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Many of the control systems used in today's industry are operated by solid-state devices as well as magnetic and mechanical devices. To install and troubleshoot control systems, an electrician must have an under standing of electronic control devices as well as relays and motor starters. Solid-state devices, such as diodes and transistors, are often called semiconductors. The word semiconductor refers to the type of material used to make solid-state devices. To understand how solid state devices operate, one must first study the atomic structure of conductors, insulators, and semiconductors.
Conductors are materials that provide easy paths for the flow of electrons. Conductors are generally made from materials that have large, heavy atoms. For this reason, most conductors are metals. The best electrical conductors are silver, copper and aluminum.
Conductors are also made from materials that have only one or two valence electrons in their atoms.
(Valence electrons are the electrons in the outer orbit of an atom, FIG. 1). An atom that has only one valence electron makes the best electrical conductor be cause the electron is held loosely in orbit and is easily given up for current flow.
Insulators are generally made from lightweight materials that have small atoms. The outer orbits of the atoms of insulating materials are filled or almost filled with valence electrons. This means an insulator will have seven or eight valence electrons as in the example in FIG. 2. Since an insulator has its outer orbit filled or almost filled with valence electrons, the electrons are held tightly in orbit and are not easily given up for cur rent flow.
Semiconductors, as the word implies, are materials that are neither good conductors nor good insulators. Semi conductors are made from materials that have four valence electrons in their outer orbits (FIG. 3).
Germanium and silicon are the most common semi conductor materials used in the electronics field. Of these materials, silicon is used more often because of its ability to withstand heat.
When semiconductor materials are refined into a pure form, the molecules arrange themselves into a crystal structure with a definite pattern (FIG. 4).
This type of pattern is called a lattice structure. A pure semiconductor material such as silicon has no special properties and will do little more than make a poor conductive material. To make semiconductor material useful in the production of solid-state components, it is mixed with an impurity. When pure semiconductor material is mixed with an impurity that has only three valence electrons, such as indium or gallium, the lattice structure changes, leaving a hole in the material (FIG. 5). This hole is caused by a missing electron. Since the material now lacks an electron, it is no longer electrically neutral. Electrons are negative particles. The hole, which has taken the place of an electron, has a positive charge; therefore, the semiconductor material now has a net positive charge and is called a P-type material.
FIG. 6 Lattice structure of an N-type material.
When a semiconductor material is mixed with an impurity that has five valence electrons, such as arsenic or antimony, the lattice structure has an excess of electrons (FIG. 6). Since electrons are negative particles, and there are more electrons in the material than there should be, the material has a net negative charge. This material is called an N-type material because of its negative charge.
All solid-state devices are made from combinations of P- and N-type materials. The type of device formed is determined by how the P- and N-type materials are connected. The number of layers of material and the thickness of each layer play an important part in deter mining what type of device is formed. For example, the diode is often called a PN junction because it is made by joining a piece of P-type material and a piece of N-type material (FIG. 7). The transistor, on the other hand, is made by joining three layers of semiconductor materials (FIG. 8).
1. The atoms of a material used as a conductor generally contain _____ valence electrons.
2. The atoms of a material used as an insulator generally contain _____ valence electrons.
3. The two materials most often used to produce semiconductor devices are ______ and ______.
4. What is a lattice structure?
5. How is a P-type material made?
6. How is an N-type material made?
7. Which type of semiconductor material can withstand the greatest amount of heat?
8. All electronic components are formed from P-type and N-type materials. What factors determine the kind of components formed?