Effect of polysilicon-emitter shape on dopant diffusion in polysilicon-emitter transistors

The shape of the polysilicon region of a polysilicon-emitter transistor fabricated with a single layer of polysilicon depends on the processing conditions and can vary considerably. When the boundary between the polysilicon and the adjacent oxide is not vertical, the anisotropic dopant diffusivity in polysilicon can limit the ability of the dopant to diffuse to the corner, restricting the effectiveness of the polysilicon as a diffusion source in this region. In the extreme case, the junction may be located in the polysilicon, rather than in the underlying single-crystal silicon, causing large leakage currents and degrading the transistor gain.<>