At very low temperatures, the semiconductor's valence band is a full band (see energy band theory), after thermal excitation bai, some of the electrons in the valence band will cross the forbidden band into the higher energy of the empty band, the existence of electrons in the empty band after becoming a conduction band, the lack of an electron in the valence band after the formation of a positively charged vacancy, known as a hole. The hole conducts electricity is not an actual motion, but an equivalent.
Electrons' conductive isoelectric holes will move along its opposite direction. They produce directional motion under the action of the external electric field and the formation of macroscopic current, known as electron conductive and hole conductive, respectively. This mixed type of conductivity due to the formation of electron-hole pairs is called intrinsic conductivity. A conductive band of electrons will fall into the hole, electron-hole pairs disappear, known as the composite.
The energy released when the composite becomes electromagnetic radiation (luminescence) or the thermal vibration energy of the lattice (fever). At a certain temperature, electron-hole pairs are generated and compound at the same time and reach dynamic equilibrium, at this time the semiconductor has a certain carrier density, which has a certain resistivity. When the temperature rises, more electron-hole pairs will be generated, the carrier density increases and the resistivity decreases.
Here is a more detailed explanation.