Analysis of current voltage characteristics of low-temperature-processed polysilicon thin-film transistors

The relationship between device performance and trap state density in polysilicon films was investigated. The density in the silicon energy gap was obtained by fitting the calculated on-state current versus gate voltage curve to the measured one for low-temperature (⩽600°C) processed polysilicon TFTs fabricated under various conditions, such as different deposition temperatures and annealing methods for crystallization. On-state current was markedly improved by reducing the density near band edges in the gap, and the improvement was realized by depositing the films at around 500°C in an LPCVD system or employing laser annealing instead of thermal annealing at 600°C. Off-state current was reduced to a great extent by reduction of the density around the midgap by using a plasma-hydrogenation technique