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We have investigated the degradation of n-channel thin-film transistors under dynamic stress. Degradation was examined for various pulse parameters such as rising time or frequency. A shortfall time led to a large degradation. This mechanism was analyzed by using a picosecond emission microscope and a device simulator to examine the transient current, experimentally and theoretically, respectively. We have successfully detected emission at the pulse fall edge for the first time. Emission intensity increased with the decrease in pulse fall time. By means of the transient device simulation, transient current corresponding to the gate pulse was obtained. From the comparison between internal field and transient current, hot carrier current generated in the pulse fall was detected for the first time. A reasonable agreement between the data obtained by the emission microscope and those obtained by the device simulator clearly indicates that hot electrons are the dominant cause of degradation under dynamic stress. On the basis of the comparison between experimental and theoretical results, we proposed a model which takes into consideration electron traps in poly-Si.