Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) with silicon nitride (SiN x ) 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 SiN x 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 SiN x and redistributions of defect states in the a-Si:H layer.

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 SiN x . It was found that TFTs with high-defect-density a-Si:H films and N-rich SiN x gate exhibit the 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 influenced by charge traps in SiN x 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.

A short H2 plasma treatment of the gate SiNx before depositing amorphous silicon (a-Si:H) is found to significantly decrease the threshold shifts in the bias stress, inverted a-Si:H thin film transistors (TFTs). The reduced threshold voltage shift is attributed to a plasma induced reconstruction of SiNx precursors leading to the removal of the weak bonds. A prolonged plasma treatment, however, degraded the TFT characteristics; this was traced H2 plasma damage which eventually generated a rough a-Si:H/SiNx interface.

The hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) with silicon nitride as a gate insulator have been stressed with negative and positive bias to realize the instability mechanisms. It is found that the threshold voltages of the a-Si:H TFTs are positively shifted under low negative bias stress and then negatively shifted for large negative gate bias. The positive threshold voltage shift is ascribed to the increased states in the band gap near the conduction band by the negative gate bias. As the negative bias continuously increases, the hole trapping in the silicon nitride or at the a-Si:H/silicon nitride interface will become dominant, resulting in the negative threshold voltage shift. A similar turnaround phenomenon is also observed with respect to the stress time. On the other hand, for the TFTs stressed with positive gate bias, the monotonic increase of the threshold voltage shift with stress time is attributed to the state creation. Nevertheless, the distributions of the created states in the energy band gap for the a-SiH TFTs after the stress will be affected by the bias polarity based on the defect pool model, reflecting the asymmetrical subthreshold swing change against the positive and negative stress bias.