Base current analysis of poly-Si emitter bipolar transistors

Polysilicon emitter bipolar transistors with clear poly/mono-interfaces have been fabricated and base currents have been measured and analyzed in detail. The sources of base current within the pn-junction, the mono-Si part and the poly-Si part of the emitter are identified and separated. Measurements have been described and carried out precisely describing the recombination phenomena within the crystalline emitter. Currently used parameters as bandgap narrowing and Auger recombination are shown to be correct, whereas the electrically active doping within the polysilicon emitter is found to be drastically lower than within commonly processed emitters. Also the interface recombination velocity is much lower than that of directly metallized emitters. Both these results explain the substantial reduction of base current in polysilicon emitters, thus removing previously reported discrepancies between experiment and simulations. The effects of the mono-Si part on base current can be identified and separated out leaving an effective recombination velocity S as a figure of merit for the poly part of the emitter. This parameter S integrally describes recombination and transport phenomena within the poly-Si part of the emitter and the interface. The quantity S is based on experimental data only and necessary as well as sufficient to describe the effects of poly-Si on base current, without the necessity to get involved into the very complex details of transport within poly-Si layers and across grain boundaries. The dependence of S on processing conditions and implications for optimization of poly-Si emitter transistors are discussed.