Relatively speaking, the number of carriers in intrinsic semiconductors is very small, and the conductivity is still very low. However, if a small amount of impurities are doped, the conductivity of the formed impurity semiconductor will be greatly enhanced. Due to different impurities, impurity semiconductors can be divided into two types: N type and P type.

The impurity doped in N-type semiconductor is phosphorus or other pentavalent elements. When phosphorus atoms replace atoms in the original crystal structure to form valence bonds, the extra pentavalent electrons can easily get rid of the constraints of phosphorus nuclei and become free electrons, so the number of free electrons in semiconductors increases greatly, and free electrons become majority carriers and holes become minority carriers.

The impurity doped in P-type semiconductor is boron or other trivalent elements. When boron atoms replace atoms in the original crystal structure to form valence bonds, holes will be formed due to the lack of a valence electron, so the number of holes in the semiconductor increases greatly, and holes become majority carriers, while free electrons become minority carriers.

Note that both N-type and P-type semiconductors have most carriers, but the whole crystal still has no charge.