Classify the materials based on the energy bands and explain them.

Classification of materials based on energy bands:

With reference to various band structures demonstrated in figure below we can generally divide solid in semiconductors, insulators and conductors. Conductors have many of electrons in the conduction band at room temperature. There no energy gaps exist and the valence and conduction bands overlap.

Insulator is a material wherein the energy gap is too large that practically no electron can be given sufficient energy to jump that gap.

These materials might conduct little electricity when their temperature is raised to very high values enabling some electrons to join the conduction band. A semiconductor is a solid along with an energy gap small sufficient for electron to cross simply from the valence band to the conduction bands. At room temperature adequate energy is available for valence electrons to bridge the energy gap to the conduction band; therefore the material sustains a few electric current.

The energy distribution of electrons in a solid are governed through the laws of Fermi - Dirac statistics.

The Fermi level is that at any temperature, the no. of electrons along with greater energy than the Fermi energy is equivalent to the number of unoccupied energy levels lower than it. In conductors, the Fermi level is put in a permitted band because the valence band and conduction band overlap with no energy gap. In insulators, this lies in the centre of the large energy gap whereas in semiconductors this lies in the relatively small energy gap.