The instability mechanisms of hydrogenated amorphous silicon thin film transistors under AC bias stress

Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) with silicon nitride (SiNx) gates have been stressed under various AC biases to investigate the instability mechanisms. The state creation is dominant at low stress voltage although the charge trapping also occurs in SiNx gates, and such cases have also been found under the DC bias stress. In addition, the degradations of a-Si:H TFTs are found to be independent of the AC frequency for the positive polarity but show frequency dependence for the negative polarity due to the RC effect. Furthermore, the threshold voltage shift is associated with the duty ratio due to the accumulation of stress time. Finally, the degradation of the a-Si:H TFTs under bipolar AC bias stress is also introduced. It is found that the instability mechanisms of devices are composed of different charge compensations in SiNx and redistributions of defect states in the a-Si:H layer.