Subject Code : ECE – 101/102 BASIC ELECTRONICS COURSE MATERIAL For 1ST & 2ND Semester B.E....

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


ELECTRONICS

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

Senior Grade LecturerDept of Electronics and Communication EngineeringMIT, Manipal

BASIC ELECTRONICS

BY


ELECTRONICS

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


ELECTRONICS

Syllabus of module 1

Module 1• What is an Atom• Structure of an Atom• Energy Band Theory• EV-Unit of Energy• Classification of Materials based on Energy Band Theory• Properties of Semiconductor• Mobility, Current Density, conductivity• Intrinsic Semiconductor• Electron and hole in Intrinsic semiconductor

• Conduction by electron and holes• Conductivity of a semiconductor

• Law of Mass action Donor and acceptor impurities• Energy band diagram for extrinsic semiconductor

• Diffusion• Drift

• PN junction• PN junction as a diode

• VI characteristics


ELECTRONICS

Reference for module 1 :

• Integrated Electronics Millman Jocobs,Halkies.C.C• Electronics Principle Robert boylsted


ELECTRONICS

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 like glass, 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 silicon are two well-known semiconductors.


ELECTRONICS

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


ELECTRONICS

The Atom


ELECTRONICS



ELECTRONICS

neutrons carry no electrical charge at all



ELECTRONICS

The protons and neutrons cluster together in the central part of the atom,called the nucleus, and the electrons 'orbit' the nucleus



ELECTRONICS

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


ELECTRONICS


ELECTRONICS


• Atoms and Elements

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


ELECTRONICS

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.



ELECTRONICS


Constituents of Atoms


ELECTRONICS



ELECTRONICS

Atomic Structure



ELECTRONICS

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 1st level 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.


ELECTRONICS

Maximum Electron Capacities of the First Four Principle Energy Levels (Shells)

58 electronsThe Seventh Level is the Highest Occupied

Ground-State Electrons in any Element now Known

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


ELECTRONICS


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.


ELECTRONICS


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.


ELECTRONICS


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.


ELECTRONICS

Silicon Lattice

The main point here is that a silicon atom has four electrons which it canshare in covalent bonds with its neighbors. These simplified diagrams do not 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.


ELECTRONICS

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 largely determine 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.


ELECTRONICS

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)×10−19 J. where one electron volt is the energy required to move an electron across a potential difference of one volt.


ELECTRONICS

The electronvolt (symbol eV, or, rarely and incorrectly, 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 53×10−19 J. (Source: CODATA 2002 recommended values)

http://en.wikipedia.org/wiki/Joule


ELECTRONICS

Band theory

Electron energy levels in an insulator


ELECTRONICS

Energy levels of an atom’s electrons

A ball bouncing down a flightof stairs provides an analogyfor energy levels of electrons,because the ball can only reston each step, not betweensteps.

Third energy level (shell)

(a)

Second energy level (shell)

First energy level (shell)

Energyabsorbed

Energylost

An electron can move from one level to another only if the energyit gains or loses is exactly equal to the difference in energy betweenthe two levels. Arrows indicate some of the step-wise changes inpotential energy that are possible.

(b)

Atomicnucleus


ELECTRONICS

—18 electrons


ELECTRONICS

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 from this 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.


ELECTRONICS

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.


ELECTRONICS


ELECTRONICS

Semiconductor Energy Bands For 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 electrons can 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.


ELECTRONICS

Conductor Energy Bands

In terms of the band theory of solids, metals are unique as good conductors of electricity. This can be seen to be a result of their valence electrons being essentially free. In the band theory, this is depicted as an overlap of the valence band and the conduction band so that at least a fraction of the valence electrons can move through the material


ELECTRONICS

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

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

Where L is the length of the conductor, measured in metersA 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.


ELECTRONICS

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


ELECTRONICS


ELECTRONICS

Band structure of a semiconductor


ELECTRONICS


ELECTRONICS

• Silicon Energy Bands At finite temperatures, the number of electrons, which reach 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.


