Stress-induced interface trap generation (ΔNit) in the mid-channel region of standard n-channel MOSFETs with ultrathin plasma-nitrided SiO2 films (2.34 and 3.48 nm) were systematically studied under static and dynamic (both bipolar and unipolar) oxide field stresses over a wide frequency range from 1 to 10[7] Hz using a direct-current current-voltage measurement. At the bipolar stresses, ΔNit increases with the stress frequency significantly when the frequency is larger than ∼104 Hz while it saturates or decreases at the frequency larger than ∼107 Hz. The frequency dependence of the ΔNit enhancement can be attributed to a charge pumping current during the dynamic stress. Nitrogen incorporation increases not only ΔNit, but also the frequency dependence.