The resistivity of a material, independent of size and shape, is represented by r . The resistance of a sample of a material is proportional to the resistivity r and the length l along which the current flows and is inversely proportional to the cross-sectional area A of the sample. Metals, in general, have a low resistance to the flow of current. The resistance increases with increased temperature. This is due to increased movement of the lattice and a greater frequency of collisions between the moving electrons and the lattice.
Some solids are very poor conductors of electrons.
Semiconductors have an energy gap between the bonding electrons and the conduction band. They do not conduct at the slightest potential difference but require a certain minimum field strength in order to overcome the energy gap between the valence band and the conduction band. Electrons promoted to the higher energy conduction band cause the semiconductor to conduct a current. Electrons move in one direction while the positive holes move in the opposite direction. In this case conduction is enhanced by an increase in temperature.