We fabricated amorphous InGaZnO thin film transistors (alpha-IGZO TFTs) with aluminum oxide (Al2O3) as a gate insulator grown through atomic layer deposition (ALD) method at different deposition temperatures (Tdep). The Al2O3 gate insulator with a low Tdep exhibited a high amount of hydrogen in the film, and the relationship between the hydrogen content and the electrical properties of the TFTs was investigated. The device with the Al2O3 gate insulator having a high H content showed much better transfer parameters and reliabilities than the low H sample. This is attributed to the defect passivation effect of H in the active layer, which is diffused from the Al2O3 layer. In addition, according to the post-annealing temperature (Tpost-ann), alpha-IGZO TFTs exhibited two unique changes of properties; the degradation in low Tpost-ann and the enhancement in high Tpost-ann, as explained in terms of H diffusion from the gate insulator to an active layer.