Metals, Semiconductors, and Insulators

1. For electrons to move within an applied electric field there have to be states available to them.
2. A totally filled band cannot contribute to current transport; neither can a totally empty band.
3. So, semiconductors at 0 K are perfect insulators.
4. With thermal or optical excitation, a number of these electrons can be excited from the valence band to the conduction band, and after that they can contribute to the current transport process.
5. At temperatures other than 0 K, the magnitude of the band gap take aparts an insulator from a semiconductor, for example at 300 K, E_g(diamond) = 5 eV (insulator), and E_g (Silicon) = 1.12 eV (semiconductor).
6. Number of electrons presented for conduction can be increased greatly in semiconductors via reasonable amount of thermal or optical energy.
7. In metals, the bands are either partially filled or they overlap => so, electrons and empty states coexist => great electrical conductivity.