Ultra-thin ZnO thin-film transistors with a channel thickness of <10 nm have disadvantages of a high threshold voltage and a low carrier mobility due to a low carrier concentration. Although these issues can be addressed by utilizing the strong reducing power of tri-methyl-aluminum, a method is required to control parameters such as the threshold voltage. Therefore, we fabricated a ZnO/Al₂O₃ thin-film transistor with a thickness of 6 nm and adjusted the threshold voltage and carrier mobility through the modulation of carrier generation by varying the growth temperature of Al₂O₃. As the growth temperature of Al₂O₃ increased, oxygen vacancies generated at the hetero–oxide interface increased, supplying a free carrier into the channel and causing the threshold voltage to shift in the negative direction. The optimized device, a ZnO/Al₂O₃ thin-film transistor with a growth temperature of 140 °C, exhibited a μ_sat of 12.26 cm²/V∙s, V_th of 8.16 V, SS of 0.65 V/decade, and I_ON/OFF of 3.98 × 10⁶. X-ray photoelectron spectroscopy was performed to analyze the properties of ZnO/Al₂O₃ thin films.

We present a ZnSnO (ZTO) thin-film transistor (TFT) with enhanced mobility, achieved through the incorporation of a metal-capping layer. The fabrication of the ZTO active layer involves the deposition of a ZnO incubation layer, followed by super cycles of depositing ZnO and SnO2 layers through atomic layer deposition (ALD). An additional 60-nm-thick Al layer between the source and drain serves as a metal-capping layer and forms a conductive region. Rich in free electrons and located in the back channel away from the gate insulator, the conductive region leads to a primary ON-current path, thereby improving electrical characteristics and stability. Thus, the Al-capped ZTO TFT with a capping length of $200~\mu $ m exhibits decent performance with a saturation mobility of 16.89 cm2/V $\cdot $ s, a threshold voltage of 0.81 V, a subthreshold swing (SS) of 0.59 V/dec, and an ON/OFF current ratio over 107. Moreover, it exhibits a minimal threshold voltage shift of 0.9 V in the positive bias test and −0.14 V in the negative bias stress test. On ALD-based ZTO TFTs, these results demonstrate the applicability of the Al-capping method, which successfully overcomes the tradeoff between mobility improvement and bias stability.