We show that efficient light emission and fast modulation at 1.54 μm can be achieved from Er-doped light emitting Si diodes. When incorporated within the depletion region of a p + -n + junction, Er ions are excited by hot carriers with a cross section of 6×10 -17 cm 2 and decay with a lifetime of 100 μsec. This allows to achieve an intense electroluminescence signal and to reach population inversion of the Er ions already at a current density of 30 A/cm 2 at room temperature. At the diode turn-off, very efficient non-radiative decay channels set in for the Er ions that, although excited in the depletion region, are now embedded within the heavily doped neutral region of the junction. Thus fast modulation of the diode can be achieved