Silicon (tetravalent) has 4 valence electrons. Pentavalent impurities (like Phosphorus, Arsenic, or Antimony) have 5 valence electrons.

• Four of these electrons form covalent bonds with neighboring Si atoms.
• The fifth electron is loosely bound and becomes a 'free' charge carrier even at low temperatures.
• Since the majority carriers are now negatively charged electrons, the material is called an n-type (n for negative) semiconductor.