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We analyzed the active layer thickness-dependent performance variations of amorphous oxide-based semiconductor thin-film transistors (AOS TFTs), which are typically operated in depletion mode by using an ATLAS 2-D device simulator. The negative shift of threshold voltage was originated from increasing the amount of intrinsic carrier as active layer thickness is increased. On the contrary, off-current level was a function of Debye length, which is in inverse proportion to the square root of carrier density and the amount of valence band deep states, as well as active layer thickness. Therefore, the relation between Debye length and active layer thickness determines the off-current level, which also enables to explain the off-current behavior of AOS TFTs under light illumination.