NUMERICAL STUDY ON THE INJECTION PERFORMANCE OF ALGAAS GAAS ABRUPT EMITTER HETEROJUNCTION BIPOLAR-TRANSISTORS

The injection performance of abrupt emitter HBT's and related effects on the device characteristics are studied by taking an Npn Al0.25Ga0.7 5 As/GaAs/GaAs HBT as an example. In order to take into account the co upled transport phenomena of drift-diffusion and tunneling-emission pr ocesses across the abrupt heterojunction in a single coupled formulati on, a numerical technique based on the boundary condition approach is employed. Compared to previous numerical investigations relying on eit her a drift-diffusion or a tunneling-emission scheme, more complete an d accurate characterization of abrupt emitter HBT's has been achieved in this study. It is demonstrated that the presence of abrupt disconti nuities of the conduction and valence bands at the emitter-base juncti on brings several different features to the injection efficiency and r ecombination characteristics of abrupt emitter HBT's compared to grade d emitter HBT's. Based on investigations of the emitter doping effects on the current drive capability and device gain, an optimum emitter d oping density is determined for a given structure. When the emitter-ba se p-n junction of the abrupt emitter HBT is slightly displaced with r espect to the heterojunction, significant changes in the electrical ch aracteristics are observed. A small displacement of the p-n junction i nto the narrow bandgap semiconductor is found to be very attractive fo r the performance optimization of abrupt emitter HBT's.