On the perimeter base leakage of double-poly self-aligned p-n-p transistors

It is shown that in the shallow junction formation for high-performance p-n-p devices, the perimeter E-B junction may be positioned inside the polysilicon due to insufficient boron dopants, causing excessive low-level base leakage current and current gain degradation. The I-V characteristic has an exp(qV/2kT) dependence consistent with carrier recombination at grain boundaries. Although the problem can be cured by using a deep emitter drive-in, the resulting AC performance will be traded off due to increased emitter charge storage. The nonuniform lateral profile limits the minimum achievable emitter junction depth for useful p-n-p devices, which in turn makes thin-base formation more difficult.< >