The Leibnitz rule of integral calculus yields a useful expression for the quasistatic capacitance versus voltage characteristic of ap/njunction space‐charge layer that holds for all doping profiles and for forward as well as for reverse applied voltages. This capacitance is the sum of two components. One accounts for the incremental mobile charge within the volume of the layer. The other accounts for the incremental mobile charge at its edges.This expression enables novel analytical characterizations and interpretations of numerical solutions.