Analysis of the short-term DC-current gain variation during highcurrent density-low temperature stress of AlGaAs/GaAs heterojunctionbipolar 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