Analysis of drain ac stress-induced hot-carrier degradation in low-temperature poly-Si TFTs

摘要

We investigated hot-carrier degradation in low-temperature (≤425 ℃) polycrystalline-silicon thin-film transistors (poly-Si TFTs). When the appropriate stress is applied to the TFTs, a decrease in on-current and an increase in off-current are induced by the drain avalanche hot carriers (DAHC). The extent of the degradation is quite large, however, when the ac stress is applied to the drain (drain ac stress). Moreover, the degradation is accelerated by an increase in the frequency of the drain ac stress. We found that the pulse number of the drain ac stress dominated this degradation. It is well known that hot carriers are generated by impact ionization, and the impact ionization rate increases with an increase in the carrier density and the electric field in the channel. When the dc stress is supplied to the drain (drain dc stress), the electric field near the drain is high, while the electron density near the drain is low because of the formation of a pinch-off region. On the other hand, for the case of drain AC stress, both the electric field and electron density near the drain become high during the transition when th voltage of the stress pulse changes from low to high. The impact ionization rate increases at this point. As this cycle is repeated, it is tought that the degradation of TFTs is accelerated.