To model high injection phenomena in P–N junction devices it is usually necessary to use numerical analysis. This is because the standard procedure of dividing the structure into neutral zones and depletion layer (regional approximation) fails. In a recent paper, Yueetal. [J. Appl. Phys.77, 1611 (1995)] proposed an extension of the Shockley theory, retaining the form of the conventional diffusion current‐only model for the conduction mechanism. Their solution was based on the resolution of the ambipolar transport equation in the base. It is shown here by a numerical simulation of the complete structure, within the framework of the drift‐diffusion model, that both the exponential current dependence:J&agr; exp(eVa/2kT) as well as the Yueetal. approximation are valid only in the limiting case of a strongly extrinsic short base diode. ©1996 American Institute of Physics.