In semiconductors physics the term donor is used to generically indicate a dopant atom that added to a semiconductor can form n-type regions.
For example, when Silicon (Si), having four valence electrons, needs to be doped as an n-type semiconductor, elements from group V like Phosphorus (P) or Arsenic (As) can be used because they have five valence electrons. When substituting a Si atom in the crystalline lattice, four of the valence electrons of Phosphorus form covalent bonds with the neighbouring Si atoms but the fifth one remains weakly bonded. At room temperature all the fifth electrons are liberated, can move around the Si crystal and can carry a current. The initially neutral donor becomes positively charged (ionised).
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