TODAY’S LECTURE

Semiconductors

p-n junction diode

Ideal diode behavior

Practical diode behavior

Changing future, one IC at a time!

SEMI CONDUCTORS

Smallest unit of an element

Three basic particles:

• Neutrons

• Protons

• Electrons

Protons and Neutrons make up the nucleus.

Electrons revolve around the nucleus

Atomic Theory

Path of electrons are called orbits or shells.

The shells are named K,L,…,Q.

Valence shell : outermost shell

Free electrons in the valence shell determine the conductivity of the atom.

Atomic Model

What’s he maximum no of

electrons in the valence shell?

What’s the relationship between

valence shell and conductivity?

ANS : Eight.

ANS: Conductivity decreases with an increase in the no. of electrons.

Atoms having four electrons in their valence shell

Neither good conductors, nor insulators

Most common semi- conductors:

• Carbon

• Silicon

• Germanium

Semi Conductors

With no external force: No. of electrons = No. of protons

Net charge on an atom = 0

If an atom gains one electron, it becomes negatively charged.

If an atom loses one electron, it becomes positively charged.

Charges

What do you call a charged atom?

ANS : Ion

Positive ion = Cation Negative ion = Anion

Each shell has an energy level associated with it.

When an electron receives energy equal to energy difference of two shells, it jumps up to the next shell.

When the electron releases the energy, it comes back to the lower level.

Energy Levels

Atomic Number : 14

Atomic Mass : 28

Group IV of the periodic table

Silicon

Atoms form covalent bonds to complete their valence shells by sharing electrons.

Covalent Bond

The completed valence shells make intrinsic (pure) silicon a bad conductor.

The process of adding impurity atoms to the intrinsic (pure) silicon or germanium to improve the conductivity of the semiconductor.

Doping

What type of elements are used for

doping?

ANS : Trivalent : 3 valence electrons

Pentavalent : 5 valence electrons

When pentavalent impurities are added to silicon or germanium, the material is known as n-type.

This results in free electrons that increase conductivity.

N-type Material

When trivalent impurities are added to silicon or germanium, the material is known as p-type.

This results in the formation of holes that increase conductivity.

P-type Material

The fusion begins!

P-N JUNCTION

Individually, p-type and n-type materials offer little use.

Their real use comes when we can control the width of the depletion region of a p-n junction.

p-n Junction

When the p-type material is at greater potential than the n-type material.

Causes a flow of current through the p-n junction, provided that applied potential difference is greater than the potential barrier of the p-n junction.

Forward Bias

The depletion region becomes very thin, or practically disappears.

When the p-type material is at lower or equal potential to the n-type material.

No current flow. The width of the depletion region effectively

increases.

Reverse Bias

TAKE A BREAK! You’re not in jail!

The one way switch

P-N DIODE

A semi-conductor device made by creating a p-n junction.

Can be used in two modes :

Forward Bias.

Reverse Bias.

Acts as an uncontrolled switch.

p-n Junction Diode

Switch

How many states does a switch have?

ANS : Two 1 : ON 2 : OFF

How does a switch behave in its states?

ANS: ON : short circuit OFF: open circuit

Closed state : Essentially acts as a short circuit. • No resistance to current

flow

• No voltage drop across it

Open state : Acts as an open circuit. • Maximum resistance to

current flow

• ALL voltage is dropped across it

Switch: Behavior

Forward Bias : Essentially acts as a short circuit.

• No resistance to current flow

• No voltage drop across it

Reverse Bias : Acts as an open circuit.

• Maximum resistance to current flow

• ALL voltage is dropped across it

Diode: Behavior

Same properties as an ideal switch:

Turns ON and OFF instantaneously.

Offers zero resistance in forward bias state.

Zero forward bias voltage drop.

Infinite resistance in reverse bias state.

Zero leakage current.

Ideal Diode: Properties

Find out which diode in forward biased and which is reverse biased.

Ideal Diode: Examples

Find out the voltage drop across the resistors.

Ideal Diode: Examples

Ans: 3 Volts Ans: 0 Volts

Ideal Diode: Use

Why do we need to represent diode ideally?

To understand and represent things better, we assume the components ideal. • Diode is supposed to be ideal in

early stages of trouble shooting, when we are only concerned with the direction of current flow.

The way things really work

PRACTICAL DIODE

To understand and represent things better, we assume the components ideal.

However, these ideal models do not represent what actually happens in life.

A real diode has a finite forward voltage drop :

Silicon : 0.7 Volts

Germanium : 0.3 Volts

Caused by the p-n junction potential barrier.

Practical Diode: Properties

For a diode to be able to conduct electricity, it must be supplied some initial voltage equal to the barrier potential.

This voltage is dropped across the diode.

Forward Voltage Drop

Diodes may vary in shapes and sizes depending upon its type and application

Real Diodes

Find out the voltage drop across the resistors.

Practical Diode: Example

Describes how current through diode varies with a change in applied voltage.

Voltage-Current Relation

Which variable should be on x-axis and why?

Ans : Voltage, because current is changing with

respect to voltage

No forward voltage drop

No resistance to current flow

Ideal Diode V-I Relation

Finite forward voltage drop

Finite resistance to current flow

0.7V for Si

0.3V for Ge

Practical Diode V-I Model

ANY QUESTIONS?

Anyone willing to present?

Time allowed : 5 mins

Topic : Any

Bonus Points : +3

• INTRODUCTORY ELECTRONIC DEVICES AND CIRCUITS – By Robert T. Paynter

P.S: Please note that I might not follow this book throughout the semester, but this book is a really good one with some nice examples that will help you understand things a little better. You may ask questions related to the book, or lectures during University hours. You know where to find me =]

Reference Book

Fun Facts Why do crackers have holes in them??