It has been observed that certain materials like germanium, silicon etc. have resistivity between good conductors like copper and insulators like glass. These materials are known as semiconductors. A material which has resistivity between conductors and insulators is known as semiconductor. The resistivity of a semiconductor lie approximately between 10-2 and 104 Ω m at room temperature. The resistance of a semiconductor decreases with increase in temperature over a particular temperature range. This behaviour is contrary to that of a metallic conductor for which the resistance increases with increase in temperature.
The elements that are classified as semiconductors are Si, Ge, In, etc.
Germanium and silicon are most widely used as semiconductors.
Valence band, conduction band and forbidden energy gap
The atoms of a solid are arranged in a regular repeated geometric pattern and the electrons of the atom revolve around the nucleus in certain permitted energy levels. The electrons in the inner shells are trongly bound to the nucleus. A band which is occupied by the valence electrons or a band having highest energy is defined as valence band (Fig). The valence band may be partially or completely filled. This band can never be empty.
In some materials, the valence electrons are loosely attached to the nucleus. Even at room temperature, some of the valence electrons can leave the valence band. These are called as free electrons. They are responsible for conduction of current in a conductor and are henceforth called as conduction electrons. The band occupied by these electrons is called conduction band. This band may be an empty band or partially filled band.
The separation between valence band and conduction band is known as forbidden energy gap. If an electron is to be transfered from valence band to conduction band, external energy is required, which is equal to the forbidden energy gap.