How does phosphorus doping create an n-type silicon semiconductor?

Phosphorus doping creates an n-type semiconductor by introducing impurities into the pure silicon crystal lattice.12

In a pure silicon crystal, each atom shares its four outer electrons with four neighboring atoms to form covalent bonds. Phosphorus, however, is a pentavalent atom with five electrons in its outer shell. When a phosphorus atom replaces a silicon atom in the lattice, four of its electrons participate in the existing bonding structure.13

The fifth electron does not have a bonding partner and becomes a free, or "conduction," electron. Because this extra electron is donated to the conduction band and is available to carry a negative charge, the material is classified as n-type (where n stands for negative). In this process, the phosphorus atom acts as a donor.132