NOVELTY - A solar cell comprises a silicon substrate (100) having a P type impurity region and a semiconductor layer (120) having a N type impurity region, which is laminated on the silicon substrate. The semiconductor layer has a porous structure consisting of several holes formed on a surface of the structure.    USE - As a solar cell such as bulk type solar cell useful as a clean energy resources and as photo diodes to directly convert solar light into electrical energy, and as electronic device for electrical power supply.    ADVANTAGE - The solar cell has porous structure with uniformly low reflectivity in various wavelengths of solar light, thereby improving quality of a device in the solar cell and enhancing photoelectric conversion efficiency of the solar cell. The semiconductor layers are laminated by different forms of lamination and fabricated with different doping concentrations so as to adjust Fermi energy levels of separate semiconductor layers, leading to high charge accumulation between the semiconductor layers. The porous structure and passivation layer reduce surface defects while improving efficiency of solar cell. The solar cell has high efficiency solar cell with minimum surface defects, which comprises a passivation layer made of stable materials laminated on a surface of a silicon substrate. The solar cell can enable mass production of the solar cell with high quality and efficiency while sufficiently utilizing any conventional process for manufacturing a solar cell to decrease production cost, thereby having an economical advantage. Because of the porous surface structure, the semiconductor substrate has a considerably larger surface area than that of a conventional semiconductor substrate, and the solar cell can be fabricated by reduced processes for optimal impurity implantation, thereby improving the efficiency of solar cell. The passivation layer formed on the porous silicon wafer can greatly reduce surface defects and improve the efficiency of the solar cell.    DETAILED DESCRIPTION - A solar cell comprises a silicon substrate (100) having a first (e.g. P type) impurity region and a semiconductor layer (120) having a second (e.g. N type) impurity region distinguished from the first impurity region, which is laminated on the silicon substrate. The semiconductor layer has a porous structure consisting of several holes formed on a surface the structure. The semiconductor layer includes an upper electrode (140) positioned on a given portion of the surface. The porous structure of the semiconductor is formed on the remaining portion of the surface except the given portion on which the upper electrode is positioned. The silicon substrate having the first impurity region is provided with a lower electrode made of at least one selected from conductive metals on a bottom portion of the substrate. The solar cell further comprises another semiconductor layer having another first impurity region between the silicon substrate having the first impurity region and the semiconductor layer having the second impurity region. The first impurity region has higher concentration than that of the first impurity region in the silicon substrate. The semiconductor layer having the first impurity region has a thickness of 100-600 nm. The hole has a depth smaller, equal to or larger than the thickness of the semiconductor layer having the second impurity region. A porous structure of the semiconductor layer having the second impurity region has surface porosity of 10-70%. The semiconductor layer has the second impurity region includes a passivation layer (180) or an anti-reflective layer on a surface of the porous structure. The passivation layer passivates the silicon substrate. The constitutional ingredients of the passivation layer have a refractive index between those of air and the silicon substrate. An INDEPENDENT CLAIM is included for fabrication of a solar cell involving doping an upper surface of a silicon substrate having a first impurity region with a second impurity to form a semiconductor layer having a second impurity region; and forming a porous structure consisting of holes on a surface of the formed semiconductor layer having the second impurity region.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of a structure of the solar cell.    Silicon substrate (100)    Semiconductor layer (120)    Upper electrode (140)    P+ Semiconductor layer (160)    Passivation layer (180)