It was recalled that the highest equilibrium free-carrier dopant concentration which was possible in a given material was limited by the so-called pinning energy. The latter exhibited a markedly universal agreement for a given class of semiconductor. Initial first-principles total energy calculations revealed that equilibrium n-type doping was ultimately limited by the spontaneous formation of closed-shell acceptor defects. These included the 3--charged cation vacancy in AlN, GaN, InP and GaAs, and the 1--charged DX center in AlAs, AlP and GaP. This explained the agreement of the pinning energies, and predicted the maximum equilibrium doping levels in various materials.

Microscopic Origin of the Phenomenological Equilibrium "Doping Limit Rule" in n-Type III-V Semiconductors S.B.Zhang, S.H.Wei, A.Zunger: Physical Review Letters, 2000, 84[6], 1232-5