Analysis of the short-term DC-current gain variation during high current density-low temperature stress of AlGaAs/GaAs heterojunction bipolar transistors

Improving the reliability of AlGaAs/GaAs heterojunction bipolar transistors (HBT´s) is one of the major issues in order to exploit their performance in future microwave systems at higher power and current densities. In this work, we show the following: (1) under very high current density (Jc>3×10⁴ A/cm²) the DC-current gain (B) of HBT´s exhibits principally two effects: an increase (burn-in) or a decrease (degradation). Burn-in and degradation are initiated by different junction temperatures. No remarkable change of B is observed if Jc is lower than 2×10⁴ A/cm² . (2) Linear dependences on the collector current density and activation energies of about 0.4 eV are extracted both for burn-in and degradation. (3) By comparing devices fabricated on epitaxial layers with different DC-current gains a correlation between the value of B and the observed phenomena (i.e., burn in and degradation) is found. (4) Consequently, we discuss the roles of the extrinsic base surface, base-emitter junction, and material quality. Based on the measured data, we propose a hydrogen-related mechanism, which allows us to explain the observed degradation mode