We have analyzed the mechanisms and the efficiency of the 1.54 &mgr;m electroluminescence from Er-doped crystalline Si. Optical doping of a 0.25 &mgr;m deepp+−n+junction was achieved by multiple Er and O implants which realize a uniform concentration of 1019Er/cm3and 1020O/cm3from 0.2 to 0.9 &mgr;m from the surface. It has been found that, for the same current density passing through the device, the room temperature electroluminescence signal is 2–10 times higher under reverse bias at the diode breakdown than under forward bias. Detailed analyses of the spectrum line shape, temperature, and current density dependencies and modulation performances under both forward and reverse bias allowed us to elucidate the reasons for this difference. In forward bias, in spite of the large effective excitation cross section (>6×10−17cm2at 300 K), the efficiency of room temperature electroluminescence is limited by the small number of excitable sites (∼1&percent; of the total Er concentration) and by the efficiency of nonradiative de-excitation processes of the Er ions. Furthermore, since in forward bias Er ions are excited by electron–hole recombination at an Er related level in the Si band gap, the electroluminescence yield is also reduced by competitive carrier recombinations at the residual defects left over after diode processing. On the other hand, under reverse bias, Er ions are excited by hot carrier impact excitation in the thin (∼70 nm) depletion layer. In this case all of the Er atoms in the depletion region are excitable and nonradiative de-excitation processes, such as Auger de-excitation to free electrons, are inhibited. This allows one to achieve an internal quantum efficiency of 1.5×10−4at 300 K. Moreover, fast modulation of the diode can be achieved. At the diode turn-off, the excited Er ions are embedded in the heavily doped (∼1019/cm3) neutral regions of the diode where Auger-type de-excitation processes produce fast decay of the Er ions thus allowing to achieve modulation frequencies higher than 80 kHz. The major limitations to the achievement of a higher efficiency under reverse bias are the thin excitable region and the limited fraction of hot carriers having enough energy to impact excite the Er ions. ©1997 American Institute of Physics.