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Department of Electronics and Communication Engineering,Manipal Institute of Technology, Manipal, INDIA BASIC ELECTRONICS
Subject Code : ECE
101/102
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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SEMI CONDUCTOR THEORY
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neutrons carry no electrical charge at all
SEMI CONDUCTOR THEORY
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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
SEMI CONDUCTOR THEORY
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Electrons carry a negative electrical charge= -1.6x10-19 Coulombs
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SEMI CONDUCTOR THEORY
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.
SEMI CONDUCTOR THEORY
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SEMI CONDUCTOR THEORY
Constituents of Atoms
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SEMI CONDUCTOR THEORY
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Atomic Structure
SEMI CONDUCTOR THEORY
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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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SEMI CONDUCTOR THEORY
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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SEMI CONDUCTOR THEORY
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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SEMI CONDUCTOR THEORY
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)
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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.html7/28/2019 1.Introduction to Semi Conductors [EngineeringDuniya.com]
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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
Charge densities in an Extrinsic Semiconductor
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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.
Charge densities in an Extrinsic Semiconductor
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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
Charge densities in an Extrinsic Semiconductor
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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