Thermal nitridation of a thin SiO2 film in purified NH3 gas at elevated temperatures produces a strong protective layer against impurity diffusion. This enables fabrication of a reliable MOS structure with a high work function gate material such as p+ polysilicon. This combination is advantageous for miniaturized MOSFETs because a reasonable threshold voltage is obtainable without a high dose of channel ion implantation which deteriorates carrier mobilities and interfacial properties. Experimental results indicate easy incorporation of a nitrified SiO2 film in the conventional MOS process.