Absorption coeffcient is a physical parameter to describe electromagnetic energy absorption of materials,which is closely related to solar cells and photodetectors.We grow a series of positive-intrinsic-negative (PIN) structures on silicon wafer by a gas source molecule beam epitaxy system and the investigate the absorption coeffcient through the photovoltaic processes in detail.It is found that the absorption coefficient is enhanced by one order and can be tuned greatly through the thickness of the intrinsic layer in the PIN structure,which is also demonstrated by the 730-nm-wavelength laser irradiation.These results cannot be explained by the traditional absorption theory.We speculate that there could be some uncovered mechanism in this system,which will inspire us to understandthe absorption process further.