A COMPREHENSIVE STUDY OF HIGH-LEVEL FREE-CARRIER INJECTION IN BIPOLARJUNCTION TRANSISTORS

This paper presents a comprehensive study on the effects of high-level free-carrier injection on the current transport of bipolar junction t ransistors (BJTs). Detailed information for the free-carrier concentra tion, electric field, and drift and diffusion current components in th e quasi-neutral base (QNB) under high-level injection are calculated u sing the modified ambipolar transport equation and using several diffe rent approximations for the majority-carrier current in the QNB. It is shown that high-level injection can create a large retarding field wh ich is in the opposite direction of the built-in field associated with the nonuniform doping concentration. High-level injection also enhanc es recombination in the QNB, thus resulting in a position-dependent mi nority-carrier current in the region even if the base is thin. Our res ults further suggest that the widely used zero majority current approx imation gives rise to a larger error compared to other lesser known ap proximations.