Mechanism of device degradation under AC stress in low-temperature polycrystalline silicon TFTs

Enhanced device degradation of low-temperature polycrystalline thin-film transistors (poly-Si TFTs) under exposure to AC stress has been quantitatively analyzed. Degradation of the device characteristics of a single-drain (SD) TFT is greater under AC stress than under DC stress over an equivalent period. Hot holes are strongly related to this greater severity of degradation. A lightly doped drain (LDD) TFT is less strongly affected, and the effect here is dominated by the effective drain avalanche hot carrier (DAHC) stress, i.e., period of stress with a duty ratio of DAHC-stress application taken into account. Differences between the electric field in the respective channel regions is responsible for the different degradation properties of SD and LDD TFTs. With substantial contribution from hot-hole and hot-electron concentrations near the drain junctions, we find the severe degradation under AC stress in SD TFTs is caused by increased DAHC stress.