AN ANALYTICAL MODEL FOR CURRENT TRANSPORT IN ALGAAS/GAAS ABRUPT HBTS WITH A SETBACK LAYER

An intrinsic setback layer (or spacer) is frequently used in abrupt heterojunctions to improve the emitter injection efficiency and to reduce the impurity out-diffusion from the heavily doped base to the emitter. We have developed an analytical model to predict the d.c. performance of the AlGaAs/GaAs abrupt heterojunction bipolar transistor (HBT) with a setback layer. The effects of different setback layer thicknesses on the collector and base currents are studied in detail. Our results suggest that the presence of the setback layer can improve injection efficiency, but it can also increase the base current. Furthermore, for the device considered and parameters used, it was shown that a setback layer with a 100 angstrom thickness can yield the highest current gain and that a setback layer thicker than 100 angstrom can actually degrade the HBT performance. The model predictions compare favorably with the results obtained from solving numerically the Poisson and continuity equations including the nonuniform spatial band distribution as well as carrier degeneracy.