A modified Schottky-contact gated-four-probe structure was applied to study the stability of the hydrogenated and deuterated amorphous silicon (a-Si:D) thin-film transistors under various bias conditions. It was found that after 10 V bias stress, the density of gap states generated in both the upper and lower part of the mobility gap of deuterated amorphous silicon is two to twenty times less than those of hydrogenated silicon. Besides, less density of states at the lower part of mobility gap of a-Si:D is generated after 20, -10, and -20 V bias stress. (C) 2003 American Vacuum Society.