A Comprehensive Study of High-Level Free-Carrier Injection in Bipolar Junction Transistors

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Abstract
This paper presents a comprehensive study on the effects of high-level free-carrier injection on the current transport of bipolar junction transistors (BJTs). Detailed information for the free-carrier concentration, electric field, and drift and diffusion current components in the quasi-neutral base (QNB) under high-level injection are calculated using the modified ambipolar transport equation and using several different approximations for the majority-carrier current in the QNB. It is shown that high-level injection can create a large retarding field which is in the opposite direction of the built-in field associated with the nonuniform doping concentration. High-level injection also enhances recombination in the QNB, thus resulting in a position-dependent minority-carrier current in the region even if the base is thin. Our results further suggest that the widely used zero majority current approximation gives rise to a larger error compared to other lesser known approximations.