The electroluminescence of vertically emitting Ge-on-Si light emitting diodes (LEDs) was investigated for tensile strain ranging from 0% to 0.24% and for heavy n-type doping ranging from 5 x 10(17) cm(-3) to 1 x 10(20) cm(-3). The tensile strain increased the electroluminescence of a Ge-on-Si pin LED by a factor of 2. For high n-type doping concentrations a distinct bandgap narrowing was observed and the electroluminescence at an optimal concentration of 3 x 10(19) cm(-3) increased by a factor of 5.5 compared to an undoped Ge-on-Si pin LED. In a lateral design the electroluminescence spectrum of the 4 x 10(19) cm(-3) n-type doped LED shows features of high intensity and narrow linewidth. (C) 2013 Elsevier B. V. All rights reserved.