ELECTRONICS


ELECTRONICS

• Germanium Energy Bands At finite temperatures, the number of electrons, which reach 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


ELECTRONICS

• Electrical conductivity

Electrical conductivity is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current. The conductivity σ is defined as the ratio of the current density to the electric field strength : . . Conductivity is the reciprocal (inverse) of electrical resistivity, and has the SI units of siemens per metre (S·m-1). It is commonly represented by the Greek letter σ, but κ or γ are also occasionally used.

.


ELECTRONICS

Mobility , Conductivity and Current density of a semi conductor • In a semi conductor , there are two charged particles. One is

negatively charged free electrons while the other is positively charged hole. These particles move in opposite direction, under the influence of an electric field but as both are of opposite sign, they constitute current in the same direction.

If E is the applied electric field and V is the velocity with which these particles move then,

V=µE …………(1)Where µ= mobility of charged particle

The mobility of free electron is denoted as µn while the mobility of holes with µp. 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


ELECTRONICS

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

J=(nµn + pµp)eE ……………(2)

Where e= magnitude of charge on one electron σ = conductivity measured in (Ω- m)-1then the current density J can be expressed interms of conductivity as ,J= σ E ………(3)Hence from the above equations (2) and (3) , we can write σ =(nµn + pµp)e ………(4)for an intrinsic conductor , n = p = ni = intrinsic concentration,

substituting in equation (4) we get ,σi =ni (µn +µp)e …………(5)the equation (5) gives the conductivity of an intrinsic semi conductor

denoted as σi

2mA


ELECTRONICS

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, where the subscript i is added for intrinsic material.


ELECTRONICS

• These concentrations(or densities) are also interrelated by the law of electrical neutrality which we shall now state. Let ND be the concentration of donor atoms, which are practically all ionized(because of electrical neutrality mentioned above);consequently, ND positive charges per cubic metre are contributed by the donor ions. Hence the total positive charge density is ND+p. In a similar manner, 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 the semiconductor is electrically neutral, the positive and negative charge densities(or Concentrations)must be equal in magnitute ,

• or ND+p=NA+n ………(2)


ELECTRONICS

• Let us take an N-type material having NA=0. Now, in an N-Type semiconductor, the number of electrons is much greater than the number of holes (i.e., n≥p) then equation (2) becomes

• n≈ND ..(3)• In an n-Type semiconductor, the free –electron

concentration 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 n or p for an N-Type or a P-Type material respectively hence equation (3) is rewritten as nn≈ND . The concentration pn of holes in the N-type semiconductor is obtained from equation(1) is now written as nnpn=ni2. thus pn = D

i

Nn2


ELECTRONICS

Similarly for p –Type semiconductor, nppp=ni2

pp≈NA

np=A

i

Nn 2


ELECTRONICS

Intrinsic Semiconductor

• A silicon crystal is different from an insulator because at any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position, leaving behind an electron deficiency called a "hole”. If a voltage is applied, then both the electron and the hole can contribute to a small current flow. • The conductivity of a semiconductor can be modeled in terms of the band theory of solids. The band model of a semiconductor suggests that at ordinary temperatures there is a finite possibility that electrons can reach the conduction band and contribute to electrical conduction.


ELECTRONICS

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


ELECTRONICS

Electrons and Holes in Intrinsic semiconductor

In an intrinsic semiconductor like silicon at temperatures above absolute 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 produces extra 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.


ELECTRONICS


ELECTRONICS

Mechanism of Hole Current in Intrinsic semiconductor


ELECTRONICS

Intrinsic Semiconductor Current

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


ELECTRONICS

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.


ELECTRONICS

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.


