The electrically active phosphorus profile resulting from segregation during thermal oxidation of silicon is important because it affects the electrical characteristics of submicron p-channel MOSFETS widely used in complementary metal oxide semiconductor circuits, for example. It is shown that the electrically active phosphorus density profile after thermally oxidation is an erfc which increases from the silicon surface to the bulk. How the true electrically active profile is extracted is described to establish the validity of this electrically active phosphorus density profile However, because phosphorus is more soluble in silicon than in SiO2, it piles up at the silicon surface. A model is proposed to explain the relation between the electrically active and pileup profiles.