Semiconductors exhibit a number of useful and unique properties related to their electronic structure. In solids the electrons tend to occupy various energy bands. The energy band associated with electrons in their ground state is called the valence band. These electrons are static. The energy band of excited electrons is called the conduction band. These electrons move freely and are usually higher energy. As the name implies, electrons in the conduction band are able to conduct electricity. The energy spacing between the valence band and the conduction band is called the band gap and corresponds to the energy necessary to excite an electron from the valence band into the conduction band. For some metals, such as magnesium, the valence and conduction bands overlap, corresponding to a negative band gap. In this situation, there are always some electrons in the conduction band and the material is highly conductive. Other metals, such as copper, have empty states in the valence band. In this case electrons in the valence band can conduct electricity by moving between the various states and again the material is highly conductive. For insulators the valence band is completely filled and the band gap is relatively large, preventing conduction. Semiconductors have an electronic structure similar to that of insulators, but with a relatively small band gap, generally less than 2 eV. Because the band gap is relatively small, electrons can be thermally excited into the conduction band, making semiconductors somewhat conductive at room temperature.
Electrons in the conduction band are free to move through the material conducting electricity. In addition, when an electron is excited into the conduction band it leaves behind an empty state in the valence band, corresponding to a missing electron in one of the covalent bonds. Under the influence of an electric field,an adjacent valence electron may move into the missing electron position, effectively moving the location of the missing electron. Thus, like the electron, this missing electron or hole is also able to move through the material, conducting electricity. Holes are considered to have a charge of the same magnitude as an electron (1.6×10-19 C), but of opposite charge. Thus, in the presence of an electric field excited electrons and holes move in opposite directions. Electrons are somewhat more mobile than holes and are thus more efficient at conducting electricity. Because both electrons and holes are capable of carrying electricity, they are collectively called carriers.
The concentration of carriers is strongly dependent on the temperature. Increasing the temperature leads to an increase in the number of carriers and a corresponding increase in conductivity. This contrasts sharply with most conductors, which tend to become less conductive at higher temperatures. This principle is used in thermistors.
