Reliability analysis of GaAs/AlGaAs HBTs under forward current/temperature stress

The reliability of GaAs/AlGaAs heterojunction bipolar transistors is investigated by accelerated life-testing of discrete devices under forward bias stress at elevated temperatures. The DC device characteristics are monitored to evaluate the effect of bias/temperature stress on a large number of devices fabricated on MBE (molecular beam epitaxy) grown material. The primary degradation observed in some devices is a reduction in the current gain which appears to be due to an electric field-aided diffusion of interstitial Be from the base into the base-emitter graded region. Other devices with optimal epitaxial material show stable current gain after DC bias stress at high temperature. Ohmic contact degradation, with or without bias, is also observed at the emitter contact, resulting in an increased emitter series resistance.<>