The instability mechanisms of passivated and non-passivated low-temperature polycrystalline silicon thin film transistors (LTPS TFTs) under various bias stress conditions have been investigated. Irrespective of plasma treatment, the degradation was more severe under negative gate bias stress than that under positive gate bias stress. This could be due to Fowler-Nordheim tunneling electron induced impact ionization. For hot carrier stress, TFTs with NH3 plasma treatment degraded more severely than those without plasma treatment. This might be attributed to collapsing of weak Si-H bonds in NH3-plasma passivated devices. For the high current stress, it showed the opposite results against hot carrier stress. (C) 2002 Elsevier Science Ltd. All rights reserved.