Emitter—Base bandgap grading effects on GaAlAs/GaAs heterojunction bipolar transistor characteristics

Emitter-base bandgap grading effects on GaAlAs/GaAs heterojunction bipolar transistor characteristics are theoretically investigated using an accurate one-dimensional numerical model including aluminum mole-fraction-dependent velocity versus field characteristics and donor energy level. The bandgap grading is shown to influence not only the electron injection but also the carrier recombination and the hole injection, resulting in a significant common-emitter current gain dependence on the graded layer thickness. The cutoff frequency dependence on the graded layer thickness is also described. Detailed discussion is given for the underlying physical mechanism that determines the device performance.