Generalized Hot-Carrier Degradation and Its Mechanism in Poly-Si TFTs Under DC/AC Operations

In the previous report, we had reported the mechanism for the degradation of poly-Si TFTs under OFF region gate ac operation with the source and drain electrodes grounded. In this paper, the study is extended to the degradation of the devices under various ac and dc operation conditions. It is discovered that, though these stress conditions are different, the corresponding degradation behaviors in their I-V and C-V curves all resemble the degradation behavior of the device under dc hot-carrier stress. Two important factors, namely, the electric field across the junction and the number of carriers flowing through the junction, are taken into discussion in this paper and comparison of these stress conditions. It is then categorized that these operation conditions can be described as the "generalized hot-carrier effect," since the degradation is found to occur near the junctions by the energized carriers, just as that under dc hot-carrier stress. The qualitative comparison of the electric field and carrier flow through the junction for the four stress conditions as well as the difference in the degradation mechanism between MOSFETS and poly-Si TFTs are also provided.