What is Band Theory?
Electrons occupy energy states in atomic orbitals. When several atoms are brought close to each other in a
solid these energy states split into a series of energy states
(molecular orbitals). The spacing between these states is so small that they
overlap to form an energy band.
 |
| Band Theory |
The furthest band from the nucleus is filled with valence
electrons and is called the valence band. The empty band is called the conduction band. The energy of the highest filled state is called Fermi energy. There is a certain energy gap, called bandgap, between
valence and conduction bands. Primarily four types of band structures exist in solids.
 |
| Fermi Energy |
In metals, the valence band is either partially filled (Cu) or the
valence and conduction bands overlap (Mg). Insulators and semiconductors have completely filled
valence bands and empty conduction bands. It is the magnitude of the band gap which separates metals,
semiconductors, and insulators in terms of their electrical
conductivity. The band gap is relatively smaller in semiconductors while it
is very large in insulators.