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.

The impact of this technology promises to be profound, from radars, to medical applications, to space and to communications. Therefore the project is supported by major industry partners such as Element-Six, MACOM, Airbus Defence & Space, IQE, European Space Agency and others. The UK has strong capabilities in electronic systems that rely on RF power ampliers (PAs); advances in underlying device performance beyond the increasingly dominant GaN-on-SiC technology will be disruptive. Companies such as Airbus Defence & Space use microwave PAs in satellite communication and space-based radar; compact and efficient subsystems are critical to their business. Similarly, military systems companies such as Finmeccanica, MBDA and Cobham require pre-matched, high-power microwave modules for phased array radar, RF seekers and electronic counter measures. Developers of hand-held medical devices would benefit from this new technology. Exploitation is also expected for mobile phone basestation applications (through project partners such as MACOM), where GaN-on-Diamond could reduce system complexity. Even a 10% share of the present GaN-on-SiC market (~$400M/year) would open opportunities for UK business, not even considering the high-value systems market. Until a few years ago, the focus of GaN development for electronic applications in the UK was based at QinetiQ. Their epitaxial GaN activities were acquired by project-partner IQE (UK). The UK also has the leading diamond supplier and project partner, Element-Six, who have recently established a £20M Diamond Innovation Centre at Harwell, Oxford. The UK has therefore an obvious, strong, industrial exploitation route. To ensure that the impacts of this Programme Grant are realised, we will integrate researchers from industry into our academic teams; this will allow key partners, especially Element-Six, IQE, MACOM and Airbus Defence & Space, to efficiently translate the knowledge and IP generated in this project into new business opportunities, including towards 100% uptake of the technology for 5G, and beyond, enhancing the GaN electronic device supply chain. We will provide discrete device demonstrators to UK industry to trial the technology, seconding our researchers into their R&D teams to support translation of this technology. We aim, by KE Fellows and through KTP, to embed outcomes of the project into a larger range of end users. We will pursue further opportunities via industry-funded collaborative PhDs. We will run workshops with NMI, the Industry Association for the UK microelectronics and defence industry, to deliver technical information for companies to implement the technology. Advice will be given to UK systems manufacturers on the opportunities for the implementation of RF GaN-on-Diamond power amplifier technology, improving their ability to act as intelligent customers, advancing the uptake of the technology, and improving their international competitiveness. This project will also train young researchers in device design and RF power amplifier operation for UK industry. This skill development of young researchers is essential to ensure the UK has the knowledge base needed in UK power, defence, space and aeronautics industry using semiconductor devices.