  Er doped Si light emitting diodes may find important applications in the
generation and storage of quantum information. These diodes exhibit an emission
efficiency two orders of magnitude higher at reverse bias than forward bias due
to impact excitation. However, physics of impact excitation in these devices
remains largely unexplored. In this work, we fabricated an Er/O/B co-doped Si
light emitting diode which exhibits a strong electro-luminescence by the impact
excitation of electrons inelastically colliding the Er ions. An analytical
impact excitation theory was established to predict the electroluminescence
intensity and internal quantum efficiency which fit well with the experimental
data. From the fittings, we find that the excitable Er ions reach a record
concentration of 1.9 x 10^19 cm-3 and up to 45% of them are in excitation state
by impact excitation. This work has important implications for developing
efficient classical and quantum light sources based on rare earth elements.
