The physical and electrical properties in metal-oxide-Si (MOS) capacitors with the gate electrodes deposited using poly-Si or amorphous-Si (a-Si) and with the gate oxide grown in O-2 or N2O have been investigated. The differences of a gate oxide grown in N2O with a conventional furnace and an oxide film annealed in N2O by a rapid thermal process (RTP) were also studied. The analysis of physical properties included the thickness variations of oxide films, the shrink ratios of gate electrode films, the nitrogen and hydrogen concentrations in oxides, and the SiO2/Si interfacial strain. The measurement of electrical properties in MOS capacitors included the interface trap density (Dit), the charge-to-breakdown, and the hot electron and radiation induced Delta Dit and flatband voltage shifts. To improve the electrical reliability of MOS devices with ultrathin gate oxides, an oxynitride should be introduced although some properties of it are slightly inferior to those of conventional oxides. The combination of a gate electrode deposited using a-Si and a gate oxide annealed in N2O using RTP is shown to exhibit excellent charge-to-breakdown performance and to reduce hot electron and radiation induced interface traps. This improvement can be explained using a mechanism based on the release of compressive stress in the oxide and the relaxation of SiO2/Si interfacial strain, which could be qualitatively analyzed using infrared spectra.