Our laboratory conducts research on core device technologies based on amorphous oxide semiconductors (AOS) for next-generation thin-film transistor (TFT) applications. Oxide semiconductors are advantageous materials for realizing ultra-low-power devices because they can be processed at low temperatures, exhibit excellent large-area uniformity, and offer extremely low off-current characteristics. To maximize the performance and reliability of oxide semiconductor transistors, we are conducting research on composition engineering, process optimization, and device structure improvement of AOS materials, including IGZO. In particular, we physically analyze how oxygen vacancies, hydrogen-related defects, and deep-level defects affect the electrical characteristics and long-term stability of devices. By controlling bias-stress-induced instability, we aim to develop high-performance and highly reliable next-generation oxide semiconductor devices.