Abstract Metal‐oxide thin‐film transistors (TFTs) have garnered much attention because of their advantages such as high transparency, low leakage current, and low processing temperature. However, there is a need to continuously improve their mobility and bias stability for application to next‐generation advanced electronics. In this study, the thickness of bilayer semiconductors is finely controlled to enhance the charge transport characteristics and bias stability in solution‐processed heterojunction oxide TFTs. The thicknesses of the top and bottom layers in the bilayer are individually adjusted by controlling solution molarity. The introduction of a bilayer channel improved the electrical performance of oxide TFTs via effective charge transport. However, trap‐limited conduction becomes dominant in the bilayer with an excessively thick top layer, thereby leading to a significant reduction in mobility and positive bias stability. Meanwhile, although increasing the bottom layer thickness contributes to improved mobility and reliability, it causes a serious negative shift in threshold voltage (V TH ). TFTs with an optimized bilayer structure show high mobility at a V TH close to 0 V and have particularly excellent positive bias stress stability. This study on bilayer channel thickness will be beneficial for developing advanced transistors with optimized bilayer or multilayer channels.