“The energy band structure of solid determines whether it is a conductor, an insulator or a semiconductor.

• The properties and parameters are mainly based upon “free electron theory” of solids. Here only free electrons are considered as the name suggest.
• In case of a single isolated atom, there are single energy levels as seen in case of hydrogen atom.
• But when two atoms are brought very near to each other there will be profound modifications in the energy levels. We know that in case of solids, the atoms are arranged in systematic space lattice.
• The closeness of atoms results in the intermixing of electrons of neighbouring atoms, of course, for the valence electrons in the outermost shells which are not strongly bounded by nucleus.
• Due to this intermixing, the number of permissible energy levels increases. Hence in case of a solid, instead of single energy level associated with the single atom, there will be bands of energy levels.
• A set of such closely spaced energy levels is called an energy band.
• Now the bands of energy levels are referred to the entire solid as a whole and not to the single individual atom. Here it should be remembered that the individual energies within the band are so close together that for many purposes, the energy band may be considered to be continuous.