Device simulations of grain boundaries in lightly doped polysilicon films have been performed. Dependence of the energy band, carrier density, potential barrier and electric conductivity on the defect density and grain size was carefully investigated. As a result, the mechanism of the carrier transportation has been clarified. The boundary defects not only trap and reduce free carriers, but also form the potential barrier and interfere with the carrier movement. As the defect density increases,

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... in the case of the small grain size, first, the space-charge regions spread over the entire grain. Next, while the potential barrier remains the same, the lowest energy of the conduction band from the Fermi level (E c -E f ) increases, and the carrier density decreases. Finally, E c -E f becomes the highest and remains the same. On the other hand, in the case of the large grain size, before the space-charge regions spread over the entire grain, E c -E f at the grain boundary reaches its maximum. Therefore, even if the defect density increases further, the potential barrier remains the same, and the carrier density remains high. By comparing the experimental and simulated electric conductivity, the defect density can be extracted.