The transport of electrons across the space‐charge region of ap‐njunction is examined by means of the exact solution of the time‐dependent Boltzmann equation in which the scattering integral is written in terms of the relaxation time approximation. The relaxation time approximation is only valid for elemental semiconductors over the range of temperatures where the device is normally used, and could also be valid for polar semiconductors but only at high temperatures. It is shown that for transition regions which are thick compared to the mean‐free path the effective mobility of the electrons is the high field mobility (exactly the same results are obtained as those derived by Gunn), however when the space‐charge region width is less than the mean‐free path the distribution function is more nearly that obtained for low fields.