Improved base current ideality in polysilicon emitter bipolar transistors by fluorine implantation

Abstract

In this paper we have shown that the presence of fluorine in a polysilicon emitter bipolar transistor can result in several important benefits. We have shown that in devices where the emitter/base junction depth is very shallow, the presence of fluorine results in an improved base current ideality factor, in both npn and pnp polysilicon emitters. For example, in npn polysilicon emitters given an emitter drive-in of 10 seconds at 1025°C, an improvement in ideality factor from 1.19 without a fluorine implant to 1.03 with a fluorine implant. Further, in the similarly treated pnp polysilicon emitters, a reduction in base current for all values of VBE was observed, by at least a factor of 2. In contrast, in devices where the emitter/base junction depth was not as shallow, a reduction in emitter resistance was observed. For example, in npn polysilicon emitter devices given an emitter drive-in of 60 minutes at 900°C, a drop in interfacial resistivity from 23.5 Ωμm2 for unimplanted devices to 12.5 Ωμm2 for fluorine implanted devices. This drop in emitter resistance is consistent with enhanced break up of the interfacial oxide layer. (5 pages)