A new theory for the junction capacitance of mathematically abrupt diodes is presented. In contrast with previous theories, Fermi–Dirac statistics are applied, and instead of using a parabolic density of states, a more appropriate function can be taken into account as this is required for heavily doped material. The main approximation is that of constant quasi‐Fermi levels. Besides the study of symmetrical junctions, the behavior of asymmetrical abrupt junctions is both analyzed and explained. Anomalies resulting from the calculations are shown to be due to the mathematical discontinuity.