A current-voltage model based on the charge sheet approach in post saturation that includes the "kink effect" for polycrystalline silicon thin film transistor is presented. To explain the kink effect, the increase in the channel charge density due to the impact ionization caused by the high electric field in the pinch-off region near the drain is evaluated. The charge density thus obtained is used to obtain the drain current in the post saturation and the results are matched with the experimental data, showing good agreement. (C) 2002 Elsevier Science Ltd. All rights reserved.