Metal-oxide thin-film transistors (TFTs) have been implanted for a display panel, but further mobility improvement is required for future applications. In this study, excellent performance was observed for top-gate coplanar binary SnO2 TFTs, with a high field-effect mobility (µFE) of 136 cm2/Vs, a large on-current/off-current (ION/IOFF) of 1.5 × 108, and steep subthreshold slopes of 108 mV/dec. Here, µFE represents the maximum among the top-gate TFTs made on an amorphous SiO2 substrate, with a maximum process temperature of ≤ 400◦C. In contrast to a bottom-gate device, a top-gate device is the standard structure for monolithic integrated circuits (ICs). Such a superb device integrity was achieved by using an ultra-thin SnO2 channel layer of 4.5 nm and an HfO2 gate dielectric with a 3 nm SiO2 interfacial layer between the SnO2 and HfO2 . The inserted SiO2 layer is crucial for decreasing the charged defect scattering in the HfO2 and HfO2 /SnO2 interfaces to increase the mobility. Such high µFE, large ION, and low IOFF top-gate SnO2 devices with a coplanar structure are important for display, dynamic random-access memory, and monolithic three-dimensional ICs.