GaN power electronics, in particular, AlGaN/GaN high electron mobility transistors (HEMT) are currently being developed and starting to be applied for power conversion, radar, satellite and communication applications. Switched mode power systems based on this will deliver improved efficiency, hence forming a key enabling technology for the low carbon economy. Although performance of these devices is fully sufficient to enable disruptive changes for many system applications, reliability is presently still in question, not only in the UK and Europe, but also in the USA and Japan. This proposal aims at developing a new electrical methodology to study and understand reliability of GaN based HEMTs, in particular to identify the nature of electronic traps generated during the operation of GaN HEMTs, and which affect their lifetime. The programme is supported by key UK, European and US industries (International Rectifier UK, Fraunhofer Institute IAF Germany, UMS Germany, TriQuint USA), and builds on leading expertise in the field of GaN HEMT reliability developed at the Center for Device Thermography and Reliability (CDTR) in Bristol, established in various research programmes in Bristol funded by EPSRC and the US Office of Naval Research (ONR). The focus of this work will lie in overcoming the challenge that the highly accurate standard Capacitance-Voltage (CV) or Conductance technique for probing electronic traps in semiconductor devices cannot be performed on transistor structures relevant to real applications. This is because these techniques require large transistor structures to have enough capacitance to be measurable. Realistic devices have short gate length with consequently too low a capacitance to be accurately measured at the typical measurement frequency of 1kHz-1MHz, also any damage introduced into a device during device operation is typically in too small an area to be easily detectable using traditional techniques. In contrast, methods which can be applied to small III-V FET devices such as current-DLTS or transconductance dispersion respectively use a non-equilibrium pulse technique which is prone to misinterpretation, or have only given qualitative information to date. A key insight which underpins this proposal is that electronic traps in or near the channel primarily generate dispersion in a device below the pinch off voltage in the sub-threshold regime of operation which will be exploited in this programme. We will develop a dynamic transconductance method for GaN HEMT reliability analysis, suitable for small HEMT devices and insensitive to gate leakage currents. The development of this new electrical methodology which delivers the advantages of the quasi-equilibrium capacitance techniques but in small devices, will allow accurate measurements of degradation induced trap properties to be made for the first time. Noise measurements will complement this novel trap analysis, in additional we will benefit from the pulsed electrical-optical trapping analysis technique we developed in the ONR funded DRIFT programme. The work will advance the understanding of GaN HEMT device degradation during operation, i.e., device reliability, and will keep the UK at the forefront of internationally leading semiconductor device reliability research. The methodologies to be developed will also have direct applicability to the burgeoning worldwide effort in III-V CMOS technology for scaled low-power logic.

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.

AlGaN/GaN high electron mobility transistors (HEMT) are a key enabling technology for future high efficiency military and civilian microwave systems. The aim of this proposal is to provide transformative insight into the underlying physical processes that cause degradation in GaN RF power amplifiers (PA). This is of strategic importance for the UK given its strong RF electronics base, due to the fact that GaN RF power electronics delivers a disruptive step change in systems capability through power densities as high as 40W/mm and frequencies exceeding 300GHz. The UK has internationally leading academic research groups in this field, including Bristol and Cardiff. The key issue addressed in this proposal is that device degradation under RF stress is distinctly different than under DC stress, often resulting in a large increase in source resistance, something that never occurs under DC stress and is not explicable by conventional models. This observation implies that a device in RF operation applies voltage/current stresses, which are inaccessible under static conditions, making it imperative to understand the interaction between the RF operating mode and the degradation mechanism. Bristol has provided seminal contributions to the international effort to understand DC GaN transistor degradation, where an understanding is slowly emerging that includes oxygen related reactions and diffusion processes, and dislocation linked breakdown in GaN transistors. This includes electroluminescence imaging for detection of leakage pathways, dynamic transconductance and transient analysis to detect trapping states, and the simulation of the effect of pulsed operation on bulk and surface traps. Over the last 15 years, Cardiff has established a world leading capability in RF PA design and measurement. In particular waveform engineering systems enable RF current/voltage waveforms to not only be measured directly but also to be manipulated almost at will. This manipulation of the waveform has allowed Cardiff to make seminal contributions to the understanding of high efficiency RF PA operation. In this project, the unique capability to 'tune' RF operation into extremely well defined states to enable 'controlled' RF stressing will be used to gain the step change understanding of RF device degradation. Reverse engineering of failed devices, electrical and electro-optical measurement before/after and during the RF stress, combined with physical device simulation, will be used to determine the RF specific degradation mechanisms. This capability to predict, engineer and measure the RF waveforms is key to achieving an understanding of the RF stresses that devices undergo during PA operation, and then to determine and specify the safe-operating-area for HEMTs. This project utilises a partnership with state-of-the-art foundries in Germany and the USA, allowing the project to use production quality devices, essential for the relevance of the work. The project will be guided in terms of its relevance through guidance and interaction with Selex for systems level issues and IQE for the materials. The key synergy of Bristol and Cardiff will address a vitally important issue for the uptake of this disruptive technology, the identification of the RF degradation mechanisms. This will enable the impact of different modes of RF operation to be predicted, and a novel robust RF reliability test methodology to be developed, thus delivering large UK benefit and international impact.

