Mechanism of device instability for unhydrogenatedpolysilicon TFTs under off-state stress

Abstract

The effects of off-state stress (Vgs = 0 V,Vds = 0 to -20 V) on unhydrogenated p-channelpolysilicon thin-film transistors (TFTs) were studied.It was observed that the post-stressed subthreshold swing isfirst improved due to the annealing effect from the interactionof tunnelling electrons and captured holes. As the stress timeincreases or as the stress bias increases, the generation oftraps caused by tunnelling electrons will cancel out theannealing effect and then degrade the subthreshold swing. Inaddition, the trapping of tunnelling electrons in the gate oxidecauses a shift of threshold voltage. However, improving thequality of the gate oxide interface by oxidation of the channelpolysilicon on submicrometre bottom-gate TFTs can reduce theimpact of the off-state stress.