Concept:Doping silicon with phosphorus, a pentavalent element, produces extra free electrons and forms an n-type semiconductor.
Explanation:Silicon has four valence electrons.
Phosphorus has five valence electrons, so it is a pentavalent impurity.
When phosphorus atoms replace some silicon atoms, four of the phosphorus electrons form covalent bonds with neighbouring silicon atoms.
The fifth electron is loosely bound and becomes a free electron for conduction.
Since the main charge carriers are free electrons (negative charges), the material is called an n-type semiconductor.
Answer:C. n-type