We propose a simulation model to explain two-dimensional field-effect operation of undoped poly-Si TFTs under small drain voltages. Our model includes both thermionic-emission and drift-diffusion conduction processes. We calculated grain-boundary potential barriers, channel currents, and various device parameters depending on grain size and defect density. In order to validate our model, we compared calculated currents with experimental data for two types of poly-Si TFTs. We could obtain the good current fits simultaneously in both subthreshold and linear regions by adopting proper densities of states in the poly-Si channels. We could also explain well the temperature dependent current changes, the current activation energy versus the gate voltage, and enhancement of device performance due to hydrogenation.
New method was designed to determine the grain-boundary trap distribution. Hydrogenation effect was considered not only as decrease in density but also as a shift of position of midgap defects. The adopted density of states in the current fit was found to be in good agreement with the evaluated value by this method. Finally, we succeeded in modelling the drain current under small drain voltages by using the combined transport process along with the proper density of states in the two-dimensional grain-boundary structure.
