The degree of degradation between the amorphous-indium-gallium-zinc oxide (a-IGZO) thin film transistor (TFT) using the top-gate only or bottom-gate only is compared. Under negative gate bias illumination stress (NBIS), the threshold voltage (V-T) after bottom-gate NBIS monotonically shifts in the negative direction, whereas top-gate NBIS operation exhibits on-state current increases without V-T shift. Such anomalous degradation behavior of NBIS under top-gate operation is due to hole-trapping in the etch stop layer above the central portion of the channel. These phenomena can be ascribed to the screening of the electric field by redundant source/drain electrodes. In addition, the device degradation of dual gate a-IGZO TFT stressed with different top gate pulse waveforms is investigated. It is observed that the degradation is dependent on the frequency of the top gate pulses. The V-T shift increases with decreasing frequency, indicating the hole mobility of IGZO is low. (C) 2016 Elsevier B.V. All rights reserved.