Recent Progress in GaN-on-Diamond Device Technology

The intrinsic properties of gallium nitride (GaN) make it an ideal semiconductor material for microwave/millimeter wave power amplifiers. Numerous groups have demonstrated AlGaN/GaN high electron mobility transistors (HEMTs) with power densities exceeding 40 W/mm [1]. Operation at their maximum potential is impractical due to the lack of a viable cooling solution. The majority of high power GaN RF devices are fabricated on semi-insulating silicon carbide (SiC) substrates. This has been shown to be a viable solution albeit even SiC with its superior thermal conductivity (~350 W/m-°K), cannot overcome the heat loads being generated by the GaN HEMTs. It has become readily apparent as GaN device technology matures that thermal impediments are limiting it from realization of its true capability. One strategy under consideration is to substitute the SiC substrate with a much higher thermal conductivity diamond substrate (~2000 W/m-°K) to enhance localized thermal management. In 2006 AFRL demonstrated the first working AlGaN/GaN HEMT on a GaN/DIA wafer fabricated by Group4 and Emcore [2]. This early technology demonstration provided a pathway for future exploration of producing GaN based devices on polycrystalline CVD diamond substrates. For the past two years, the DARPA Near Junction Thermal Transport (NJTT) Program has explored the development of passive cooling approaches through integration of high thermal conductivity diamond in close proximity to the active transistor junction. In this work, AFRL provides an assessment of the electrical and thermal performance of diamond integrated GaN devices fabricated under the NJTT Program.