Integrated GaN-Diamond Microwave Electronics: From Materials, Transistors to MMICs

Global demand for high power microwave electronic devices that can deliver power densities well exceeding current technology is increasing. In particular Gallium Nitride (GaN) based high electron mobility transistors (HEMTs) are a key enabling technology for high-efficiency military and civilian microwave systems, and increasingly for power conditioning applications in the low carbon economy. This material and device system well exceeds the performance permitted by the existing Si LDMOS, GaAs PHEMT or HBT technologies. GaN-based HEMTs have reached RF power levels up to 40 W/mm, and at frequencies exceeding 300 GHz, i.e., a spectacular performance enabling disruptive changes for many system applications. However, transistor reliability is driven by electric field and channel temperature, so self-heating means in practice that reliable devices can only be operated up to RF power densities of 10 W/mm in contrast to the 40 W/mm hero data published in the literature. Considerable concern also exists in the UK and across Europe that access to state-of-the-art GaN microwave technology is limited by US ITAR (International Traffic in Arms Regulation) restrictions. The most advanced capabilities for all elements of GaN HEMT technology, using traditional SiC substrates, epitaxy and device processing currently reside in the US, with restricted access by UK industry. The vision of Integrated GaN-Diamond Microwave Electronics: From Materials, Transistors to MMICs (GaN-DaME) is to develop transformative GaN-on-Diamond HEMTs and MMICs, the technology step beyond GaN-on-SiC, which will revolutionize the thermal management which presently limits GaN electronics. Challenges occur in terms of how to integrate such dissimilar materials into a reliable device technology. The outcome will be devices with a >5x increase in RF power compared to GaN-on-SiC, or alternatively and equally valuably, a dramatic 'step-change' shrinkage in MMIC or PA size, and hence an increase in efficiency through the removal of lossy combining networks as well as a reduction in power amplifier (PA) cost. This represents a disruptive change in capability that will allow the realisation of new system architectures e.g. for RF seekers and medical applications, and enable the bandwidths needed to deliver 5G and beyond. Reduced requirements for cooling / increased reliability will result in major cost savings at the system level. To enable our vision to become reality, we will develop new diamond growth approaches that maximize diamond thermal conductivity close to the active GaN device area. In present GaN-on-Diamond devices a thin dielectric layer is required on the GaN surface to enable seeding and successful deposition of diamond onto the GaN. Unfortunately, most of the thermal barrier in these devices then exists at this GaN-dielectric-diamond interface, which has much poorer thermal conductivity than desired. Any reduction in this thermal resistance, either by removing the need for a dielectric seeding layer for diamond growth, or by optimizing the grain structure of the diamond near the seeding, would be of huge benefit. Novel diamond growth will be combined with innovative micro-fluidics using phase-change materials, a dramatically more powerful approach than conventional micro-fluidics, to further aid heat extraction. An undiscussed consequence of using diamond, its low dielectric constant, which poses challenges and opportunities for microwave design will be exploited. At the most basic level, the reliability of this technology is not known. For instance, at the materials level the diamond and GaN have very different coefficients of thermal expansion (CTE). Mechanically rigid interfaces will need to be developed including interdigitated GaN-diamond interfaces.