Si light emitting diodes were realized by implanting boron ions into Czochralski-n-Si wafers. The Si surface was not textured. It is shown that the light emission comes from a large region of the n-wafer and not, as generally assumed, from the upper highly doped p-Si layer. Enhancing the implantation dose yields an optimized boron-doped region, which increases hole injection into the n-wafer. The result is an improved internal efficiency of about 1%. (c) 2006 The Electrochemical Society.