A COMPARISON OF THE KINK EFFECT IN POLYSILICON THIN-FILM TRANSISTORS AND SILICON-ON-INSULATOR TRANSISTORS

Polysilicon thin film transistors (TFTs) differ from conventional sili con on insulator (SOI) transistors in that the TFT exhibits a fundamen tal gate length dependence of the voltage at which a kink occurs in th e output characteristics. This difference is shown to be caused by the peak lateral electric field being strongly dependent on the doping de nsity in an SOI transistor, but relatively insensitive to trap distrib ution in a TFT. Source barrier lowering which occurs in SOI transistor s is absent in a TFT, where the increase in current is the result of a held redistribution along the channel. For very short gate lengths, t he TFT exhibits a small pseudo-bipolar gain. Estimates of this bipolar gain can be made by simulation of TFT characteristics with and withou t impact ionisation. The magnitude of the gain is shown to be approxim ately inversely proportional to gate length. Copyright (C) 1996 Elsevi er Science Ltd