Why are phosphorus and arsenic called donor atoms in semiconductors?

Phosphorus and arsenic are referred to as donor atoms because of their role in the doping process, which modifies the electrical conductivity of semiconductors like silicon.

In a pure silicon crystal, each atom uses its four outer electrons to form covalent bonds with its four nearest neighbors. Phosphorus and arsenic are pentavalent elements, meaning they possess five electrons in their outer orbit. When one of these atoms is substituted into the silicon lattice, four of its electrons participate in the standard bonding arrangement with neighboring silicon atoms.

The fifth electron, however, does not participate in a bond and is instead "donated" to the conduction band, where it becomes available for electrical conduction. Because these atoms contribute an extra electron to the crystal structure, they are classified as donor atoms, and the resulting material is known as an n-type semiconductor.123