We report on unusually abrupt drain current change observed in polysilicon thin-film transistors (TFT's) with a channel length and width of 1 μm or smaller. The polysilicon used to fabricate the devices was deposited by LPCVD and the grain size of the film was enhanced by silicon ion implantation followed by a low-temperature anneal. The TFT's exhibited an abrupt drain current change of more than five orders of magnitude for a corresponding gate voltage change of less than 40 mV. A self-limiting positive feedback loop due to impact ionization currents and/or a parasitic bipolar effect are suggested as possible explanations.
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