This study investigates the effects of drain-bias and atmosphere on the hump formation in the transfer curves for RF-sputtered bottom-gate Mg0.05Zn0.95O thin film transistors (TFTs) after the gate-bias stressing at an elevated temperature (80 degrees C). TFTs with two different annealing conditions are tested: one is 200 degrees C for 5 h; the other is 350 degrees C for 30 min. Both 200 degrees C and 350 degrees C-annealed TFTs exhibit typical shift of transfer curves when stressed at 25 degrees C. However, we observe a hump shape in the subthreshold region of the transfer curve for the 200 degrees C-annealed TFT after positive gate-bias stressing at 80 degrees C; yet a hump is not identified for the TFT annealed at 350 degrees C. This suggests that the hump formation in the transfer curves should relate to the material properties of TFTs, modified by the annealing condition. The formation of the hump in the transfer curves is not much influenced by the additional application of the drain-bias (5 V) to the positive gate-bias (20 V). However, TFTs stressed under dry O-2 or N-2 atmosphere show some mitigation or deferring effects on the development of humps in the transfer curves, yet humps still form. (c) 2012 Elsevier B.V. All rights reserved.