Thin base formation by double diffused polysilicon technology

The method of double-diffused emitter-base formation is characterized. It is shown to be a viable technique for the fabrication of advanced bipolar transistors. The use of amorphous instead of polycrystalline silicon as the emitter contact material results in a shallower emitter-base junction and little effect of the boron diffusion on the obtained arsenic profile. The narrowing of the base yields a higher intrinsic base resistance for the same number of carriers leading to a decrease in current gain for the same intrinsic base resistance. The different processing of double diffusion compared to base implantation may also lead to a loss in emitter efficiency. Nevertheless uniformity of the basewidth seems to be excellent and good high-frequency characteristics (up to f_T=15 GHz) are obtained