Positive Gate-Bias Temperature Stability of RF-Sputtered Mg0.05Zn0.95O Active-Layer Thin-Film Transistors

This paper investigates the positive gate-bias temperature stability of RF-sputtered bottom-gate Mg0.05Zn0.95O active-layer thin-film transistors (TFTs) annealed at 200 degrees C for 5 h and 350 degrees C for 30 min. Although the TFT devices initially exhibited similar electrical characteristics, the TFTs annealed at 350 degrees C demonstrated stability characteristics superior to those annealed at 200 degrees C. This result is due to the improved crystallinity and more stable phase with greater proportion of Zn replaced by Mg in the ZnO crystals. The results also reveal a hump shape in the subthreshold region of the transfer characteristics, which is induced by the positive gate-bias stress at elevated temperatures. The hump phenomenon was suppressed in the TFT annealed at 350 degrees C. The hump disappeared shortly after removing the positive gate bias, suggesting that this phenomenon was meta-stable and resulted from gate-bias-induced electric field. One possible mechanism responsible for the hump formation in the transfer curve is the gate-field-induced back-channel parasitic transistor. Alternatively, this hump phenomenon might have been due to the creation of meta-stable vacancies in which the neutral defects were thermally excited and released electrons into the active layer to form a leakage path when the TFTs were subjected to gate-bias stress at elevated temperatures.