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 Al _0.25Ga_0.75As/GaAs/GaAs HBT as an example. In order to take into account the coupled transport phenomena of drift-diffusion and tunneling-emission processes across the abrupt heterojunction in a single coupled formulation, a numerical technique based on the boundary condition approach is employed. Compared to previous numerical investigations relying on either a drift-diffusion or a tunneling-emission scheme, more complete and accurate characterization of abrupt emitter HBT´s has been achieved in this study. It is demonstrated that the presence of abrupt discontinuities of the conduction and valence bands at the emitter-base junction brings several different features to the injection efficiency and recombination characteristics of abrupt emitter HBT´s compared to graded emitter HBT´s. Based on investigations of the emitter doping effects on the current drive capability and device gain, an optimum emitter doping density is determined for a given structure. When the emitter-base p-n junction of the abrupt emitter HBT is slightly displaced with respect to the heterojunction, significant changes in the electrical characteristics are observed. A small displacement of the p-n junction into the narrow bandgap semiconductor is found to be very attractive for the performance optimization of abrupt emitter HBT´s