In this study, negative-bias-temperature-instability (NBTI) stress induced interface, and bulk states of polycrystalline silicon (poly-Si) film were identified by using gated poly-Si P-intrinsic-N (PIN) diodes. The generation of reverse current was proportional to the quantity of defects in the poly-Si of the gated diode device. Experimental results revealed NBTI degradation has a spatial distribution in the poly-Si film and demonstrated that the resultant generation of poly-Si bulk states causes the elevated drain leakage current of thin-film transistors (TFTs). We conclude that grain boundaries, with enriched hydrogenated silicon bonds in the bulk of poly-Si, interact electrochemically with hole-carriers. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3551463] All rights reserved.