Impact of processing parameters on base current in polysilicon contacted bipolar transistors

A number of models have been presented to explain the lower base current in bipolar transistors with polysilicon emitter contacts: the effects of oxides (native and chemically grown) at the polysilicon-silicon interface; dopant segregation at this interface; and minority carder transport in the polysilicon. To differentiate between these models and to determine the process sensitivity of polysilicon contacted BJT´s, a series of controlled experiments correlating physical structures with electrical characteristics have been performed. We find that for anneals at low temperatures and times (e.g., 900°C/1hr) the unintentional native oxide at the polysilicon-silicon interface dominates the electrical characteristics, resulting in extremely low base currents. For higher temperature anneals or doping levels above 2×10²⁰/cm³, the native oxide layer breaks up and epitaxial regrowth of the polysilicon layer occurs. This reduces the blocking action of the interface and allows more carriers to enter the polysilicon, increasing the base current by as much as a factor of four.