High Performance Buffers for RF GaN Electronics

AlGaN/GaN high electron mobility transistors (HEMTs) are a key enabling technology for future power conditioning applications in the low carbon economy, and for high efficiency military and civilian microwave systems. GaN-on-Si is highly attractive as a low cost, medium performance technology platform which has been proved to be usable even up to the W-band. The main down-sides of Si are the low bandgap and hence resistive lossy substrate especially at modest elevated temperatures, the vulnerability of the Si to unintentional doping with gallium during epitaxy causing RF losses, and the somewhat restricted power handling resulting from the relatively low thermal conductivity of the Si compared to the 4" SiC growth substrates currently used. However the cost benefits are dramatic allowing 6" or even 8" high volume wafer processing. 6" GaN-on-Si epitaxy is already available driven by the emerging GaN-on-Si power switch market, however it is optimised for high voltage, switched-mode operation. Improved RF power amplifier (PA) efficiency using GaN-on-Si, which is the focus of this proposal, would reduce the transistor temperature rise, reduce the substrate losses and deliver a low-cost high-performance technology as it would reduce the transistor temperature rise and reduce the substrate losses. The advance that is required is an optimised RF specific GaN-on-Si transistor architecture, which requires detailed understanding of electronic traps introduced into the GaN buffer of these devices by iron, carbon and carbon/iron co-doping, which is presently lacking. The key aim of this proposal is to control and model the device capacitances and conductances using novel epitaxial design of the GaN buffer, as this is key to delivering improved efficiency, gain and linearity in RF amplifiers.

GaN-on-Si for RF is a game-changer, with low cost high RF power capability likely to be available shortly, and this is only just beginning to be recognised by industry. Aiding the implementation of GaN-on-Si for RF, which is the aim of this proposal, would therefore result in major economic benefit for the UK from this emerging GaN HEMT technology either directly through epitaxial wafer supply, or indirectly through its application in the microwave systems industries. The availability of high performance GaN is of direct benefit to M/A-COM UK who have device manufacture in the US but system design and manufacture in the UK. UK aeronautics, space & defence companies (Selex UK, Astrium, Airbus, MBDA, BAESYSTEMS etc) will similarly benefit from the dramatic fall in RF component costs that will follow. These companies need to understand and de-risk any new technology before it can be implemented cost-effectively. It is expected that GaN-on-Si technology will become the dominant solid-state PA technology in the future, but this has not yet happened essentially due to component cost, investment hurdles, and a lack of experience and confidence on the part of systems companies. GaN-on-Si for power switching electronics will sweep aside those investment hurdles and it has been estimated that at maturity GaN-on-Si for RF will cut the cost to a third that of current high-volume GaAs RF technology. This project uses direct links between the Universities of Bristol and Cardiff to two world leading enterprises with highly advanced GaN activities, giving a tangible benefit to those companies. IQE, based in Cardiff, is the world's largest specialist epitaxy supplier with more than 50% of the market, and has GaN-on-Si epitaxial development underway in both Cardiff and the USA at 100mm, 150mm and 200mm scale. M/A-COM is a leading supplier of high performance analogue RF, microwave, millimeterwave and photonic semiconductor products that enable next-generation internet and modern battlefield applications. They have established GaN-on-SiC and Si processes for RF, both in-house and outsourced. IQE and M/A-COM have put in place a strategic supply agreement for GaN epitaxial products.