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 s ilicon nitride (SiNx) gates have been stressed under various AC biases to i nvestigate the instability mechanisms. The state creation is dominant at lo w stress voltage although the charge trapping also occurs in SiNx gates, an d such cases have also been found under the DC bias stress. In addition, th e degradations of a-Si:H TFTs are found to be independent of the AC frequen cy 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. Fin ally, the degradation of the a-Si:H TFTs under bipolar AC bias stress is al so introduced. It is found that the instability mechanisms of devices are c omposed of different charge compensations in SiNx and redistributions of de fect states in the a-Si:H layer.