On the basis of electrical conductivity the solids can be broadly classified into three types:
(a) Metals
(b) Insulators
(c) Semiconductors
Electrical conductivity of metals is very high and is of the order of 104 to 107 ohm-1 while that of insulators is one of the order of 10-20 to 10-10 ohm-1 m-1. Semiconductors have intermediate conductivity which lays in the range 10-8 to 104 ohm-1 m-1.
Electrical conductivity of solids may arise through the motion of electrons and positive holes (electronic conductivity) or through the motion of ions (ionic conductivity). The conduction through positive holes is called n-type conduction and through positive holes is called-type conduction. Electrical conductivity of metals is due to motion of electrons and it increases with the number of electrons available to participate in the conduction process. Pure ionic solids are those where conduction can take place only from motion of ions are insulators. Though, the presence of defects in the crystal structure raises their conductivity.
Unlike metals, the conductivity of semiconductors and insulators is mainly due to the presence of interstitial electrons and positive holes in the solids due to imperfections. The conductivity of semiconductors and insulators increases with increase in temperature whereas that of metals decreases.