We investigated a method of suppressing both negative bias temperature instability (NBTI) in PMOSFETs and initial characteristic degradation in NMOSFETs, using test structures constructed using several SiN films. It was found that the thickness and area of the SiN films play a crucial role in order to suppress both of these effects. In this work, two experiments, in which SiN films were deposited all over the wafer and the films were patterned, were carried out. In the former, the thickness of SiN films is strictly limited to a range from 6 nm to 7 nm in order to suppress both NBTI in PMOSFETs and initial characteristic degradation in NMOSFETs. In the latter, the thickness is not strictly limited. NBTI in PMOSFETs and initial characteristic degradation can be suppressed by constructing the SiN film area so that hydrogen reaches the gate oxide but water does not reach the gate oxide. The area of the SiN films can be easily determined by taking water diffusion into account.