The effects of top-insulator on the instability problems of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) have been studied. In a-Si:H TFT with top-insulator (E/S type), charge trapping into the both of top-insulator and gate insulator has been shown under the bias stress.
In order to investigate the charge trapping effects of top-insulator, we proposed a new method of Measurement. By this Method, we observed that trapped charges in top-insulator increased drain currents for positive gate bias stress, and this increment of drain currents was more serious with increasing the ratio of source/drain overlap length to channel length. It has founded that the instability problems of a-Si:H TFTs was attributed to the effects of top-insulator as well as that of gate insulator.