Effects of nitrogen in pi polysilicon gates on boron penetration into a silicon substrate through the gate oxide are studied. Four factors that might be involved in how nitrogen in the polysilicon gate suppresses boron penetration were investigated : first, boron diffusivity in the interior of the nitrogen-doped polysilicon film; second, segregation of boron at the polysilicon/SiO2 interface; third, pileup of nitrogen at the polysilicon/SiO2 interface; and fourth, boron diffusivity in SiO2 decreased by incorporation of nitrogen into SiO2 from polysilicon. From several experimental results, it was confirmed that the decrease in the boron diffusivity in the interior of the polysilicon gate is responsible for the suppression of boron penetration.