Criteria for occurrence of the quasineutral diffusion mode have been investigated in terms of a generalized approach that takes into account the dependences of the electron and hole velocities on the electric field. These criteria should be used to describe the carrier transport in bases of forward biased bipolar semiconductor devices (diodes, thyristors, and power bipolar transistors in the saturation mode). The criteria appreciably differ from the previously obtained commonly accepted criteria. It is demonstrated that equations describing the carrier transport in the quasineutral approximation in n- and p-type semiconductors are different. These equations are used to obtain analytical conditions in which the diffusion mode is operative. The applicability limits of the diffusion approximation in simulation of semiconductor structures are found. The analytical results are confirmed by a numerical experiment. (C) 2010 Elsevier Ltd. All rights reserved.