This Phase-I SBIR proposal proposes for the first time ever, the use of a new class of materials - Gallium Nitride-on-diamond - in the manufacture of very high efficiency, solid-state Power Amplifier MMICs that will operate in X-band (8.4GHz), and Ka-band (26-38GHz). In this particular proposal, the first ever 80% efficient 8.4GHz and 34-38GHz GaN-on-Diamond FETs will be demonstrated, exhibiting a record 5-10 W/mm at record temperature levels. Arrays of these FETs will be used to form 60%+ efficient 150W-1KWatt Power Amplifiers (PA) MMICs in Phase-II. Polycrystalline free standing CVD diamond nature's most efficient thermal conductor enables nearly perfect heat extraction from a "hot" device (Thermal conductivities of GaAs, Si, and SiC are 35W/m/K, 150W/m/K and 390W/m/K respectively; diamond ranges from 1200-2000 W/m/K depending on quality). In the proposed scheme, the device's active epitaxial layers are removed from their original host substrate and transferred to a specially treated low-cost CVD diamond substrate using a proprietary low-cost manufacturable scheme. The active junction rests just 20-nm from diamond. The diamond technology proposed here may be applied to GaAs, SiC, SiGe, etc.