For semiconductors, Ohm’s law and the described temperature dependence apply only partially, or in some cases not at all. In a semiconductor, the electrons are bound in the crystal structure at absolute zero temperature, similar to an insulator. By supplying energy, for example in the form of heat, the electrons can be released from the crystal lattice and then move freely, as in a conductor. At temperatures above 0 K there are always some electrons that can overcome this binding energy, with the conductivity increasing not linearly, but exponentially with temperature.
At the location where the electron is released, a positive charge is left behind, which in semiconductors is referred to as a hole. The creation of an electron–hole pair is therefore called generation. When an electron falls into a hole of a neighboring atom, annihilating both, this process is called recombination.
When an external voltage is applied, the electrons move along the electric field, while the holes move in the opposite direction.