We studied nitrogen incorporation in Al2O3 gate dielectrics by nitrogen plasma and examined the dependence of the electrical properties on the nitrogen incorporation. We found that the nitrogen concentration and profile in Al2O3 films thinner than 3 nm can be controlled by the substrate temperature and the plasma conditions. The electrical characterization showed that the plasma nitridation suppresses charges in Al2O3 films and prevents dopant penetration through the gate dielectric without increasing the leakage current or the interfacial trap density. We also demonstrated the improved performance of a metal-oxide-semiconductor field effect transistor by using a plasma nitrided Al2O3 gate dielectric. These results indicate that plasma nitridation is a promising method for improving the electrical properties of Al2O3 gate dielectrics.