Minority-carrier hole diffusion length in heavily-doped polysilicon and its influence on polysilicon-emitter transistors

The minority-carrier hole diffusion lengths in heavily doped polysilicon with and without hydrogen passivation were determined by analyzing the photon-induced current in polysilicon-emitter diode structure. After hydrogen passivation, the minority-carrier hole diffusion length in polysilicon was increased by a factor of approximately 3.5. Such an increase in diffusion length should result in an increase in polysilicon-emitter transistor current gain if the transport model of T.H. Ning and R.D. Issac (1980) is valid. However, by comparing transistors with and without hydrogen passivation, it is found that the polysilicon-monosilicon interface properties are more important than the minority-carrier transport properties in the polysilicon layer in determining the current gain for transistors with emitter doping >1×10²⁰ cm^-3. Nevertheless, for an emitter doping ⩽1×10²⁰ cm^-3, the diffusion and recombination inside the polysilicon layer has to be taken into account