Thermally Efficient GaN Devices for High Performance Microwave Amplifiers

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Over the last decade a significant effort has been made in the development of Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMTs). This has resulted in the demonstration of clear benefits of this technology with GaN based electronic devices offering up to 5 times higher power density than devices produced in Gallium Arsenide (GaAs). It has also been shown that this high power density can be traded for improved efficiency or increased total power compared to conventional GaAs based products. The higher power density, however, also means that waste heat energy per unit area in the devices is higher and without its efficient removal results in devices operating at high channel temperatures, which both degrades the device performance and reliability. Indeed, today's GaN devices are thermally limited and have to be significantly de-rated to limit channel temperatures to under 200 degC for improved RF performance and a significantly improved mean-time-to-failure (MTTF). Nonetheless, it is now also clear that GaN based RF transistors will be the microwave technology of choice for many future applications. Indeed, GaN amplifiers already offer unparalleled performance for base stations, industrial scientific & medical, and aerospace & defence applications, but improvement in thermal management is required to reap the benefits.Aims and objectives:This project seeks to develop thermally enhanced GaN-based HEMT RF electronic devices grown on the high thermal conductivity Silicon Carbide (SiC) substrates. It has the following objectives: 1) Design, fabrication and characterization small gate periphery GaN-based HEMT devices with integrated thermal pathways and with: a) a power density of over 20 W/mm and b) unity current gain and power gain current cut-off frequencies in excess of 50 GHz; and 2) Realization and characterisation of devices with large gate periphery capable of delivering 10 - 100 W RF power per device.Novelty of the research methodology:On this project, various thermal management strategies including the use of integrated thermal vias and distributed gate designs will be investigated. Preliminary experimental and simulation results indicate that significant improvements in device performance can be achieved. Device characterisation using load pull and S-parameter measurements will provide the feedback required for technology optimisation.Alignment to EPSRC's strategies and research areas:The research topic is aligned to EPSRC priority areas of RF and Microwave Devices, Non-CMOS Device Technology and Manufacturing the Future.Its potential applications and benefits:RF and microwave applications are becoming increasingly important as the world becomes more reliant upon wireless communications and radar systems. RF GaN transistors have the potential to be the RF enabling technology for advancing systems in these sectors; GaN devices with improved thermal management will further accelerate these developments. Depending on which market report you read, the RF and microwave components sector is estimated at $1 billion dollars, with 40% of devices purchased for communications base stations. Industrial and emerging commercial markets will use these transistors for example in high lumen lighting for industrial processes, architectural design, stage and stadia, industrial/domestic high-efficiency heating, as well as medical irradiation.