An active frequency-doubler MMIC achieving an output frequency of 300 GHz and its monolithic integration with a 300 GHz resistive mixer is presented. The frequency-doubler provides a broadband source with an average output power of -9.5 dBm and better than 10 % conversion efficiency in the frequency range from 250 to 310 GHz. At 300 GHz, a non-saturated output power of -6.4 dBm is measured at an input power of 1 dBm. A 300 GHz down-conversion mixer MMIC, combining the frequency-doubler with a resistive mixer, achieves a conversion loss of 20 dB in the RF range from 246 to 300 GHz. Both MMICs are realized in a metamorphic HEMT technology with 50 nm gate-length.