In this work, the characteristics of a p-type polysilicon thin-film transistor (poly-Si TFT) with dynamic bias stress were investigated. The ac stress is operated with the constant drain voltage (15 V) and the varying gate voltage (0 to 15 V) to degrade the devices. There are some phenomena which cannot be completely explained by the typical negative bias temperature instability (NBTI) mechanism in the experiment. In addition to NBTI, we suggest that the self-heating effect might be involved, because the self-heating effect could raise the channel temperature and cause the dissociation of the Si-H bonds at the poly-Si/SiO2 interface due to Joule heating. The released hydrogen reacts with SiO2 and causes the fixed charge in the gate oxide. Thus, the degradation of the electrical characteristics of device is mainly dominated by the self-heating-induced NBTI effect. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2990726] All rights reserved.