The ever-increasing combined carbon footprint of information and communications technology (ICT) is unsustainable - more efficient devices must be developed. Thermal characterisation, which feeds into design optimisation, is one of the key steps for ensuring the efficiency and reliable operation of the new electronic devices being developed. However, accurately measuring the temperature of leading-edge electronic devices is becoming increasingly difficult or impossible because of their small size, and that is the challenge addressed in this proposal. Wide bandgap electronic devices including GaN have great proven potential for the next generation of sustainable ICT and power electronics, contributing to the needed carbon emissions reduction. Miniaturization is one of the routes to further increase the efficiency and performance of wide bandgap electronic devices, decreasing the active region size to <200 nm, similar to the technology pathway that silicon (Si) electronics has taken, using concepts such as the FinFET. Thermal management, which is the efficient extraction of waste heat from the active part of the device, is especially important for achieving efficient reliable nanoscale electronic devices; thermal resistance increases as they are "scaled" to nanometre dimensions because of a thermal conductivity reduction and heat confinement in 3-D device structures, e.g. in a fin shape. While self-heating can be mitigated reasonably easily for lower power density Si FinFETs, it is potentially a significant roadblock for "scaled" wide bandgap devices which operate at enormous power densities. However there is currently no thermal imaging technique with a sufficiently high spatial resolution (e.g. Raman thermography has a diffraction limited resolution of about 0.5 micrometer, >10x the hotspot size) to be able to accurately measure the hotspot temperature of these novel nanoscale wide bandgap electronic devices. Instead we currently rely on complex electrothermal models to estimate the temperature of nanoscale devices, with inherent uncertainties - measurement is needed. A step change is required, namely a sub diffraction limit (super resolution) thermal imaging technique, which is addressed by the Future thermal Imaging with Nanometre Enhanced Resolution (FINER) project. We will develop a transformative nano quantum dot based thermal imaging (nQTI) technique to deliver nanometre resolution thermal imaging for the first time. To demonstrate the newly developed technique our application focus is on scaled wide bandgap electronic devices supplied by our national and international partners, however this technique will be widely applicable. Quantum dots are ideal for this application: They can be deposited as a nm-thickness film on the surface of the device being tested, and the emission colour is temperature dependent, which is what we exploit for thermal imaging. Structured Illumination Microscopy (SIM) and Stimulated Emission Depletion (STED) super-resolution techniques which were originally developed for fluorescence microscopy, but are presently unsuitable for thermal imaging, will be exploited to achieve a resolution as small as 50nm for nQTI. nQTI will enable nano-scale electrothermal models to be developed and experimentally verified. Accurate models will further our understanding of nano-scale self-heating and heat diffusion, feeding back into improved device designs and novel thermal management solutions. This work will be done at the Centre for Device Thermography and Reliability (CDTR) which has an international reputation for being at the forefront of high spatial and temporal resolution thermal imaging, pioneering Raman thermography. This expertise makes the CDTR ideally placed to deliver this project successfully. The generous industrial support for this programme demonstrates that there is a great need for this and their belief in our ability to successfully deliver it.