Ultrahigh-speed photodetectors and printed-circuit antennas construct photonic transmitters. In this letter, we studied the saturation behaviors of an edge-coupled membrane photonic transmitter based on low-temperature-grown GaAs. The saturation behaviors determine the optimized operation condition of photonic transmitters. Ultrahigh external light-terahertz (THz) conversion efficiency of 0.11 % was achieved with 645-GHz radiation. According to our knowledge, this value is the highest reported external conversion efficiency of all photonic transmitters with radiation higher than 500 GHz. The high conversion efficiency and the edge-coupled structure of our devices release the power burden imposed on tunable semiconductor laser sources and imply their applications as compact all-solid-state THz radiation sources.