Base-dopant density variation with emitter size in polysilicon-emitter transistors

Fabrication of polysilicon-emitter bipolar transistors with submicron emitter dimensions leads to diminished arsenic concentration in the emitter polysilicon. As a result, these small transistors are found to have significantly higher base dopant density, lower peak frequency response, and lower base current density than transistors with larger emitters made on the same silicon chip. Data indicate that these effects are caused, at least in part, by the steep topography of the polysilicon emitter. This topography results in lower arsenic concentrations in polysilicon of smaller emitters and, consequently, shallower emitter depths and wider base widths