We report on the effect of Fabry-Pérot (FP) resonance on hot-carrier electroluminescence (EL) spectra and the optical power efficiencies of silicon (Si) avalanche-mode (AM) LEDs in the wavelength range from 500 nm to 950 nm. The LEDs, fabricated in a silicon-on-insulator photonics technology, consist of symmetric p-n junctions placed within a 0.21 µm thick Si micro-ring of varying width and radius. We show that the peak wavelength in the EL-spectra can be tuned within a range of 100 nm by varying the ring width from 0.16 µm to 0.30 µm, which is explained by FP resonance. The measured EL-spectra features relatively narrow bands (with a spectral width of ∼50 nm) with high intensities compared to conventional Si AMLEDs. By varying the ring radius and using a high doping level, we obtain a record high optical power efficiency of 3.2×10−5. Our work is a breakthrough in engineering the EL spectrum of Si, foreseen to benefit the performance of Si-integrated optical interconnects and sensors.