The dependency of the inverter voltage gain on the current level (I-Level) of depletion mode (D-mode) thin-film transistors (TFTs) has been investigated with only n-type oxide semiconductor-based TFTs. It is clear that the voltage gain strongly depends on the D-mode I-Level. To investigate the dependency, photo stress was applied to the D-mode TFT to compare the inverter characteristics depending on the D-mode I-Level. As the photo stress time increased, the D-mode I-Level increased, and the voltage gains were degraded as a result. This was mainly because the I-Level of the D-mode is formed in the high section of the subthreshold slope (S.S) of the enhancement mode (E-mode) TFT when the photo stress was applied. By designing an inverter with a low D-mode I-Level, a high voltage gain of 9.85 was obtained at V-DD = 3 V. It is important to note that the S. S value of the E-mode and the I-Level of the D-mode should be optimized for high voltage gain for the application of next generation integrated circuits and highly sensitive photodetectors.