Monitoring the AlGaAs/GaAs HBT Current Gain Long-Term Instability based on Noise and Leakage Current Characteristics

A simple model is proposed to monitor the dc current gain long-term instability in the AlGaAs/GaAs heterojunction bipolar transistor (HBT). It is derived from the theory that the recombination current at the extrinsic base surface increases with time due to the surface degradation process. Furthermore, the initial 1/f noise and base leakage current characteristics have been used to provide the needed model parameters for the HBT surface recombination mechanism and surface quality, respectively. The current gain long-term variations calculated from the model for four HBTs compare favorably with those obtained from measurements. The model proposed is potentially useful to screen unreliable HBT lots without having to carry out the long-term stress test.