The widening of thep‐njunction of a semiconductor is viewed as due to simultaneous diffusion and drift of the impurities. Rigorous first‐order differential equations for the variation of the space‐charge width with time are given. The equations are only dependent, except for an integral term, on the concentrations at the edges of the space‐charge region. Exact solution of the system is always possible for short time if the initial concentration profile is known. This has been done for a few cases.