Fabrication of bottom-gate and top-gate transparent ZnO thin film transistors

Transparent zinc oxide thin film transistors (ZnO-TFTs) with bottom-gate and top-gate structures were constructed on 50 mm silica glass substrates. The ZnO films were deposited by RF magnetron sputtering and SiO₂ films served as the gate insulator layer. We found that the ZnO-TFTs with bottom-gate structure have better electrical performance than those with top-gate structure. The bottom-gate ZnO-TFTs operate as an n-channel enhancement mode, which have clear pinch off and saturation characteristics. The field effect mobility, threshold voltage, and the current on/off ratio were determined to be 18.4 cm²/(V·s), -0.5 V and 10⁴ respectively. Meanwhile, the top-gate ZnO-TFTs exhibit n-channel depletion mode operation and no saturation characteristics were detected. The electrical difference of the devices may be due to the different character of the interface between the channel and insulator layers. The two transistors types have high transparency in the visible light region.