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Summary form only given. In the course of exploring the transfer characteristics in small-dimensional polysilicon TFTs (thin-film transistors), the authors found an extremely abrupt drain current change which cannot be explained by the existing TFT models. The measured devices are coplanar-type, n-channel polysilicon TFTs with channel widths and lengths, W and L, ranging from 20 mu m to half a micrometer. The authors measured the (drain current)-(gate voltage) characteristics in TFTs with various L and W applying a drain voltage of 5.0 V. It was found that the subthreshold slope became steeper as the channel dimension was reduced. The TFTs with dimensions smaller than 1.5 mu m showed transfer characteristics which were qualitatively different from those observed in the larger-dimension TFTs. In the TFT with W=L=0.5 mu m, the drain current changed from 50 pA to 15 mu A, corresponding to the gate voltage change of 40 mV. It is suggested that the observed switching phenomenon in the small-dimension TFTs may be related to avalanche-type breakdown at the grain boundary potential barrier, the height and width of which can be controlled by the gate voltage.