Introduction To Electronics

Chapter 1Semiconductors

McGraw-Hill 2008 The McGraw-Hill Companies Inc. All rights reserved.

Materials used in Electronics

Semiconductors (N-type & P-type)

Majority and Minority Carriers

INTRODUCTION

Materials used in electronics In terms of their electrical properties, materials can be

classified into 3 groups1. Conductor is a material that easily conducts electrical

current. The best conductors are single-element materials, such as copper (Cu), silver (Ag), gold (Au), and aluminum (Al).

2. Insulator is a material that doesnt conduct electrical current under normal conditions. Most god insulators are compounds rather than single-element materials, such as rubber, plastics, glass, mica, and quartz. Valence electrons are tightly bound to the atoms.

3. Semiconductor is a material that is between conductors and insulators in its ability to conduct electrical current.

Silicon (Si) is the most commonly used semiconductor.

Materials used in electronics Band gap

When an electron acquires enough additional energy, it can leave the valence shell, become a free electron, and exist in what is known as the conduction band.

The difference in energy between the valence band and the conduction band is called an energy gap or band gap.

This is the amount of energy that a valence electron must have in order to jump from the valence band to the conduction band.

*** In chemistry, a valence electron (shell) is an electron that is associated with an atom, and that can participate in the formation of a chemical bond

Materials used in electronics

Energy diagrams for the three types of materials.

Concept Preview The nucleus of any atom is positively charged

(neutron & proton). Negatively charged electrons orbit the nucleus. The net charge on any atom is zero because the

protons and electrons are equal in number. The valence orbit is the outermost orbit. Copper has only one valence electron and is an

excellent conductor. Materials with a full valence orbit act as insulators. Materials with 8 electrons in the valence orbit act as

insulators.

NN

NN

The center of an atom is called the nucleus.Most atoms have neutrons which have no charge.A nucleus also has protonsand they have a positive charge.

Negative electronsorbit the nucleus.

NN

NN

This is a copper atom.

It has 29 protons.

It has 29 electrons.

Its net charge = 0.

Valence electron

The valence electron is the important feature.

NN

NN

Valence electron

Its attraction to the nucleus is relatively weak.

The valence electron

The nucleus plus the inner electron orbits

A simple model of the copper atom looks like this:

Copper wire is usedto conduct electricitybecause the valence

electrons move freelythrough its structure.

Remember, the valenceelectrons are weakly

attracted to the nuclei.

So far, we know that coppers single valence electron makes it a good conductor.

It acts as an electrical insulator.

The rule of eight states that a material like thiswould be stable since its valence orbit is full.

No Vacancy

Concept Review The nucleus of any atom is positively charged. Negatively charged electrons orbit the nucleus. The net charge on any atom is zero because the

protons and electrons are equal in number. The valence orbit is the outermost orbit. Copper has only one valence electron and is an

excellent conductor. Materials with a full valence orbit act as

insulators. Materials with 8 electrons in the valence orbit act

as insulators.

Repeat Segment

Semiconductors Silicon has 4 valence electrons. Silicon atoms can form covalent bonds with each

other. Covalent silicon satisfies the rule of 8 and acts as an

insulator at room temperature. Donor impurities have 5 valence (pentavalent)

electrons. N-type silicon has been doped with a donor impurity

to make it semiconduct. Acceptor impurities have 3 valence (trivalent)

electrons. P-type silicon has been doped with an acceptor

impurity to make it semiconduct.** Increasing the number of current carriers (electrons & holes) is

called doping

Atoms of the same type can join together and form covalent bonds.This is an electron sharing process.

Silicon atoms have four valence electrons.

The covalent sharing satisfies the rule of eight.

In this structure, one bond is formed with each neighbor.

This is a silicon crystal.

It does not conduct because its valence electrons are captured by covalent bonds.

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.

For every electron raised to the conduction band by external energy, there is one hole left in the valence band, creating what is called an electron-hole pair.

Recombination occurs when a conduction-band electron loses energy and falls back into a hole in the valence band.

Thermal carriers

Covalent bonds can be broken by heating a silicon crystal.

Free electron

Hole

The thermal carriers support the flow of current.

Heating silicon crystals to make them conduct is not practical!

This is an arsenic atom.

A silicon crystal can be doped with a donor impurity.

1

2 3

45

Each donor atom that enters the crystal adds a free electron.

Free electron

Silicon that has been doped with arsenic is called N-type.

The free electrons in N-type silicon support the flow of current.

This is a boron atom.

A silicon crystal can be doped with an acceptor impurity.

1

2 3Each acceptor atom that enters the crystal creates a hole.

Hole

Silicon that has been doped with boron is called P-type.

The holes in P-type silicon support the flow of current.

What are two practical methodsof making silicon semiconduct?

HoleFree electron

Add a pentavalentimpurity.

Add a trivalentimpurity.

(N-type) (P-type)

This is an N-type crystal.

Due to heat, it could have a few free holes.

Holes are called minority carriers.Electrons are called the majority carriers.

This is a P-type crystal.

Due to heat, it could have a few free electrons.Conduction (free) electron are called minority carriers.Holes are called majority carries.

Silicon is the workhorse of the semiconductor industrybut compound semiconductors help out in key areas.

Gallium arsenide Indium phosphide Mercury cadmium telluride Silicon carbide Cadmium sulfphide Cadmium telluride

Concept Review Silicon has 4 valence electrons. Silicon atoms can form covalent bonds with each

other. Covalent silicon satisfies the rule of 8 and acts as

an insulator at room temperature. Donor impurities have 5 valence electrons. N-type silicon has been doped with a donor

impurity to make it semiconduct. Acceptor impurities have 3 valence electrons. P-type silicon has been doped with an acceptor

impurity to make it semiconduct.

Repeat Segment

REVIEW Conductors and Insulators Semiconductors N-type Semiconductors P-type Semiconductors Majority and Minority Carriers