The impurity atoms which provide conduction electrons to the crystal are called donor atoms.
When an impurity atom with 5 valence electrons is introduced in a germanium crystal, four of the five valence electrons of impurity atom form covalent bonds with one each valence electron of four germanium atoms and fifth valence electron becomes free to move in the crystal. This acts as charge carrier. Such impurity atoms are called donor atoms because they provide conduction electrons to the crystal. Hence, it is an n-type semiconductor.
While p-type semiconductors consist of impurity atoms called acceptor atoms, because they accept electrons from the pure semiconductor in the process of creating positive holes.
