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 thin film transistors (TFTs) with different defect densities of hydrogenated amorphous silicon (a-Si:H) films and compositions of SiNx. It was found that TFTs with high-defect-density a-Si:H films and N-rich SiNx gate exhibit the turnaround phenomenon while TFTs with other conditions of a-Si:H and SiNx films do not. Results reveal that the turnaround phenomenon is greatly influenced by charge traps in SiNx and state creation in the a-Si:H layer. When state creation is dominant at low bias stress, the turnaround phenomenon occurs. In contrast, if charge trapping is dominant at low bias stress, the turnaround phenomenon does not occur.