CH322 - Semiconductors & Insulators

Describe the differences between an insulator and a semiconductor and metals

Insulators have a large band gap, making it difficult for electrons to move into the conduction band and conduct electricity. Semiconductors have a smaller band gap, allowing electrons to move into the conduction band more easily, resulting in conductivity that falls between conductors and insulators. Metals are excellent conductors of electricity and heat. They have a band structure where the valence and conduction bands overlap, allowing for easy movement of electrons.

Explain the variation in resistivity with temperature for a semiconductor.

In semiconductors, resistivity generally decreases as temperature increases. This is because elevated temperatures lead to more free charge carriers in the semiconductor, increasing conductivity and thus lowering resistance.

P-type semiconductor

Electrical conductivity arises from thermal population of an acceptor level which leaves vacancies in the lower band.

N-type semiconductor

A donor level is close in energy to the conduction band

representation of the band levels (valence and conduction bands) for an insulator and for a p-type and n-type semiconductor

Band levels

The relative energies of occupied and empty bands in (a) an insulator, (b) a metal in which the lower band is only partially occupied, (c) a metal in which the occupied and empty bands overlap, and (d) a semiconductor.

Draw in one diagram (conductivity vs temperature) the behaviour of a metal (conductor), of a semiconductor and of a superconductor.