The importance of neutral base recombination in compromising the gain of Si/SiGe heterojunction bipolar transistors

Si/SiGe heterojunction transistors were fabricated with 21.4 nm, heavily doped (5*10/sup 19/ cm/sup -3/) bases. Electrical results from these transistors are presented, and compared with results from comparable silicon homojunction control devices. These results indicate that the collector current is enhanced by a factor of 13, but the potential gain enhancement is compromised by a six-fold increase in base current. The base current is shown to be strongly dependent upon the collector/base voltage, indicating that recombination in the neutral base is the mechanism responsible for the increased base current. The recombination can only be modeled if the lifetime near the collector/base junction is significantly lower than elsewhere in the base. A simple two-region lifetime model predicts a value of 3.0*10/sup -13/ s close to the collector-base junction.<
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