Investigation of the importance of high-level injection in abrupt HBTs

This note examines the assertion made in the literature that in HBTs high-level injection effects are negligible due to the high doping levels in the base relative to the emitter. The extent of applicability of this assertion to abrupt HBTs is investigated by deriving an expression for the hole concentration at the space-charge region boundary in the base and using it to compute the collector current that is valid for all injection levels. The results from this new approach are compared with those from existing models. It is established that high-level injection effects do indeed become negligible beyond a high enough base-doping level relative to the emitter. However, significant contribution from such effects is observed at reasonable base-doping levels for abrupt HBTs. Copyright (C) 1996 Elsevier Science Ltd.