Anomalous Mixed-Mode Damage Effects in SiGe HBTs at Cryogenic Temperatures

This paper investigates the mixed-mode (MM) reliability of fourth-generation silicon germanium heterojunction bipolar transistors at temperatures ranging from 300 K to 25 K. Results show that the classical MM damage mechanism, which results in increased base leakage current as a result of hot carriers generated in the CB junction, becomes more prevalent as temperature is decreased, but that at temperature below 100 K the damage rate begins to decrease. This enhanced degradation with cooling is not a result of increased hot carrier generation in the CB junction, but rather an increase in mean free path, which leads to more hot carriers reaching oxide interfaces with sufficient energy to cause damage. Measurement results also show that at temperatures below 200 K, an unexpected increase in the collector current as a function of stress becomes visible.