Degradation mechanisms for polycrystalline silicon thin-film transistors with a grain boundary in the channel under negative gate bias stress

The negative gate bias stress (NBS) reliability of n-type polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with a distinct defective grain boundary (GB) in the channel is investigated. Results show that conventional NBS degradation with negative shift of the transfer curves is absent. The on-state current is decreased, but the subthreshold characteristics are not affected. The gate bias dependence of the drain leakage current at V _ds of 5.0 V is suppressed, whereas the drain leakage current at V _ds of 0.1 V exhibits obvious gate bias dependence. As confirmed via TCAD simulation, the corresponding mechanisms are proposed to be trap state generation in the GB region, positive-charge local formation in the gate oxide near the source and drain, and trap state introduction in the gate oxide.