A high efficiency and wideband 300 GHz frequency doubler based on six Schottky diodes is presented in this paper. This balanced doubler features a compact and robust circuit on a 5-mu m-thick, 0.36-mm-wide, and 1-mm-long GaAs membrane, fabricated by LERMA-C2N Schottky process. The conversion efficiency is mainly better than 16% across the wide bandwidth of 266-336 GHz (3 dB fractional bandwidth of 24%) when pumping with 20-60 mW input power (P (in)) at the room temperature. A peak output power of 14.75 mW at 332 GHz with a 61.18 mW P (in), an excellent peak efficiency of 30.5% at 314 GHz with 43.86 mW P (in) and several frequency points with outstanding efficiency of higher than 25% are delivered. This doubler served as the second stage of the 600 GHz frequency multiplier chain is designed, fabricated, and measured. The performance of this 300 GHz doubler is highlighted comparing to the state-of-art terahertz frequency doublers.