Lateral encroachment of extrinsic-base dopant in submicrometer bipolar transistors

This letter examines the encroachment of extrinsic-base dopants in submicrometer n-p-n transistors. We measured Iband Icin devices defined by the same emitter mask on two wafers which were identically processed except for a split on the sidewall spacer. It was found that Icwas identical although the actual emitter area differed by a factor of 1.8 due to different sidewall thickness. In contrast, the base current Ibwas proportional to the actual emitter area, conforming to the common belief that Ibis dominated by surface recombination at the poly/ monosilicon interface in poly-emitter transistors. In addition, the reverse emitter-base (E-B) leakage was found to be higher on the wafer with the thinner sidewall. These results were attributed to the lateral encroachment of the extrinsic-base dopants upon the active region, which limited the effective area for Icinjection, and caused increasing leakage along the perimeter.