ON THE IMPURITY PROFILES OF DOWN SCALED BIPOLAR TRANSISTORS.

The design and reliability issues unique to bipolar scaling are discussed. They are caused by the close proximity of the heavily doped emitter and extrinsic base regions and by the contradicting heat-cycle requirement for forming the extrinsic base and the intrinsic base. An insufficient lateral diffusion from the extrinsic base leads to premature punch-through at the link region between the intrinsic base and the extrinsic base, while an excessive lateral diffusion reduces the current gain and cutoff frequency in addition, causes emitter-perimeter leakage and accelerates stress-induced current-gain degradation. The tradeoffs between the emitter-base reverse leakage current and the control of the thin base profile are also discussed.