Conductivity properties of narrow-channel polysilicon thin-film transistors

The effect of the narrow channel width on performance of thin-film transistors (TFTs) is evaluated. To this end, the authors fabricated coplanar, n-channel TFTs with a fixed gate length of 20 mu m and channel widths ranging from 20 mu m to half a micrometer. The dependences of threshold voltage, grain-boundary trap density, and transconductance on the channel width were measured. A drastic decrease in the threshold voltage was observed as the channel width W was reduced to a few micrometers. The grain-boundary trap density was also found to decrease sharply when W was decreased to below a few micrometers. The transconductance, on the other hand, was found to be slightly higher than the value expected from its linear relationship with W. This translated into an increase in the effective mobility for TFTs with small channel widths. These results demonstrate the existence of regions in the channel near the polysilicon film edges where the grain-boundary trap density is much less than in the rest of the channel.