Semiconductors are materials that do not conduct electricity as freely as conductors, but still have some free charge to move around unlike insulators. Some more common semiconductors are silicon and germanium. Semiconductors typically have resistances between 10^-4 and 10^8 ohms.

The characteristics of conductors, semiconductors, and insulators can be explained using band theory. The lowest band that is occupied as zero temperature is the valence band and does not contribute to the conduction of electricity. The next highest unfilled band is called the conduction band. The energy level between the valence band and the conduction band is very high for insulators so that the electrons cannot bridge the gap normally at normal temperatures.

Conductors have their valence and conduction bands overlap so that free electrons participate in the flow of current.

Semiconductors on the other hand have a small energy gap between the bands and typically require around one electron-volt to jump over the gap.

There are some different types of semiconductors around, including elemental and intrinsic. Some elemental semiconductors are silicon and germanium. Compound semiconductors are made up of two or more atoms that combine to make a semiconductor. GaAs and InP are examples of compound semiconductors.

In addition to the makeup differences in different types of semiconductors, there are also differences between intrinsic semiconductors and extrinsic semiconductors. Intrinsic semiconductors are made up of a pure material and do not require any additional doping or additions to make the material a semiconductor. Intrinsic semiconductors have what may seem like an unusual property that as temperature is increased their conductivity increases as well, unlike metals that decrease conductivity with increases in temperature.

The reason is that as temperature is increased more electrons have the energy to get kicked up to an energy state so that they are available to move freely in the material. As electrons move around in an intrinsic semiconductor they leave what is called a hole. The holes move in the opposite directions of the electrons and have opposite charge. There are as many holes as free moving electrons in an intrinsic semiconductor.