The incremental change in the resistance of any material as a result of a change in thermodynamic temperature. In general conductors exhibit a positive coefficient of resistance; semiconductors and insulators have a negative coefficient of resistance.

In a conductor the distribution of electronic energy levels in the material is such that conduction levels are always available (see energy bands). An increase in the thermodynamic temperature causes an increase in the vibration of nuclei in the crystal lattice. This leads to an increase in the amount of scattering of conduction electrons as they drift through the material and causes the resistance to increase.

In a semiconductor and an insulator the existence of a forbidden gap between the valence and conduction bands has the effect that as the temperature increases more charge carriers become available for conduction by crossing the forbidden gap. The resistance therefore decreases. The increased numbers of carriers more than offsets the effect of scattering by lattice nuclei.

For a given material at thermodynamic temperature T the resistance R_T is given by

where R₀ is the resistance at absolute zero (T = 0) and α and β are constants characteristic of the material. In general β is negligible and the temperature coefficient of resistance is given by α.