An impurity that can accept an electron from the valence band, leaving a mobile hole. A shallow acceptor creates an impurity level in a crystal near and above the valence-band edge. Donor and acceptor levels are descriptions of appropriate dopants, not universal features of every impurity species.
A shallow acceptor can bind an electron hole through the screened Coulomb potential of a negatively charged impurity. A scalar band effective mass approximation gives a hydrogen-like bound-state ladder. Actual valence-band degeneracy and anisotropy can split levels and limit that approximation.
In gallium nitride, incorporated hydrogen can neutralize magnesium acceptor impurities. Activation treatments restore electrical activity; adding magnesium alone does not establish a mobile electron hole population.
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