Physical models for degradation effects in polysilicon thin-film transistors

Experimental data showing the degradation in performance of polysilicon thin-film transistors (TFTs) under a variety of bias stress conditions are presented. A model is proposed to explain these effects whereby device performance degrades due to changes in the effective density of defect states in the material. Unlike single-crystal devices which degrade from hot-carrier effects, poly-Si TFTs are believed to degrade primarily due to the presence of high carrier densities in the channel. Good agreement between computer simulations of the device characteristics and experimental data ia demonstrated. It is shown that stressing under transient conditions leads to a more severe performance degradation than stressing under comparable steady-state conditions.>