How does a donor atom provide a conduction electron in a silicon crystal?

In a pure silicon crystal, each atom shares its four outer electrons with four neighbors to form covalent bonds. To increase electrical conductivity, a process called doping is used to introduce specific impurities into the lattice.123

When a silicon atom is replaced by a donor atom—such as phosphorus or arsenic, which possess five outer electrons—four of those electrons form covalent bonds with the surrounding silicon atoms. The fifth electron is not involved in these bonds and is instead "donated" to the conduction band. This extra electron becomes a conduction electron available to participate in electrical current, effectively turning the material into an n-type semiconductor.123