A monolithic 120 GHz free-space, continuous-wave source capable of providing 12.6 dBm of EIRP at 18% DC-to-EIRP conversion efficiency is presented. We utilize both the excellent high frequency performance of the InP based HEMT technology developed by HRL along with the newly developed surface wave assisted antenna concept. Because the antenna itself prefers to be fabricated on a high dielectric constant substrate, monolithic integration with MMIC circuits becomes straightforward