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Atomic layer deposition (ALD) has been studied extensively to employ oxide semiconductor thin film transistor (TFT) including both active layer and gate insulator (GI). Herein, we developed an ALD sandwich structure, which deposits both semiconductor and GI by ALD. In contrast to the previous results using sputter In-Ga-Zn-O (IGZO), ALD sandwich structure IGZO TFT exhibited severe deterioration in its electrical performance when the Al2O3 was adopted for both buffer layer and GI application. Through measurement of hydrogen permeability of ALD insulators and secondary ion mass spectroscopy of each sandwich structure after annealing, we found a hydrogen accumulation effect between Al2O3 and ALD IGZO interface layer, which caused deterioration of electrical performance. In contrast, TFTs with ALD SiO2, which has proper hydrogen diffusivity, chosen as the buffer and GI had favorable electric properties of 28.17 cm2/ $\text{V}\cdot \text{s}$ , 0.20 V/dec, 0.96, and 0.12 V for the mobility, ${V}_{\text {th}}$ , subthreshold swing (SS), and hysteresis. In this regard, an optimized GI structure via the ALD SiO2 and Al2O3 in situ process based on excellent interface formation with the semiconductor and hydrogen barrier performance, respectively, was developed. This functional GI structure consisting of SiO2 and Al2O3 exhibited excellent TFT characteristics (27.52 cm2/ $\text{V}\cdot \text{s}$ , 0.24 V/dec, and 1.07 V for the mobility, SS, and ${V}_{\text {th}}$ , respectively) with improved stability even after hydrogen annealing, which was used to examine the resistance to external hydrogen, showing a threshold voltage shift of −0.15 V and a SS shift of 0.01 V/dec. [ABSTRACT FROM AUTHOR]