Progress update on GaN-on-diamond microwave technology

In workshop ‘Diamond D-Day’ organized by the Centre for Device Thermography and Reliability (CDTR, part of the University of Bristol’s School of Physics), scientists from around the world heard progress on gallium nitride   GaN-on-diamond microwave technology.

It is reckoned that this next-generation technology will underpin future high-power radio frequency and microwave communications, space and defence systems, paving the way towards 5G and 6G mobile phone networks and much more comprehensive radar systems.

Bristol is working with a consortium of four other UK universities (Cardiff, Glasgow, Cambridge and Birmingham) as well as industry partners in the five-year (2017-2021) program GaN-DaME (Integrated GaN-Diamond Microwave Electronics: From Materials, Transistors to MMICs), which received a £4.3m grant from the UK Engineering and Physical Sciences Research Council (EPSRC) at the beginning of 2017.

Plenary speakers at the workshop included the USA’s Akash Systems Inc, the Universities of Bristol and Cardiff, the European Space Agency USA in The Netherlands/UK, Germany’s Fraunhofer Institute, Qorvo Inc and Raytheon in the USA, RHFIC Corp of SouthKorea/USA and Austria’s RHP-Technology GmbH.

“120 participants, with more than 50% from outside the UK as far away as the US, China, Japan and Australia, learned about the latest developments in this exciting new device technology for future 5G and security applications,” said Bristol’s professor Martin Kuball, the academic lead for the project.

The vision of GaN-DaME is to develop transformative GaN-on-diamond high-electron-mobility transistors (HEMTs) and monolithic microwave integrated circuits (MMICs) as the technology step beyond existing microwave devices (such as GaN-on-SiC), revolutionizing the thermal management that presently limits GaN electronics.

Energy flows in these can be as high as the heat flux on the surface of the sun, and diamond – due to its ultra-high thermal conductivity - is the only material that can handle them. These devices can allow the implementation of future communications networks and radar systems with capabilities beyond what is presently possible, it is expected.

“To enable our vision to become reality, we develop new diamond growth approaches that maximize diamond thermal conductivity close to the active GaN device area,” says Kuball. “In present GaN-on-diamond devices, a thin amorphous 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,” he adds. “Any reduction in this thermal resistance would be of huge benefit. This we achieved by using new crystalline seeding layers for the diamond. Novel diamond growth is combined with innovative micro-fluidics using phase-change materials, a dramatically more powerful approach than conventional micro-fluidics, to further aid heat extraction.”

“The outcome are devices with a spectacular >5x increase in RF power compared to the current state-of-the-art GaN-on-silicon carbide HEMTs, presently commercially available,” Kuball continues. “Alternatively, and equally valuably, a dramatic ‘step-change’ shrinkage in MMIC or power amplifier (PA) size is possible, delivering an increase in efficiency through the removal of combining networks as well as a reduction in power amplifier cost,” he adds.

“This represents a disruptive change in capability that will allow the realisation of new system architectures – for example, for radio-frequency 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,” Kuball concludes.