Monolithic integration of optical and electrical functions in crystalline Si would have a large impact in the field of chip interconnections and optical communications. This work demonstrates that the excellent electrical properties of c-Si and the engineering of its optical properties by ion implantation provides a promising approach to a Si-based optoelectronics. In particular it is shown that an efficient light source at 1.54 um can be obtained in Er-implanted crystalline Si. The efficiency of this process can largely enhanced by O-coimplantation which is fundamental in order to obtain the incorporation of a high concentration of Er ions and to maintain them excitable also at room temperature. The crucial issue of the energy transfer from the electronic system of silicon to the internal 4f shell of Er has been investigated by deep level transient spectroscopy measurements which allow us to determine those Er-related levels which are a gateway for this energy transfer. The modification in the deep level spectra due to the presence of O also provides a clue for understanding the role of this impurity in the process. The achievement of sharp and intense room temperature electroluminescence from Er-doped crystalline Si diodes under both forward and reverse bias is reported. The temperature and current density dependences of the electroluminescence yield provide interesting information on the excitation cross sections and on the efficiency of the process. Finally the possibility of high frequency operation using a silicon modulator integrated on the same chip is discussed.