Comparison of top-gate and bottom-gate amorphous InGaZnO thin-film transistors with the same SiO₂/a-InGaZnO/SiO₂ stack

We demonstrate top-gate and bottom-gate structures of amorphous indium-gallium-zinc-oxide thin-film transistors and compare their device operation. A replica material stack is fabricated for depth profile characterization to correlate with device results. We mainly focus on the oxygen content at the top and bottom. Key process factors that affect device reliability are determined based on material analysis, subgap density-of-states extraction by monochromatic photonic capacitance-voltage technique, and device simulations. We found that top-gate devices are influenced by higher deep acceptor-like states under positive gate bias-temperature stress, whereas the bottom-gate devices suffer reliability degradation under negative gate bias-temperature stress due to the decrease in oxygen content at the bottom interface.