The authors report an investigation of the effects of channel dimensions on the properties of amorphous-InGaZnO4 (a-IGZO) thin-film transistors (TFTs) that are associated with surface depletion and surface/volume ratio. As the channel width decreased below a critical value of around 100 nm, the on current abruptly decreased and the threshold voltage abruptly increased. The magnitude of hysteresis behavior also depended on the channel size. Both of these effects result from the change of channel resistance that can be explained by surface changes due to adsorption of oxygen or water vapor. The authors also investigated short channel a-IGZO TFTs and observed short channel effects below a critical value of channel length of 900 nm. The breakdown electric field of a-IGZO was found to be in the range of 0.4–0.8 MV/cm.