Band Theory
Band theory can help characterize different insulators, semi-conductors, and conductors. It provides a way to visualize how electrons behave in a material and why certain materials have specific electrical and thermal properties.
Principles
Electrons around an atom are organized into orbitals according to their discrete energy levels. These energy levels are quantized and orbitals are filled starting from the lowest. However, in solid materials, these energy levels overlap causing energy bands to form. How far apart and how large these bands are depend on the nature of the material and its atomic structure. Since there are infinite energy levels in an atom, there are an infinite number of bands. Two certain bands surrounding gap between bands are given names:
- Valence band: The valence band is the closest band above the band gap. This band is composed of the valence orbitals of the atom.
- Conduction band: Conversely, the conduction is the closest band below the band gap.
In insulators, the band gap between the valence and conduction bands is "large" and the bands do not overlap the Fermi level. In metals, the Fermi level overlaps at least one band. However, in semi-conductors, the Fermi level is contained within a band gap. While band gaps in insulators may contain the Fermi level, in semi-conductors, the gap between bands is "small".