The diffusion of such an impurity was examined analytically, by assuming that there was local equilibrium between the states, plus local electrical neutrality. It was considered that the situation could be described by an equation for concentration-dependent diffusion in which the diffusion coefficient was taken to be the product of a diffusion factor and a drift factor. Both of these depended upon the distribution of impurities between the states. It was noted that the diffusion factor varied monotonically, and increased or decreased with the impurity concentration. On the other hand, the drift factor could also vary in a non-monotonic manner. Consequently, either a maximum, or a maximum and a minimum, were expected to appear. The conditions under which the behavior of the drift factor was monotonic or non-monotonic were determined, and were found to be associated with the amounts of charge in the 2 states and with the size of the coefficient of exchange between states.

E.D.Gornushkina, R.S.Malkovich: Fizika i Tekhnika Poluprovodnikov, 1995, 29[5], 908-23 (Semiconductors, 1995, 29[5], 471-8)