Off-current in polycrystalline thin film transistors: an analysis of the thermally generated component

The thermal generation component of polycrystalline silicon TFTs off-current is analysed experimentally and theoretically. In order to minimize the field-enhanced component of the leakage current, hot-hole injection, obtained by stressing the device at negative gate voltage and high source-drain voltage, has been used to reduce the electric field at the drain junction. After stress, the electrical characteristics in the off-regime are channel length independent and do not depend on gate voltage. This behaviour has been associated with the thermal generation-recombination processes occurring at the drain junction. Two-dimensional numerical simulations have been carried out with the program HFIELD, which has been modified to take into account the presence of gap states in polysilicon, and to incorporate the thermal generation-recombination processes by using the Shockley-Read-Hall statistics. Numerical simulations confirm that the generation occurs in the depletion region of the drain junction. The experimental I-d-V-ds characteristics measured at negative gate voltage have been compared with the calculated characteristics. The best fit with the experimental data was obtained only by using a rather short carrier lifetime (10(-12) s). The simulations indicate that a decrease of the density of states produces a lower off-current owing to a longer carrier lifetime and to a reduction of the drain junction depletion layer.