In a MIS (Metal/Insulator/Semiconductor) structure consisting of two terminals, a systematic analysis of the electrical charge transport mechanism through an insulator is essential for advanced electronic application devices such as next-generation memories based on resistance differences. Herein, we have verified the charge transfer phenomenon in MIOS (Metal/Insulator/Oxide Semiconductor) diodes through a defect engineering of the insulator. By selectively generating the oxygen vacancies in the insulator (Al2O3), the MIOS diode rectification of the P++-Si anode/Al2O3/IGZO cathode reached 107 at 1.8 V and considerably suppressed the leakage current. Studying the current-voltage characteristics of MIOS diodes shows that the charge carrier transport mechanism can vary depending on the defect density as well as the difference between the CBM (conduction band minimum) of the semiconductor and the oxygen vacancy energy level of the insulator.