In intrinsic semiconductor, when an electron moves from valence band to conduction band, hole is formed in place of that electron. Why does that not happen in case of n type semiconductor when 5th electron of pentavalent impurity moves to conduction band?
When you dope a semiconductor the dopants contribute a mobile charge to the semiconductor of one type and this leaves behind an immobile charge of the opposite charge, this is the ionized dopant atom. If you heat a semiconductor, or expose it to photons of a high enough energy you can generate electron-hole pairs. Since this charge is not created from the ionization of a dopant atom, there is no fixed charge.