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This letter reports the study of the reliability behavior of poly-Si thin-film transistors (TFTs) with the pulsed gate voltage lower than the threshold voltage. First, the equivalent circuit model for poly-Si TFT is proposed. Considering the voltage drop for each element in the circuit model during the OFF-region gate dynamic stress, it is proposed that the main voltage drop occurs at the source and drain junctions, which could in turn degrade the device during stress. Based on this assumption, the gated p-i-n device fabricated on the same glass with the identical process conditions is stressed and analyzed. The similarity between the capacitance curves of the TFTs and gated p-i-n devices after stress proves that the main reason for degradation of poly-Si TFTs under gate OFF region ac stress is the large voltage drop across the source and drain junctions.