Tuning the band-gap of photonic crystals has been a challenge because there are few parameters that can be varied to modify the band structure. In this paper we modeled the effect of well controlled complementary metal oxide semiconductor (CMOS) compatible processing techniques, viz. oxidation and oxide etching, on the band-gap and mid-gap frequency of a silicon photonic crystal (PC). By adding silicon dioxide as the third dielectric, oxide thickness serves as a new parameter that influences the band structure. In the case of a PC of Si atoms in air, if silicon is etched to reduce the original atom radius from 0.28a (a = lattice constant) to 0.1a, the band-gap and mid-gap normalized frequencies increase from 0.1 to 1.6 and from 0.28 to 0.49 respectively. During oxidation, silicon is consumed to form silicon dioxide. Thus, if the Si atom of 0.4a radius is oxidized until it reaches 0.1a, the band-gap and mid-gap vary in the range of (0.1-1.5) and (0.28-0.41) respectively. When the oxide is etched back, the band-gap can be tuned in the range of (0.1-1.6) for a new mid-gap frequency in the range of (0.41-0.49). Results are also presented for a triangular lattice of air holes in silicon.