Stability of beryllium-doped compositionally graded and abrupt AlInAs/GaInAs heterojunction bipolar transistors

Stability of base-emitter (BE) junction characteristics and dc current gain is analyzed for high-performance AlInAs/GaInAs heterojunction bipolar transistors (HBTs). HBT devices with heavily beryllium-doped base layers were stressed with a constant collector current at elevated temperatures (180 and 208 °C). Devices with compositionally abrupt and graded BE junctions were compared. The compositionally abrupt devices showed signs of significant Be diffusion under bias stress (i.e., drift in turn-on voltage and drop in dc current gain). The graded junction devices, however, remained extremely stable under bias stress. The compositional grading at the BE junction was accomplished by a nine-period AlInAs/GaInAs superlattice and is believed to be responsible for the stability of the graded devices.