For example, when "silicon (Si), having four "valence electrons, needs to be doped as a "n-type semiconductor, elements from "group V like "phosphorus (P) or "arsenic (As) can be used because they have five valence electrons. A dopant with five valence electrons is also called a pentavalent impurity.  Other pentavalent dopants are "antimony (Sb) and "bismuth (Bi).
When substituting a Si atom in the "crystal lattice, four of the valence electrons of phosphorus form "covalent bonds with the neighbouring Si atoms but the fifth one remains weakly bonded. At "room temperature, all the fifth electrons are liberated, can move around the Si crystal and can carry a current and thus act as "charge carriers. The initially electroneutral donor becomes positively charged (ionised).