INTERACTIONS IN SEQUENTIAL DIFFUSION PROCESSES IN SEMICONDUCTORS

The effect of an internal electric field and the variation of equilibrium lattice vacancy concentration was incorporated into the continuity equations for the treatment of impurity diffusion in semiconductors. After the explanation of dominant mechanisms involved in the interactions, formulations of the sequential diffusion process and its solution are outlined and numerical results are given. It is shown that the base impurity profile generally experiences a strong retardation by the internal field, resulting in a profile much different from what is generally conceived. Important physical and process parameters of this effect are the extrinsicity factor α (defined as the ratio of impurity surface concentration to twice the intrinsic carrier concentration), the base to emitter diffusivity ratio, and the relative diffusion distance of the base initial profile and the emitter profile. Effect of interaction on the emitter profile is mainly due to the variation of vacancy concentration with doping concentration. In the cases considered, comparisons are made between interacting and noninteracting impurity profiles. One implication particularly emphasized is that sequentially diffused transistor profiles constructed by the superposition method can involve large errors even with accurate experimental data on the single diffused emitter and base profiles. The computed transistor profiles from a boron-arsenic sequential diffusion process, based on the model combining the effect of internal field and vacancy concentration variation, agreed well with the experimental results. © 1968 The American Institute of Physics.