We identified that the instability (low reliability) in the electrical conductivity characteristics of oxide film transistors lies in the instability of oxygen vacancies, and found that this can be controlled by adding elements with high oxygen bond dissociation energy. Generally, material selection is made with an emphasis on ease of thin film fabrication and mobility, but this results in low reliability. Using indium oxide films, which inherently have high mobility, as a substrate, we significantly improved device reliability by adjusting the added elements and their amounts using sputter deposition. With ALD deposition, which enables low-temperature processes, we were able to control and retain carbon, which has a high oxygen bond dissociation energy, making it possible to create highly reliable transistors even with ultra-thin films of about 5 nm thickness. Furthermore, we use our own ALD precursor and have made unique improvements to the deposition structure. Reducing device resistance through electrode metal selection based on current injection mechanism analysis is also useful for improving reliability.