The effects of NH3 and H2 plasma passivation on the characteristics of poly-Si thin-film transistors with source/drain extensions induced by a bottom sub-gate were studied. Our results show that significant improvements in device performance can be obtained by both passivation methods. Moreover, NH3-plasma-treatment appears to be more effective in reducing the off-state leakage, subthreshold swing, compared to H2 plasma passivation. NH3 plasma treatment is also found to be more effective in reducing the anomalous subthrehold hump phenomenon observed in non-plasma-treated short-channel devices. Detailed analysis suggests that all these improvements can be explained by the more effective passivation of the traps distributed in both the front and back sides of the channel by NH3 plasma treatment.