ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition

Chapter 2Semiconductors

©1999 Glencoe/McGraw-Hill

Charles A. Schuler

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

INTRODUCTION

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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.

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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.

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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 copper’s 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

Atomic quiz

The dense and central part of any atom iscalled the ________. nucleus

Atom net charge is 0 since the number of protonsequals the number of ______. electrons

The outermost orbit of all atoms is calledthe ________ orbit. valence

Good electrical conductors have freevalence ________. electrons

The magic number for valence stabilityis ________. eight

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 held by covalent bonds.

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.

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2 3

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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 a P-type crystal.

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

These are called minority carriers.

This is an N-type crystal.

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

These are called minority carriers.

Semiconductor quiz

A pure silicon crystal, at room temperature,acts as an electrical ________. insulator

The current carriers, in N-type silicon, arecalled ________. electrons

The current carriers, in P-type silicon, arecalled ________. holes

The minority carriers, in N-type silicon, arecalled ________. holes

The minority carriers, in P-type silicon, arecalled ________. electrons

REVIEW

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