donor
/ DOH-ner /
Think of silicon's atoms as people who each have exactly four hands and join hands with four neighbors — every hand busy, nobody spare. Now drop in an atom with five hands, like phosphorus. Four hands clasp the neighbors as usual, but the fifth has no one to hold. That lonely fifth hand is barely attached and easily lets go.
A donor is an impurity atom that carries one more outer electron than the host needs for bonding, so it donates that spare electron to the crystal. Because the extra electron is held very loosely, even the gentle warmth of room temperature is enough to shake it free into the conduction band, where it can move and carry current. The donor atom is left behind as a fixed, positively charged site, but it cannot move — only the freed electron does.
Donors are how engineers add mobile electrons on purpose, producing what is called an n-type semiconductor (n for the negative charge of electrons). The honest subtlety: the donor does not create charge from nothing. The crystal stays neutral overall, because the positive donor ion and the negative electron balance — it is only that the electron is now free to wander while the ion stays put.
Phosphorus, arsenic, and antimony are common donors in silicon — each has five outer electrons against silicon's four. The fifth electron is bound so weakly that almost all of them are already free at room temperature.
Group-V atoms like phosphorus donate one loosely held electron each to silicon.
A donor's mirror image is an acceptor, which has one too few outer electrons and so creates a hole instead. Donors give electrons; acceptors take them.