Principles & Applications Electronics Principles & Applications Sixth Edition Charles A. Schuler Chapter 2 Semiconductors ©2003 Glencoe/McGraw-Hill

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

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

This is a copper atom. Its net charge = 0. Valence electron It has 29 protons. N N N N It has 29 electrons. This is a copper atom. Its net charge = 0.

The valence electron is the important feature. Its attraction to the nucleus is relatively weak.

A simple model of the copper atom looks like this: The valence electron The nucleus plus the inner electron orbits

attracted to the nuclei. to conduct electricity Remember, the valence electrons are weakly attracted to the nuclei. Copper wire is used to conduct electricity because the valence electrons move freely through its structure.

It acts as an electrical insulator. So far, we know that copper’s single valence electron makes it a good conductor. The rule of eight states that a material like this would be stable since its valence orbit is full. No Vacancy It acts as an electrical insulator.

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 captured by covalent bonds.

Covalent bonds can be broken by heating a silicon crystal. Free electron Hole Thermal carriers

The thermal carriers support the flow of current.

Heating silicon crystals to make them conduct is not practical!

Each donor atom that enters the crystal adds a free electron. A silicon crystal can be doped with a donor impurity. Free electron 5 1 4 This is an arsenic atom. Each donor atom that enters the crystal adds a free electron. 2 3

Silicon that has been doped with arsenic is called N-type. The free electrons in N-type silicon support the flow of current.

Each acceptor atom that enters the A silicon crystal can be doped with an acceptor impurity. Hole 1 This is a boron atom. Each acceptor atom that enters the crystal creates a hole. 2 3

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? Free electron Hole (N-type) (P-type) Add a pentavalent impurity. Add a trivalent impurity.

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.

Silicon is the workhorse of the semiconductor industry but compound semiconductors help out in key areas. Gallium arsenide Indium phosphide Mercury cadmium telluride Silicon carbide Cadmium sulfphide Cadmium telluride

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