Numerical study of emitter-base junction design for AlGaAs/GaAs heterojunction bipolar transistors

The DC common-emitter current gains of abrupt- and graded-junction heterojunction bipolar transistors (HBTs) are studied and compared by numerical simulation. Electron injection across abrupt emitter-base heterojunctions is evaluated by numerically solving the wave equation. A numerical model based on drift-diffusion carrier transport is used for graded-junction HBTs and to assess recombination currents in both the structures. The results demonstrate that the increase in injected electron current afforded by compositional grading is accompanied by an increase in recombination within the space-charge layer. As a result, neither design option has a clear advantage with respect to common-emitter current gain.>