In this work, two configurations (planner and sandwich) structures of (Al/n PSi/n-Si/Al) porous silicon (PSi) gas sensor have been fabricated and tested extensively for CO 2 gas molecules. Two laser wavelength infrared and violet of (810 nm) and (405 nm) was used in the laser assisting etching process on the n-type silicon substrate. The gas detection characteristic of planner and sandwich configuration were studied under different condition. The resistance measurement, for the planner and J-V characteristic for sandwich structures, which was analyzed based on the nano-sized silicon, porosity, layer thickness, and the effective dielectric constant of the PSi layer. The SEM image of the PSi layer showed the formation pore-like structure with cylindrical and rectangular pore shape with different dimensions for infrared illuminated PSi and crossed pores-like structure with randomly distributed for violet illuminated PSi. The sensing mechanism for sandwich structure configuration is governed by the porous silicon parameters, while for planer configuration the silicon channel among the pore has a significant role in sensing process.