This work investigates the asymmetric negative bias temperature instability (NBTI) degradation of poly-Si thin film transistors (TFTs). Electric measurements of normal and reverse modes are employed to analyze degradation of threshold voltage, current, leakage current, and subthreshold swing. The results indicate that a nonuniform vertical electric field at the poly-Si/SiO2 interface results in an asymmetric NBTI degradation. The trap generation density gradually diminishes from source to drain. This paper also presents energy diagrams to explain the experimental data; these indicate that asymmetric TFT degradation results from the distribution of the trap state induced by an asymmetric NBTI. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3244596] All rights reserved.