An investigation of the tradeoff between enhanced gain and base doping in polysilicon emitter bipolar transistors

In this paper, we investigate the tradeoff between enhanced gain and base doping for polysilicon emitter bipolar transistors fabricated with an intentional oxide layer at the polysilicion-silicon interface. To assist in our investigation, we have fabricated a range of transistors of this type with identical processing apart from the boron implant for the active base. The results of detailed electrical measurements on these devices are presented, and it is demonstrated that the enhanced gain can be traded for a considerable reduction in base resistance and, hence, for a potential improvement in circuit performance. At very high base doping, an unexpected increase in base current has been observed which leads to a rapid fall-off in gain. Possible mechanisms to account for this behavior have been investigated, and the results indicate that a strong correlation exists between the increase in base current and the high concentrations of boron present in the single crystal region of the emitter.