Semiconductors

A semiconductor is a material whose conductivity lies between that of good conductor & good insulator . Silicon and germanium are the examples of semiconductors widely used in the electronics industry .

According to the band theory, semiconductors are a class of material in a narrow forbidden band. The energy gap is the order of 1 eV . At absolute zero, the valance band energy levels are completely filled by by electrons, the conduction band is empty, & the material is then an insulator. At normal temperature, the thermal energy of some electrons is sufficient for them to reach the conduction band, where they become conduction electrons. The gap left in the valance band by the movement of an electron is called a HOLE. A hole acts as if it were positively charged carrier . Both hole and conduction electrons take part in electrical conduction in a semi conductor.

In insulators, the valance band is energy is completely filled by electrons. The conduction band is empty but the two bands are separated by a wide energy gap (6-9 eV ), called forbidden band. Under an applied electric field, the valance electrons cannot gain enough energy to jump to the conduction band. so, the material is insulator.