Bias temperature stress stabilities of thin-film transistors (TFTs) using In–Ga–Zn–O (IGZO) channels prepared by the atomic layer deposition process were investigated with varying channel thicknesses (10 and 6 nm). Even when the IGZO channel thickness was reduced to 6 nm, the device exhibited good characteristics with a high saturation mobility of 15.1 cm² V⁻¹ s⁻¹ and low sub-threshold swing of 0.12 V dec⁻¹. Excellent positive and negative bias stress stabilities were also obtained. When positive bias temperature stress (PBTS) stability was tested from 40 to 80 °C for 10⁴ s, the threshold voltages (V_TH) of the device using the 6 nm-thick IGZO channel shifted negatively, and the V_TH shifts increased from −0.5 to −6.9 V with the increasing temperature. Time-dependent PBTS instabilities could be explained by a stretched-exponential equation, representing a charge-trapping mechanism.