The influence of post-emitter processing on the current gain of bipolar transistors

The effects of post-emitter processing on the current gain of n-p-n and p-n-p transistors used in linear integrated circuits, is the subject of this paper. It is seen that during the emitter diffusion deep level impurities segregate into the bulk and space-charge regions of the emitter. The aluminum alloy process at 500°C provides a powerful scavenging action that getters the impurities. The post-alloy current gain increases by a factor of ten. This gettering is effective in phosphorus diffused emitters if the dielectric is either a CVD oxide, a plasma deposited oxide, or phosphorus glass under Si
3
N
4
. With thermally oxidized emitters there is no improvement in current gain. Boron diffused emitters, however, respond to the aluminum alloy process regardless of the dielectric. This study also identifies two processes that contribute to bandgap narrowing (
\DeltaE_{g}
) these are the emitter diffusion and the post-emitter thermal oxidation. A value of
\DeltaE_{g}
= 0.04 eV measured after the emitter diffusion was found to increase to 0.09 eV after thermal oxidation at 975°C. This additional component of
\DeltaE_{g}
is a result of the anomalous accumulation of phosphorus at the silicon surface during oxidation.