Definition of acceptor level
When acceptor impurities are doped into pure semiconductor materials, the migration of valence electrons will form positively charged holes and negatively charged centers. Due to electrostatic attraction, holes will be weakly bound by the negatively charged centers to form bound states. Moving around, after the holes get energy, they will be free from binding to form conductive holes. The energy state of holes bound by acceptor impurities is called the acceptor energy level.