ELECTRONICS

Introducing Dopants:

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 isdoped 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 much less than the band gap energy so it is shown close to the valence band. Holes are created without creating electrons


ELECTRONICS


ELECTRONICS

Dopingn-type:• replace few Si atoms by: e.g. As • Si has 4 valence electrons needed for

covalent bond• 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

ÎF

donor levels

acceptor levels

ÎF


ELECTRONICS


ELECTRONICS

Band Diagram: Donor Dopant in Semiconductor

• 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 energy level 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

http://jas.eng.buffalo.edu/education/semicon/fermi/bandAndLevel/fermi.html


ELECTRONICS

Band Diagram: Acceptor Dopant in Semiconductor

• For Si, add a group III element to “accept” an electron and make p-type Si (more positive “holes”!)

• “Missing” electrons trapped in acceptor energy level EA just above EV.

– Resultant holes in valence band increase conductivity.

• Fermi level EF moves down because there are fewer carriers.

EA

EC

EV

EF

p-type Si


ELECTRONICS

The Doping of Semiconductors

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


ELECTRONICS


ELECTRONICS

P- and N- Type Semiconductors


ELECTRONICS

The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).


ELECTRONICS

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 typical to use B2H6 diborane gas to diffuse boron into the silicon material.


ELECTRONICS

Charge densities in an Extrinsic Semiconductor • 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. The

densities of free electrons and holes are related by the law of electrically neutrality


ELECTRONICS

• Let ND be concentration of donor atoms. Since all donor atoms get ionized at room temperature, ND immobile positive charges per volume are contributed by the donor ions. Thus, the total positive charge density is p+ND. Similarly, let NA be concentration of acceptor atoms and let it contribute NA immobile negative charges per volume. Thus the total negative charge density is n+NA but the semi conductor is electrically neutral. Hence the magnitude of positive charge density must be equal to the magnitude of negative charge density.Charge densities in an Extrinsic Semiconductor


ELECTRONICS

Charge densities in an Extrinsic Semiconductor• 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

n≈ND

Thus we conclude that in an n type material, the free electron concentration is approximately equal to the density of donor atoms


ELECTRONICS

Charge densities in an Extrinsic Semiconductor

• Similarly in a P-type semiconductor with no donor doping (i.e., ND=0), the concentration of holes is very much greater than concentration of free electrons(i.e., p>>n) then the above equation reduces to p≈NA

• Thus we conclude that in a p-type material the hole concentration is approximately equal to the density of acceptor atoms.

• If a semiconductor is doped with equal donor and acceptor densities, then it remains intrinsic. In this case, the holes produced by the acceptor combines with the electron produced by the donors, thus resulting in no free charge carriers


ELECTRONICS

Diffusion: • Diffusion, being the spontaneous

spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Diffusion is the movement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration).


ELECTRONICS

Diffusion:


ELECTRONICS

Diffusion• In addition to the conduction current , there is another type of current due to the transport of charge carriers in a semi conductor. This mechanism is called diffusion and the resulted current is called diffusion current. The diffusion is a flow of charge carriers from a region of high density to s region of low density due to non uniform distribution of it. The current density due to this diffusion is proportional to the carrier density gradient. The constant of proportionality called diffusion constant or diffusion co-efficient D which has a unit of m 2/sec.


ELECTRONICS

Drift Current

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 intoconsideration 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.


ELECTRONICS

P-N Junction

One of the crucial keys to solid-state electronics is the nature 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 in one 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


ELECTRONICS


ELECTRONICS

The open circles on the left side of the junction above 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-type dopant. 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 diagram represents increasing electron energy.


ELECTRONICS

Depletion Region

When a p-n junction is formed, some of the free electrons in the n-region diffuse across the junction and combine with holes to form negative ions. In so doing they leave behind positive ions at the donor impurity sites.


ELECTRONICS

Depletion Region


ELECTRONICS

The P-N Junction Diode

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 for rectification in the building of DC power supplies.


ELECTRONICS


ELECTRONICS

End of module 1

Subject Code : ECE – 101/102 BASIC ELECTRONICS COURSE MATERIAL For 1ST & 2ND Semester B.E....

Documents

Transcript of Subject Code : ECE – 101/102 BASIC ELECTRONICS COURSE MATERIAL For 1ST & 2ND Semester B.E....