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The degradation mechanism in p-channel polysilicon thin-film transistors under negative-bias temperature (NBT) stress and pulse stress, which alternates NBT stress and drain-avalanche hot carrier (DAHC) stress, was investigated. An analysis of recovery effects and activation energy suggests that the device degradation under dc-NBT stress is explained by a reaction-diffusion model and limited by hydrogen diffusion. These features are also observed in the case of the device degradation under pulse stress. Pronounced degradation occurs not after DAHC stress application (electron injection) but after NBT stress application (hole injection). NBT stress degradation is locally accelerated after DAHC stress application because the effective gate voltage negatively increases due to trapped electrons during DAHC stress. The trap states and positive charges that were generated by this accelerated NBT stress are considered to be the main cause of device degradation under pulse stress.