We fabricated asymmetric metal/Ge/metal diodes with p-type Ge-on-insulator (GOI) and bulk Ge substrates, from which direct band gap electroluminescence (EL) was observed at room temperature. The integrated EL intensity for p-GOI diodes is approximately 16 times that for bulk p-Ge diodes, owing to the structure of GOI that confines carriers in the very thin Ge layer, and consequently enhances both the electron density in the conduction band and the efficiency of radiative recombination. Micro-photoluminescence spectra of p-GOI show defect-related peaks in the range of 0.70-0.75 eV, of which the intensity and energy depend on the measurement position, which implies fluctuation of defect type and density in p-GOI. Those defects contribute to the low-energy part of the broad EL spectra in p-GOI diodes.