Abnormal Hump and Two-Step Degradation of Top Gate a-InGaZnO TFTs Under Positive Bias Stress

In this study, the electrical characteristics of top-gate amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) under positive bias stress (PBS) are investigated. Abnormal two-step degradation and hump effects are observed under PBS. When the self-aligned process is applied during the interlayer dielectric (ILD) deposition,
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IGZO is formed by hydrogen doping to define the source and drain, which is accompanied by hydrogen diffusion not only into the channel to influence the initial electrical performance but also into the gate insulator (GI), which leads to the degradation under PBS. The diffusion distance in the length and width directions is calculated. Also, the differences in the diffusion distance in the width and length directions can be ascribed to the geometric structure of the top-gate a-IGZO TFTs. The hump occurs after PBS because of the impact of hydrogen diffusion in the width direction. The two-step degradation and its sequences can be ascribed to two mechanisms, including electron trapping and hydrogen diffusion.