Physics The intrinsic conductivity of a semiconductor crystal is related to the number of intrinsic charge carriers (electrons or holes). When a conduction electron is freed, i.e. when it goes from the valence band to the conduction band, it leaves a hole in the valence band, and this hole takes part in electrical conduction like an electron with a positive charge.
The lower the energy difference between the conduction band and the valence band (or gap), the easier it is to form an electron-hole pair.
Intrinsic semiconductors have a characteristic property: thermal agitation is sufficient to provide the energy necessary for the electron to go from the valence band to the conduction band. The resulting intrinsic conductivity increases with temperature, unlike what happens with a metallic conductor. Electromagnetic radiation can also cause an electron to go to the conduction band (photoconduction).
Extrinsic semiconduction is obtained through the introduction of impurities, by n doping (negative) or p doping (positive), and this greatly increases the conductivity of intrinsic semiconductors.
The free carriers introduce an extra energy band into the forbidden band of the semiconductor (with acceptors or donors, according to whether there is p or n doping).