A conduction model for semiconductor-grain-boundary-semiconductor barriers in polycrystalline-silicon films

A quantitative trapping model is introduced to describe the electrical properties of a semiconductor-grain-boundary-semiconductor (SGBS) barrier in polysilicon films over a wide temperature range. The grain-boundary scattering effects on carrier transport are studied analytically by examining the behavior of the height and width of a rectangular grain-boundary potential barrier. The model also verifies the applicability of a single-crystal band diagram for the crystallite within which an impurity level exists. Carder transport includes not only thermionic field emission through the space-charge potential barrier resulting from trapping effects and through the grain-boundary scattering potential barrier but also thermionic emission over these barriers. Thermionic emission dominates at high temperatures; however, at low temperatures, thermionic field emission becomes more important and the grain-boundary scattering effects are an essential factor. By characterizing the experimental data of the characteristics, resistivity, mobility, and carrier concentration, this model enhances the understanding of the current transport in polysilicon films with grain sizes from 100 Å to 1 µm, doping levels from 1 × 10¹⁶to 8 × 10¹⁹cm^-3, and measurement temperatures from -176 to 144°C. The limitations of the model are also discussed.