In this article, the conceptual model of a hybrid energy converter consisting of a thermoradiative cell (TRC) and a thermionic cell (TIC) is established, where the TRC is made of p-type and n-type semiconductor materials of group III–V, and two metal-based electrodes, while the TIC is composed of an anode and a cathode (one electrode of the TRC). According to the semiconductor physics and the Planck’s radiation theory, the expressions for the overall power output density and energy conversion efficiency of the hybrid device are given. The output voltages of the two subcells are optimized to obtain the optimal power output density and efficiency. Furthermore, the optimum thicknesses of the TRC are determined to obtain the maximum power output density and efficiency of the hybrid energy converter. Making tradeoff between the power output density and efficiency, the parametric optimum regions are determined. This article provides a route to efficiently convert the medium-grade thermal energy into electricity.