Plasma‐enhanced atomic layer deposition (PEALD)‐based bilayer IZO (back channel)/IGZO top‐gate thin‐film transistors (TFTs) with different IZO and IGZO layer thicknesses are fabricated to evaluate the correlation between thickness and electrical characteristics/reliability caused by dual‐channel modulation. The dual‐channel formed by IZO stacked on the backchannel improves both mobility and reliability of devices as the IZO layer thickness increases. In the TCAD simulation, as the thickness of IZO increases, the current flowing through the IZO channel among the dual channels increases and the main channel transition from IGZO to IZO occurs above a certain IZO layer thickness. The main channel transition to IZO, which has high mobility and is located in the backchannel away from the gate insulator (GI), leads to a mobility increase with a lower threshold voltage (Vth) shift and a remarkable improvement of reliability deteriorated by the GI. As a result, PEALD‐based IZO/IGZO TG TFTs exhibit both high mobility (≈40 cm2 V−1 s−1) and high stability (ΔVth = ‐0.07 V) of a positive bias temperature stress up to 10 800 s. This suggests that ALD‐based dual‐channel regulation by nanoscale thickness control of the stacking oxide semiconductor can overcome the trade‐off between mobility and reliability.