Effects of high-level free-carrier injection on the base transit time of bipolar junction transistors

This paper presents a detailed study on the effects of high-level free-carrier injection on the base transit time of bipolar junction transistors (BJTs). The results are calculated using the modified ambipolar transport equation and using several different approximations for the majority-carrier current in the quasi-neutral base (QNB). It is shown that high-level injection can create a large retarding field in the QNB which is in the opposite direction of the built-in field associated with the nonuniform doping concentration, thus increasing the base transit time. Our results further suggest that the widely used zero majority-carrier current approximation gives rise to a larger error compared to other lesser known approximations