Turnaround phenomenon of threshold voltage shifts in amorphous silicon thin film transistors under negative bias stress

The turnaround phenomenon of threshold voltage shifts is investigated in th in film transistors (TFTs) with different defect densities of hydrogenated amorphous silicon (a-Si:H) films and compositions of SiNx. It was found tha t TFTs with high-defect-density a-Si:H films and N-rich SiNx gate exhibit t he turnaround phenomenon while TFTs with other conditions of a-Si:H and SiN x films do not. Results reveal that the turnaround phenomenon is greatly in fluenced by charge traps in SiNx and state creation in the a-Si:H layer. Wh en state creation is dominant at low bias stress, the turnaround phenomenon occurs. In contrast, if charge trapping is dominant at low bias stress, th e turnaround phenomenon does not occur.