Modeling of laser-annealed polysilicon TFT characteristics

A model based on two-dimensional (2-D) simulation, for a polysilicon thin-film transistor (poly-Si TFT) with large grains, fabricated in laser recrystallized material, is presented. The importance of differentiating between the density of states of traps within grains and traps localized at grain boundaries is demonstrated. It is shown that the observed lack of saturation in the TFT output characteristics arises due to the effect of high interface trap density within the grain boundaries, whereas the subthreshold slope has a strong dependence on the trap density within the grains. Only by differentiating in this way between grain and grain boundary parameters can both output and subthreshold characteristics of an n-channel poly-Si TFT be accurately modeled using the same set of parameters. Appropriate values for the density of states in both grains and grain boundaries are suggested for laser-annealed TFTs.