EEE231 Electronics 1

Lecture 1Course Introduction

&Basic Concepts

Instructor Details(Muhammad Rizwan Azam)

• Educational Backgroundo PHD .. (Control Systems)o MS (Control Systems)o BSc Eng. (Electronics)

• Contact Detailso Room # 113, EE Blocko rizwan.azam@comsats.edu.pk

Course Prerequisites1. Electric Circuit analysis 1 (EEE 121)2. Calculus and Analytic Geometry (MTH104)3. Applied Physics

Marks Distribution (Theory)Sessional -1 10%Sessional 2 15%Quizzes (4) 15%Assignments (4) 10%Terminal Exam 50%

Note: Quizzes will be announced as well un announced.Expect a quiz after submission of assignment.Copied assignments and assignments submitted after the due date will be marked zero.

Course Objectives1. To make students understand construction, physical

characteristics and operation of the major semiconductor devices.

2. To study, analyze and design circuits using semiconductor devices.

3. To illustrate how the device characteristics are utilized in switching, digital, and amplification applications.

Topics to be Covered1) Semiconductor material and properties 2) Diode model, equivalent model and circuit analysis 3) Analysis of diode based circuits (rectifiers, clipper, clampers,

etc.) 4) Special diodes characteristics and applications 5) Bipolar transistors characteristics and modes of operations 6) DC analysis, dc load line, biasing of bipolar transistor

circuits 7) Small signal analysis of bipolar transistor circuits 8) Design and analysis of common bipolar transistor amplifiers

Course Material• Textbooks:

o Electronic Circuits Analysis and Design, 3rd Edition, by Donald A Neamen.

o Electronic devices and circuit theory, 10th ed. Boylestad.• Reference Books:

o Electronic Devices 7th (or 9th ) Edition by Floyd.o Electronics Devices and Circuits, 6th Edition by Bogart.o Introductory Electronic Devices and Circuits, 4th Edition

by Robert T. Paynter.o Microelectronic Circuits 6th Edition by Sedra, Smith.

Why Electronics ?• The giant strides that we have made in the areas of

communications and computers are possible only because of the great successes that we have achieved in the field of electronics.

Some Basic Concepts• Electronics :

o Science of the motion of charges in a gas, vacuum, or semiconductor.

• Ohm’s Law:o The current through a conductor is directly proportional

to the potential difference.

• Electrical Power:o Power is how much work is done over time.

AC and DC• Direct Current (DC):

o Flow of charge in one direction,o Current maintains the same polarity,o DC is produced by sources such as batteries, solar cells

etc.• Alternating Current (AC):

o An alternating voltage source periodically alternates or reverses in polarity.

o The resulting current, therefore, periodically reverses in direction.

Electronic Circuits• In most electronic circuits

o There are two inputs.o One input is from a power supply that provides dc

voltages and currents to establish the proper biasing for the circuit.

o The second input is a signal that can be amplified by the circuit.

o The output signal can be larger than the input signal.

Analog and Digital Signals• Analog Signals

o The magnitude of an analog signal may have any value.o The amplitude may vary continuously with respect to

time.o Electronic circuits that process such signals are called

analog circuits.

Analog and Digital Signals• Digital Signals

o An alternative signal is at one of two distinct levels and is called a digital signal.

o The digital signal has discrete values, it is said to be quantized.

o Electronic circuits that process digital signals are called digital circuits.

Atom• An atom is composed of ;

o A nucleus, which contains positively charged protons and neutral neutrons,

o And negatively charged electrons that, orbit the nucleus. • The electrons are distributed in various “shells” at

different distances from the nucleus, • Electron energy increases as shell radius increases.• Electrons in the outermost shell are called valence electrons,

Electronic Materials

• The basic goal of electronic materials is to generate and control the flow of an electric current.

