Abstract The differential capacity of a semiconductor electrode with multiple doping states has been investigated through model calculations. Deep states in the band gap that are not ionized at flatband conditions become ionized as a function of increasing potential drop in the space charge layer. The creation of an additional source of charge in the form of these ionized states causes an increase in the capacity near the potential at which these states are ionized. The charge of the deep donors is treated as a potential-dependent function. It is shown that the slope of a Mott-Schottky plot can be used to evaluate the dopant concentration only when the charge density function is constant.