Positive gate-bias temperature instability of ZnO thin-film transistor

The positive gate-bias temperature instability of a radio frequency (RF) sputtered ZnO thin-film transistor (ZnO TFT) is investigated. Under positive gate-bias stress, the saturation drain current and OFF-state current decrease, and the threshold voltage shifts toward the positive direction. The stress amplitude and stress temperature are considered as important factors in threshold-voltage instability, and the time dependences of threshold voltage shift under various bias temperature stress conditions could be described by a stretched-exponential equation. Based on the analysis of hysteresis behaviors in current-voltage and capacitance-voltage characteristics before and after the gate-bias stress, it can be clarified that the threshold-voltage shift is predominantly attributed to the trapping of negative charge carriers in the defect states located at the gate-dielectric/channel interface.