Electronics Principles & Applications Fifth Edition Chapter 2 Semiconductors ©1999 Glencoe/McGraw-Hill Charles A. Schuler

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

N N N N The center of an atom is called the nucleus. Most atoms have neutrons which have no charge. A nucleus also has protons and they have a positive charge. Negative electrons orbit the nucleus.

N N N N 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. N N N N 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 used to conduct electricity because the valence electrons move freely through its structure. Remember, the valence electrons 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 this would be stable since its valence orbit is full. No Vacancy

Atomic quiz The dense and central part of any atom is called the ________. nucleus Atom net charge is 0 since the number of protons equals the number of ______. electrons The outermost orbit of all atoms is called the ________ orbit. valence Good electrical conductors have free valence ________. electrons The magic number for valence stability is ________. 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 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 Each 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 methods of making silicon semiconduct? Hole Free electron Add a pentavalent impurity Add a trivalent impurity (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, are called ________. electrons The current carriers, in P-type silicon, are called ________. holes The minority carriers, in N-type silicon, are called ________. holes The minority carriers, in P-type silicon, are called ________. electrons

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