ANOMALOUS BIAS-STRESS-INDUCED UNSTABLE PHENOMENA OF HYDROGENATED AMORPHOUS-SILICON THIN-FILM TRANSISTORS

Thin-film transistors (TFTs) with different silicon nitride (SiNx) gat e compositions and various hydrogen concentrations in their amorphous silicon films (a-Si:H) have been stressed with positive and negative b iases to realize the instability mechanisms. For both stress polaritie s, it is found that the threshold voltage shifts significantly increas e due to the trap sites in the SiNx gate. As the effects of the trappe d charges in the SiNx films are reduced, the anomalous threshold volta ge shifts under negative stress voltage are observed. The creation of the states near the conduction band in the a-Si:H films can be enhance d according to the defect pool concept and stimulated by the hydrogen contents in the a-Si:H films, which can be confirmed by the subthresho ld swing change. Therefore, the negative threshold voltage shifts caus ed by the hole trapping in the SiN, gate are positively compensated by the created states, reflecting the turnaround behaviors for the thres hold voltage shifts. As for the positive stress, the state creation in the a-Si:H films can be suppressed via the defect pool mechanism and thus the threshold voltage shifts are dominantly affected by the elect ron trapping in the SiNx gates. (C) 1995 American Institute of Physics .