We have obtained room-temperature electroluminescence (EL) at ~1.54 µm from Er and O co-doped crystalline p-n Si diodes fabricated by ion implantation, under both forward and reverse bias conditions. Under forward bias, the EL intensity decreases by a factor of ~15 on going from 110 to 300 K, where a weak peak is still visible. In contrast, we report the first sharp luminescence peak obtained under reverse bias conditions in the breakdown regime. In this case the EL intensity decreases only by a factor of 4 on going from 110 to 300 K and the room-temperature yield is more than one order of magnitude higher than under forward bias. The data suggest that Er excitation occurs through electron-hole mediated processes under forward bias and through impact excitation under reverse bias.