Investigations on the Negative Shift of the Threshold Voltage of Polycrystalline Silicon Thin-Film Transistors Under Positive Gate Bias Stress

Different from the conventional degradation phenomenon under positive bias stress (PBS), the shift of the transfer characteristic curve of polycrystalline silicon thin-film transistors (TFTs) to the negative gate bias direction after PBS is observed and reported. The PBS degradation is found to be recoverable and the recovery proceeds at a faster rate first and then continues at a much slower rate. The recovery can be further accelerated at a higher temperature or by applying a negative gate bias. After detailed data analysis, the degradation mechanism is proposed to be the generation of protons in the gate oxide and its accumulation at the channel/gate oxide interface. The proposed degradation model could explain both the degradation phenomena and the recovery behaviors of PBS degradation.