Lateral encroachment of extrinsic-base dopant in submicrometer bipolar transistors

Abstract

This letter examines the encroachment of extrinsic-base dopants in submicrometer n-p-n transistors. We measured I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> and I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> in 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 I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> was identical although the actual emitter area differed by a factor of 1.8 due to different sidewall thickness. In contrast, the base current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> was proportional to the actual emitter area, conforming to the common belief that I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> is 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 I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> injection, and caused increasing leakage along the perimeter.