Impurity in semiconductor

Impurity know as donors or acceptors. The differences between them is that the former impurities provide extra electrons to the conduction band. Whereas, the acceptor impurities provide extra holes to the valence band. Also, the chemical valence helps to determine the impurity type if it is donor or acceptor. The donor impurities have high chemical valence, while the acceptors have low chemical valence.(Ashcroft and Mermin,1976)