Sub-micron 3-D Electric Field Mapping in GaN Electronic Devices

AlGaN/GaN high electron mobility transistors (HEMTs) are a transformative technology for high-power density radio frequency applications, including radar, satellite and mobile communications. In addition, efficient power conversion systems based on GaN devices are a key enabling technology for the low carbon economy, including renewable energy generation and transport electrification. However, their full potential has not yet been realised because performance is de-rated to ensure stable long-term device operation. Experimental characterisation of the electric field distribution in these devices has been lacking, despite being identified as a primary driver of degradation phenomena including breakdown, charge trapping and self-heating. These processes occur in and around the device channel and particularly the sub-micron region under the gate and field plate where peak electric fields are located. The aim of this proposal is a step-change in electric field imaging of semiconductor devices, by developing an optical three dimensional (3-D) device analysis technique with nanometre-scale spatial resolution. The primary focus will be on electric field induced second harmonic generation (EFISHG) combined with solid immersion lenses (SILs). This will enable us to investigate key performance and reliability challenges including (i) the effect of buffer doping on the dynamic distribution of charge in the device layers which causes an undesirable memory effect, (ii) optimization of field plate geometry to manage peak electric fields, (iii) comparing electric field distributions during RF and DC operation to improve reliability forecasts. These are on the critical pathway to achieving a high performance reliable GaN HEMT device technology which exploits the full benefits of the material properties of GaN.

The performance advantages of GaN microwave high power amplifiers are proven, exceeding the power density and power added efficiency of the existing Si LDMOS, GaAs PHEMT or heterojunction bipolar transistor (HBT) technologies. The compactness and high power density afforded by GaN high power amplifiers (HPAs) is unrivalled, which has attracted large development funding from the performance driven space and military applications sector. GaN HPAs are now entering larger civilian commercial markets such as radar (aerospace) and communications. This technology is also an enabler for emerging applications such as vehicle automation (radar) and next generation communications (5G cellular). The UK has roadmaps in employing RF and microwave GaN electronics in defence as well as satellite communication, with the key industrial players in this field such as MACOM, Selex, MBDA, Astrium & others relying on reliable GaN RF and microwave electronics. GaN RF electronics is of strategic UK defence interest for phased array radars in next generation military systems. Although the technological advantages of GaN electronics have been proven, the full potential of GaN has not yet been reached. Manufacturers have to prove reliability through extensive testing. Operating voltage / power is de-rated in most applications to ensure long-term reliability, mitigating highly localised electric field driven effects which degrade reliability and performance, including breakdown, charge trapping, and self-heating. Improving the design of epitaxy and device structure will unlock the full potential of GaN. This requires a combination of device simulation and detailed quantitative measurements, which this project addresses. The methodologies developed in this project on 3-D imaging of electric field distribution in GaN HEMTs will not only be of benefit to UK RF device industries directly supporting this project such as MACOM, but will also directly benefit the UK AlGaN/GaN device industry in general, with its strong GaN electronics and opto-electronics base (e.g., Astrium, RFMD, MBDA, Nexperia). MACOM manufactures GaN electronics in the US, but has a research and development centre in the UK. MACOM expertise in RF design and test expertise will benefit this project, equally knowledge we gain on potential device design improvements is being fed back to MACOM for potential inclusion into their future device designs. Correspondingly Qorvo and UMS will use knowledge we learn on their devices in terms of electric field distribution and potential re-design benefits. We will actively engage with the Cardiff CSC and IQE as the knowledge we learn in this project would be highly beneficial for UK production of GaN electronics devices.

The expertise gained by young researchers in this programme is an essential key knowledge base needed in UK research and industry due to its strong manufacturing focus on power, defence and aeronautics applications. We are actively involved in Automated RF & Microwave Measurement Society (ARMMS) and the UK Nitride Consortium (UKNC) as the primary UK industry/academic forums in the microwave RF and GaN materials communities, respectively, providing further dissemination of the results. Public engagement via events at the Dana Centre/Science Museum, the annual National Science and Engineering Week, the annual British Science Festival and the Royal Society Summer Exhibition will be part of this programme. We also aim to work directly with schools to inspire the next generation of engineers and scientists. We will engage with Soapbox Science, a grass-route organization, to help to draw more female scientists into science and engineering.