It has been experimentally observed that the incorporated fluorine will greatly improve the reliability of high-permittivity gate dielectric based transistors, but the role of fluorine passivation on leakage current change through gate is still a debated issue. The authors performed first-principles calculations to study the role of fluorine in oxygen vacancies in HfO2 and HfSiO4. They found that fluorine behaves completely differently in these two gate dielectrics. Fluorine can passivate the gap states of HfO2 completely but has no effect on the passivation of oxygen vacancies in HfSiO4, which is a possible explanation to the controversy surrounding varying leakage current.