With the help of energy bands explain how conduction takes place in semiconductors.

Semiconductors: Substances as carbon, germanium and silicon that electrical conductivity lies in among the conductors and insulators are termed as semiconductors. The valence band of such substances is approximately filled, but the conduction band is approximately empty. The forbidden energy gap between valence and conduction band is very small (1ev) as shown in figure. Thus comparatively a smaller electric field is needed to push the valence electrons to the conduction band. It is the reason, why these materials under ordinary conditions do not conduct current and behaves as an insulator. Even though at room temperature, while some heat energy is imparted to the valence electrons, some of them cross over to the conduction band imparting minor conductivity to the semiconductors. Since the temperature is raised, more valence electrons cross over to conduction band and conductivity of the material raises. Therefore these materials have negative temperature co-efficient of resistance.

Energy Band Diagrams