Novel High Thermal Conductivity Substrates for GaN Electronics: Thermal Innovation

AlGaN/GaN high electron mobility transistors (HEMT) are a key enabling technology for future power conditioning applications in the low carbon economy, and for high efficiency military and civilian, microwave and RF systems. Although the performance of AlGaN/GaN HEMTs presently reaches RF powers up to 40W/mm, at frequencies exceeding 300 GHz, their long-term reliability, often thermally limited, is still a serious issue, in the UK & Europe, but also in the USA & Japan. Corresponding challenges exist for power conditioning applications. To mitigate the present thermal device challenges, the aim of this proposal is innovation and step change in thermal management of AlGaN/GaN HEMT devices by developing novel substrates, in particular (1) high value substrates that have higher heat extraction capability than high cost SiC substrates commonly used for GaN RF applications, and (2) low cost substrates that have improved heat extraction capability to GaN-on-Si substrates for more cost sensitive power electronics markets. The resulting step-change in improvement in heat spreading will improve reliability, circuit efficiency and ease system constraints of GaN electronics. To enable the optimization of the thermal substrate properties key enabling new thermal analysis technologies will be developed. The UK has roadmaps for employing RF and microwave GaN electronics in defence as well as satellite communication. The key UK industrial players in this field include Selex, MBDA, Astrium & others, all requiring reliable and efficient GaN RF and microwave electronics, which the proposed work will advance and enable via the new heat extracting substrate technologies and improved methods of thermal characterisation, furthermore with opportunities for IQE UK, supporter of this proposal, of being a key component in the supply chain for RF GaN applications. The corresponding roadmap for power electronics requires cost-effective GaN presently on Si substrates power devices with UK based manufacture at NXP, supporter of this project, and International Rectifier (IR) which the outcome of this proposed work can innovate. Further business opportunities will emerge with the substrate development itself, such as via Element-6, at IQE through the developments of III-Nitride epitaxial growth for best heat extraction, or spin-out companies. Dissemination of results and insights from this project will be via publications in internationally leading journals, via conferences, via the UK Nitrides Consortium, i.e., established dissemination routes will be used to transfer knowledge into academia, and directly with the industrial supporters of this project, as well as other companies Bristol and Bath have links to (e.g. Selex, MBDA). The CDTR in Bristol and the III-Nitride group in Bath have both a strong track record in being successful using these dissemination routes, in particular with companies. The field of thermal management of semiconductor devices is an important academic research field, and is especially topical and useful at the current stage of implementation of this genuinely disruptive technology. It not only trains UK workforce for industry, but also it is essential to help maintain the present high level of device physics and engineering in the UK. It provides stimulus for an efficient interaction between universities and industry to maximize benefit of EPSRC research investment. This includes in this project interaction with UK industry, in particular, IQE, NXP, and Plessey in this project.

This proposal is focused on a transformative development in improved heat-sinking substrates for GaN RF and for GaN power electronics, enabling higher powers and improved reliability. It is now clear that GaN technology will replace GaAs in most microwave applications, and increasingly likely that it will displace Si in many power electronic systems over the next few years. GaN based devices will tend to have a smaller footprint and can operate at higher current densities. This places a more stringent requirement on heat extraction from GaN RF and power devices. With better heat extraction, RF and power systems become more energy efficient (for example >70% of the electrical power input of the mobile telephone base station is wasted as heat). Therefore, the proposed heat extracting substrate technologies will be genuinely disruptive and will provide UK industry and academia with a new underpinning capability in device performance that will enable further innovation in systems to bring even greater advances in efficiency. This will help the UK to maintain and advance its capability in key GaN technologies like high power amplifiers (HPAs) in terms of power level, power density, power-added efficiency (PAE), bandwidth and robustness. Similarly for the power switching market, improved heat extraction will help GaN switches to deliver up to the predicted 10x improvement in key figures of merit such as specific on-resistance compared to Si, giving higher efficiency, smaller size or higher operating frequency for application in a wide range of power conversion products. Companies around the world are gearing up to exploit the opportunity to support the low carbon economy which flows from the adoption of this exciting new technology. However, device reliability, often thermally limited, is still a major issue of concern. GaN HEMT thermal management therefore needs to be addressed to achieve full uptake of this new technology, and gain the associated UK economic benefit, which therefore is the key aim of this proposal. The CDTR in Bristol and the III-Nitrides Group in Bath have been one of only a few UK academic groups involved in major European projects aimed at establishing an independent supply chain for GaN microwave technology (KORRIGAN, MANGA and GREAT2 funded by European Defence Agency and European Space Agency, MORGaN funded by FP7). All those contributions have arisen due to the demonstrated benefit to industry over the last decade of the techniques developed in Bristol for thermal/strain measurement and reliability assessment, in Bath for novelty in growth and processing of III-nitride materials. While achieving reliable GaN HEMT technology is naturally an industry focused activity, these successes show that there exist significant opportunities for UK academia to contribute to obtaining high performance and reliable devices, in terms of understanding and optimizing thermal device limitations as well as growth innovation. This approach is embodied in this proposal and will not only directly benefit the UK industrial partners of this project with whom regular meetings will be established to achieve direct industrial impact, but also other UK industries Bristol and Bath have links with (e.g. Selex, MBDA). Furthermore this project will train young researchers in device thermal management, and III-nitride growth and processing. The expertise gained by young researchers in this programme is an essential key knowledge base needed in UK power, defence and aeronautics industry.