Both direct and indirect band gap transitions are observed in Ge by photoluminescence and electroluminescence. The relative emission intensity of direct band gap transition with respect to indirect band gap transition increases with the increase of the n type doping level, optical pumping power, injection current density, temperature, and strain. The enhancement of direct band gap transition is due to the increase of electron population in the direct valley by reducing the difference between direct and indirect band gaps. The reduction of direct and indirect band gaps can be extracted from the emission spectra with direct and indirect transition models. The defects and the thickness dependent reabsorption are responsible for the relatively strong direct band gap transition in the Ge-on-Si sample as compared to bulk Ge. (C) 2011 Elsevier B.V. All rights reserved.