical potential tolower chemical potential (chemical

potential can in most cases of diffusion be

represented by a change in

concentration).

Diffusion:


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Diffusion


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In addition to the conduction current , there isanother type of current due to the transport ofcharge carriers in a semi conductor. Thismechanism is called diffusion and the resultedcurrent is called diffusion current. The diffusionis a flow of charge carriers from a region of highdensity to s region of low density due to nonuniform distribution of it. The current density dueto this diffusion is proportional to the carrier

density gradient. The constant of proportionalitycalled diffusion constant or diffusion co-efficientD which has a unit of m2/sec.

Drift Current


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Drift is, by definition, charged particle motion in response to an

applied electric field. When an electric field is applied across asemiconductor, the carriers start moving, producing a current.

The positively charged holes move with the electric field,

whereas the negatively charged electrons move against the

electric field. The motion of each carrier can be described as

a constant drift velocity, vd. This constant takes into

consideration the collisions and setbacks each carrier has

while moving from one place to another. It is considered a

constant though, because the carriers will eventually go the

direction they are supposed to go regardless of any setbacks,especially if you look at the direction of all the carriers, instead

of each one individually.

P-N Junction


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One of the crucial keys to solid-state electronics is thenature of the P-N junction. When p-type and n-type

materials are placed in contact with each other, the

junction behaves very differently than either type of

material alone. Specifically, current will flow readily inone direction (forward biased) but not in the other

(reverse biased), creating the basic diode. This non-

reversing behavior arises from the nature of the

charge transport process in the two types of materials


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The open circles on the left side of the junctionabove represent "holes" or deficiencies of electrons

in the lattice, which can act like positive charge

carriers. The solid circles on the right of the junction

represent the available electrons from the n-typedopant. Near the junction, electrons diffuse across to

combine with holes, creating a "depletion region".

The energy level sketch above right is a way to

visualize the equilibrium condition of the P-N

junction. The upward direction in the diagramrepresents increasing electron energy.

Depletion Region


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When a p-n junction is formed, some of the free

electrons in the n-region diffuse across thejunction and combine with holes to form

negative ions. In so doing they leave behind

positive ions at the donor impurity sites.

Depletion Region


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The P-N Junction Diode


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The nature of the p-n junction is that it will conduct

current in the forward direction but not in the

reverse direction. It is therefore a basic tool forrectification in the building of DC power supplies.


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End of module 1

1.Introduction to Semi Conductors [EngineeringDuniya.com]

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Transcript of 1.Introduction to Semi Conductors [EngineeringDuniya.com]