In elementary texts, derivations of the equationI=I0[exp(qV/kT)−1] (the ‘‘simple diode’’ equation) all depend on conditions valid when no current flows, an apparently contradictory situation. Because introductory texts justify this only qualitatively, it is desirable to do an explicit calculation which explores the limits of the equation when current actually does flow—a procedure carried out in this paper by manipulating the current density equations and integrating them across the space‐charge layer (SCL). The calculation shows the ‘‘simple diode equation’’ to be valid if the width of the SCL is small compared to the diffusion length of either an elecron or a hole. This implies relatively few collisions in the SCL and thus clarifies why SCL recombination may also be neglected in the ‘‘simple diode’’ regime.