A donor is an impurity atom that contributes a free electron to the semiconductor, producing n-type material; an acceptor is an impurity atom that captures an electron, leaving behind a free hole and producing p-type material. These are the two kinds of dopant, and which one you add decides the sign of the majority carrier.
Donor (group V)
Silicon is group IV (four valence electrons). A donor comes from group V — phosphorus or arsenic — with five valence electrons. Four bond covalently with the four silicon neighbours; the fifth electron is not needed for bonding and is held only loosely. At room temperature thermal energy is more than enough to liberate it, so the donor effectively gives away one mobile electron and itself becomes a fixed, positively-charged ion (it lost an electron). Add donors at concentration and you get free electrons; the material is n-type.
Acceptor (group III)
An acceptor comes from group III — boron or aluminium — with only three valence electrons. It can complete just three of the four surrounding bonds; the fourth is short an electron. The acceptor readily accepts an electron from a neighbouring silicon bond to fill its own, which transfers the vacancy to that neighbouring bond — a mobile hole. The acceptor, having gained an electron, becomes a fixed, negatively-charged ion. Add acceptors at concentration and you get holes; the material is p-type.
The naming is from the electron’s point of view: a donor donates an electron to the conduction band; an acceptor accepts an electron out of the valence bonds.
Ionised at room temperature
Both dopant types sit on shallow energy levels very close to a band edge — donor levels just below the conduction band, acceptor levels just above the valence band — separated by far less energy than the full Bandgap. The room-temperature thermal energy () easily covers that small gap, so in normal operation we assume complete ionisation: every donor has released its electron, every acceptor has captured one. That is exactly why we can write for n-type and for p-type — each dopant atom reliably contributes exactly one carrier. The fixed ions they leave behind are immobile; when carriers diffuse away near a junction, these uncovered donor and acceptor ions are the charge that forms the Depletion region and its Built-in voltage.