RESOLVING DEGRADATION MECHANISMS IN ULTRA-HIGH PERFORMANCE N-P-N HETEROJUNCTION BIPOLAR-TRANSISTORS

The conduction band barrier caused by base dopant outdiffusion is investigated for MBE-grown AlxGa1-xAs/GaAs N-p-n linearly graded heterojunction bipolar transistors (HBT's). The change of the B-E heterojunction conduction band barrier can be directly revealed by a novel technique based on the diffusion-thermionic emission current model. This is accomplished by measuring the inverted collector current radio at two different heterojunction reverse biases. In addition, this ratio is found to correlate with an anomalous base current component measured at low temperature. The heterojunction potential barrier and the anomalous base current component are attributed to beryllium redistribution during MBE growth and forward current stress. This suggests that the diffusion and incorporation of beryllium dictate V(BE) uniformity and long-term reliability of HBT's.