In a conductor, the conduction and valence bands essentially form a continuous series of states. Some of the valence states have the same energy as some of the conduction states, so electrons can move freely between the bands, without needing to gain any energy. Graphically, a conductor can be represented as:
Band theory diagram of a conductor.
Unlike conductors, the bands in insulators do not overlap. There is a large energy gap between the highest energy state in the valence band, and the lowest state in the conduction band.
Band theory diagram of an insulator.
The energy gap between the valence and conduction bands is large enough to prevent the electrons from moving into the conduction band. Because the electrons are always in valence states, which are bound to a specific atom, they are not free to move around. As a result of having no moveable electrons, these materials do not conduct electricity.
Semiconductors are an "in between" case - they are not good conductors, nor are they good insulator. A look at the band theory diagram for semiconductors can help us understand why this is so.
Band theory representation of a semiconductor.
Here we see a relatively small energy gap (1.09 eV for Silicon) between the valence and conduction bands. The small energy gap means that at room temperature, some electrons can gain enough thermal energy to get into a conduction band state. The presence of just a few electrons in the conduction band means that these materials will conduct electricity, but poorly.
By themselves, semiconductors cannot take the place of either conductors or insulators. However, by adding carefully selected
impurities to the semiconductor, their electrical properties can be changed. This process is known as "doping" and is a way to make
the semiconductors more conductive. Depending on the number of electrons in the outer shell of the impurity, it can be classified
as either a donor impurity or an acceptor impurity. In the next sections, we'll
examine how these impurities bring about their effects.
This document last updated April 20, 2007.