Charge Carriers in Semiconductors

1. In a metal, the atoms are imbedded in a "sea" of free electrons, and these electrons can move as a group under the influence of an applied electric field.
2. In semiconductors at 0 K, all states in the valence band are full, and all states in the conduction band are empty.
3. At T > 0 K, electrons get thermally excited from the valence band to the conduction band, and contribute to the conduction process in the conduction band.
4. The empty states left in the valence band can also contribute to current conduction.
5. Also, introduction of impurities has an important effect on the availability of the charge carriers
6. Considerable flexibility in controlling the electrical properties of semiconductors.