Punchthrough phenomena in submicron polysilicon thin-film transistors

The punchthrough phenomena of submicron polysilicon thin-film transistors (TFTs) with and without hydrogenation have been investigated in detail. Unhydrogenated TFT shows better punchthrough immunity. At high drain bias, the subthreshold swing of unhydrogenated TFTs is much smaller than that of hydrogenated TFTs. This is contrary to the phenomenon of TFTs operated under normal operated voltage (|V_ds|<8 V). Furthermore, as the channel width of unhydrogenated TFTs becomes comparable with or smaller than the polysilicon grain size, the subthreshold swing at low drain bias is continuously improved with decreasing channel width. The trend of subthreshold swing is in the opposite direction for drain biases above 8 V. A model, which takes the effect of charged traps into account, is proposed to interpret the punchthrough phenomena of TFTs.