The dependence of off-leakage current on channel film quality in poly-Si thin-film transistors has been analyzed using two-dimensional device simulation. It is found that the off-leakage current decreases as the intragrain trap density decreases for the low V-gs. This is because the Phonon-assisted tunneling with Poole-Frenkel effect is the dominant mechanism of the carrier generation and the generation rate of carrier pair decreases as the intragrain trap density decreases. On the other hand, the off-leakage current slightly increases as the intragrain trap density decreases for the high V-gs. This is because the band-to-band tunneling is the dominant mechanism and the influence of the intragrain trap density to the carrier conductance is larger than that to the generation rate. (C) 2010 Elsevier Ltd. All rights reserved.