• Electronic materials include:o Conductors: have low resistance which allows electric

current flowo Insulators: have high resistance which suppresses electric

current flowo Semiconductors: can allow or suppress electrical current flow

Insulators• Insulators have a high resistance so current does not flow

in them.• Have 8 valence electrons• Good insulators include:

o Glass, ceramic, plastics, & wood• Most insulators are compounds of several elements. • The atoms are tightly bound to one another so electrons

are difficult to strip away for current flow.

Insulators

Insulators have tightly bound electrons in their outer shellThese electrons require a very large amount of energy to free them for conduction

Let’s apply a potential difference across the insulator above…

The force on each electron is not enough to free it from its orbit and the insulator does not conduct

Insulators are said to have a high resistivity / resistance

Conductors• Good conductors have low resistance so electrons flow

through them with ease.• Best element conductors include:

o Copper, silver, gold, aluminum, & nickel • Alloys are also good conductors:

o Brass & steel • Good conductors can also be liquid:

o Salt water

Conductors

Conductors have loosely bound electrons in their outer shellThese electrons require a small amount of energy to free them for conduction

Let’s apply a potential difference across the conductor above…

The force on each electron is enough to free it from its orbit and it can jump from atom to atom – the conductor conducts

Conductors are said to have a low resistivity / resistance

Conductor Atomic Structure

• The atomic structure of good conductors usually includes only one electron in their outer shell.

• It is called a valence electron. • It is easily striped from the

atom, producing current flow.

Copper Atom

Semiconductors• A material whose properties are such that it is not quite a

conductor, not quite an insulator.

• Semiconductors have a resistivity/resistance between that of conductors and insulators.

• Their electrons are not free to move but a little energy will free them for conduction

• Some common semiconductorso elemental

• Si - Silicon (most common)• Ge - Germanium

o compound• GaAs - Gallium arsenide• GaP - Gallium phosphide• AlAs - Aluminum arsenide• AlP - Aluminum phosphide• InP - Indium Phosphide

(The resistance of a semiconductor decreases as the temperature increases – Negative Temp. Coefficient)

Semiconductor Valence Orbit

• The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

Crystal Lattice Structure

• The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice.

• The valence electrons are shared between atoms, forming what are called covalent bonds.

2D Crystal Lattice Structure

The Silicon (Si) AtomSilicon has a valency of 4 i.e. 4 electrons in its outer shell

Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms

These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms

This picture shows the shared electrons

Silicon – the crystal latticeIf we extend this arrangement throughout a piece of silicon…

We have the crystal lattice of silicon

This is how silicon looks when it is cold, i.e. T = 0 K

It has no free electrons – it cannot conduct electricity – therefore it behaves like an insulator

Energy Bands• When silicon atoms come together to form a crystal, the

electrons occupy particular allowed energy bands.

• Minimum energy required to break the covalent bond is Bandgap Energy ().

Free Electrons

Electrons Can not exist here

Electron Hole Pair• When an electron jumps to the conduction band, a

vacancy is left in the valence band within the crystal. This vacancy is called a hole.

Electron Movement in SiliconHowever, if we apply a little heat to the silicon….

An electron may gain enough energy to break free of its bond…

It is then available for conduction and is free to travel throughout the material

Hole Movement in SiliconLet’s take a closer look at what the electron has left behind

There is a gap in the bond – what we call a hole

Hole Movement in SiliconThis hole can also move…

An electron – in a nearby bond – may jump into this hole…

Effectively causing the hole to move…

Like this…

In semiconductors, the negatively charged free electron, and the positivelycharged hole contribute to the current.

Doping• Relying on heat or light for conduction does not make

reliable electronics.• To make the semiconductor conduct electricity, other

atoms called impurities must be added.• “Impurities” are different elements, normally from III

or V group of periodic table. • This process is called doping.

Semiconductor Types• An intrinsic semiconductor, also called an undoped

semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present.

• Since the electron and hole concentrations in an intrinsic semiconductor are relatively small.

• These concentrations can be greatly increased by adding controlled amounts of certain impurities (Doping).

• An extrinsic semiconductor is a semiconductor that has

been doped.

Semiconductors can be Conductors• An impurity, or element like

arsenic, has 5 valence electrons.

• Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms.

• The one electron left over for each arsenic atom becomes available to conduct current flow.

The Phosphorus AtomPhosphorus is number 15 in the periodic table

It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell

Doping – Making n-type SiliconSuppose we remove a silicon atom from the crystal lattice…and replace it with a phosphorus atom

We now have an electron that is not bonded – it is thus free for conduction

Doping – Making n-type SiliconLet’s remove another silicon atom…and replace it with a phosphorus atomAs more electrons are available for conduction we have increased the conductivity of the material

If we now apply a potential difference across the silicon…

Phosphorus is called the dopant, or donor impurity,

Extrinsic Conduction – n-type SiliconA current will flow

Note:

The negative electrons move towards the positive terminal

From now on n-type will be shown like this.N-type Silicon

• This type of silicon is called n-type • This is because the majority charge carriers are

negative electrons• A small number of minority charge carriers – holes –

will exist due to electrons-hole pairs being created in the silicon atoms due to heat

• The silicon is still electrically neutral as the number of protons is equal to the number of electrons

The Boron AtomBoron is number 5 in the periodic table

It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell

Doping – Making p-type SiliconAs before, we remove a silicon atom from the crystal lattice…

This time we replace it with a boron atom

Notice we have a hole in a bond – this hole is thus free for conduction

Doping – Making p-type SiliconLet’s remove another silicon atom…

and replace it with another boron atomAs more holes are available for conduction we have increased the conductivity of the material

If we now apply a potential difference across the silicon, hole current starts to flow.

Boron is the dopant here, also called acceptor impurity.

P-type Silicon

• This type of silicon is called p-type • This is because the majority charge carriers are positive

holes• A small number of minority charge carriers – electrons –

will exist due to electrons-hole pairs being created in the silicon atoms due to heat

• The silicon is still electrically neutral as the number of protons is equal to the number of electrons

From now on p-type will be shown like this.

The p-n JunctionSuppose we join a piece of p-type silicon to a piece of n-type silicon

We get what is called a p-n junction

Remember – both pieces are electrically neutral

The p-n JunctionWhen initially joined electrons from the n-type migrate into the p-type – less electron density there

When an electron fills a hole – both the electron and hole disappear as the gap in the bond is filled

This leaves a region with no free charge carriers – the depletion layer – this layer acts as an insulator

The p-n JunctionAs the p-type has gained electrons – it is left with an overall negative charge…

As the n-type has lost electrons – it is left with an overall positive charge…

Therefore there is a voltage across the junction – the junction voltage – for silicon this is approximately 0.6 V

0.6 V

The Reverse Biased P-N JunctionTake a p-n junction

Apply a voltage across it with the

p-type negative

n-type positive

Close the switch

The voltage sets up an electric field throughout the junction The junction is said to be reverse – biased

The Reverse Biased P-N JunctionNegative electrons in the n-type feel an attractive force which pulls them away from the depletion layer

Positive holes in the p-type also experience an attractive force which pulls them away from the depletion layer

Thus, the depletion layer ( INSULATOR ) is widened and no current flows through thep-n junction

The Forward Biased P-N JunctionTake a p-n junction

Apply a voltage across it with the

p-type postitive

n-type negative

Close the switch

The voltage sets up an electric field throughout the junction

The junction is said to be forward – biased

The Forward Biased P-N JunctionNegative electrons in the n-type feel a repulsive force which pushes them into the depletion layer

Positive holes in the p-type also experience a repulsive force which pushes them into the depletion layer

Therefore, the depletion layer is eliminated and a current flows through the p-n junction

The Forward Biased P-N JunctionAt the junction electrons fill holes

They are replenished by the external cell and current flows

Both disappear as they are no longer free for conduction

This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V

The Forward Biased P-N JunctionIf we apply a higher voltage…

The electrons feel a greater force and move faster

The current will be greater and will look like

The p-n junction is called a DIODE and is represented by the symbol…

The arrow shows the direction in which it conducts current

this….