Over 15 TW of power is continually lost worldwide in the form of waste heat. Thermoelectric generators (TEGs) offer one method of reducing this waste, by harvesting the heat and using it to create electrical power. While the conversion efficiency of TEG devices is often <10%, the sheer abundance of waste heat, offering a free fuel source, makes TEGs appealing for many diverse applications. This proposal is aimed at thin-film TEGs (active thickness, 1-20 micrometres), forecast to be a core market sector in the future, with the advent of flexible/wearable electronics, and with the increased uptake of sensors, all of which require low-power. If TEGs can be produced at low-cost and with increased functionality (e.g. flexible), their potential is significant to act as a power source for future electronic devices that improve our quality of life. As an alternative to generators, the same thin-film technology can also be used in reverse for small-scale heating/cooling applications, with thin-film modules already used for chip-cooling in high-performance electronics (space, military and aerospace applications). Silicon-based technologies underpin the global electronics industry due to their many practical advantages. These same benefits would extend to TEGs were it not for the poor thermoelectric conversion performance of silicon. This project will undertake pioneering materials work in the area of "vacancy-rich silicon" - essentially silicon with many atoms removed at the atomic level - building on initial work carried-out by us, which has shown vacancy-rich silicon to be competitive with other state-of-the-art thermoelectric materials. The realisation of flexible thin-film TEGs based on vacancy-rich silicon will represent a transformative step applicable to numerous applications, including power generation and heating/cooling within clothing, as targeted specifically by us in co-operation with our industry partners.

Ubiquitous, high-performance communication is the backbone of our society, and promises to play an increasing role not only in individual's daily lives, but just as importantly in the background with communication among devices (e.g. vehicle-to-infrastructure for mobility, process control and monitoring in industrial and manufacturing, virtualization of full environments for the metaverse, among others). The resulting explosion in data that must be processed and communicated requires extraordinary bandwidth and network ubiquity, which in turn demands supporting electronics that is high performance, power efficient, and low cost. This EPSRC - NSF proposal targets a great leap forward in the most critical link, the wireless power amplifier, that is essential to realizing a vision of ubiquitous, high-speed, transparent mobile communication. Power amplifiers are among the most critical elements in any communication system as they dictates the overall efficiency of the system. GaN-based HEMTs are especially promising for high-performance power amplifiers, but current GaN-based systems suffer from limited frequency coverage, efficiency and linearity due to a combination of factors, including device design e.g. use of field plates effectively limits operation to 30 GHz and below, and materials issues e.g. deep level traps, self-heating means that gain and efficiency degrade rapidly both with output power as well as frequency. We leverage in this programme transformative advances in both GaN-based transistor design and novel circuit topologies to dramatically improve the efficiency, bandwidth, linearity, and cost of the key wireless elements of a communication system, through co-design. The technology is based on polarization-engineeered graded channel GaN HEMTs that show a substantial improvement in linearity in comparison to conventional HEMTs. By combining with thorough investigation of their underlying device physics including trap states and thermal management, we address major effects that degrade the performance of GaN at increasing frequencies (i.e. Ka band up to 40 GHz) by optimizing device design and fabrication. We will design harmonically terminated amplifiers based on our new class of contiguous modes, that allow designers wider choice of impedances for desired characteristics of efficiency, linearity and output power. The project brings together world leading experts in the Universities of Notre Dame, Bristol and Sheffield, working alongside supporting industry in UK and US, that completes the entire supply chain from substrate growers, device/chip fabrication to circuit designer in both countries. The targeted enabling millimetre-wave communication technology is expected to be the next frontier in emerging applications that play a critical role in the levelling up agenda to drive prosperity in all regions of the UK, the US and worldwide. For example 5G is expected to underpin new industries worth $13.2T in goods and services in the UK alone by 2035.

This project will result in methods to detect boreal recruitment failure (RF) due to fire, an explanatory model of RF, and quantification of climate feedbacks from RF that are not currently accounted for in any climate or vegetation model. The associated data collection and research outputs will benefit models of climate-fire-vegetation feedbacks. Presently all models that incorporate fire disturbance assume forest recovery. Research Questions 1. When and where does boreal RF occur? 2. What are the factors that cause boreal RF? 3. What climate feedbacks are likely to result from boreal RF? Forest loss due to the failure of new trees to survive (recruitment failure) post-fire occurs in boreal forests in Eurasia and North America. The existence of ecological thresholds, or "tipping points" that cause abrupt ecological shifts, is well-known in ecosystems theory but where and when ecosystems are approaching such dramatic changes is difficult to predict. One such extreme ecological shift has been observed in boreal forests that fail to recover after multiple fires within a short time interval (< 10 years). These areas are dominated by grass and are similar to steppe vegetation. Transition from forest to steppe is consistent with predicted changes in vegetation composition in response to regional climate change, and is consistent with global observations of forest loss in response to climate. Preliminary analyses of these sites indicate causes related to changing fire regimes effected by climate. Firstly, although vegetation indices have been used to identify forest loss, there is currently no method to detect RF using remotely sensed data. We address here the likelihood that RF produces a unique signature that can be detected remotely. The total area affected by RF in Eurasia and North America is at present unknown. Using RF locations provided by the Sukachev Institute (see letter of support), we have developed preliminary methods to differentiate between successfully recovery from fire and RF using remotely-sensed vegetation indices. The proposed research would refine these methods and develop an automated approach to detect RF. The lengthening satellite data record permits a new focus on the impact of climate change on boreal forests (the largest terrestrial biome) and its potential consequences. Remotely sensed imagery to date have yielded "snapshots" of ecosystems and disturbance events. With more than a decade of daily imagery from the MODIS sensors, we can begin to monitor processes like disturbance-recovery cycles. This new focus is critically important to the study of climate-ecosystem interactions and climatic "tipping points". Secondly, the causes of RF have not been identified. RF has been observed in areas of Siberia where the length of time between fire disturbances was extremely low. Initial field observations of RF sites indicate that high soil temperature and low moisture create a seedbed unsuitable for recruitment of trees following a fire. Additional field data will provide the inputs for an explanatory model of RF that includes characteristics of the fire (such as intensity and fire weather), pre-fire vegetation (e.g., stand age and density), and post-fire environment (e.g., soil temperature and moisture). Thirdly, the effect of RF on carbon, water and energy fluxes that impact climate has not been quantified. The replacement of forests with steppe vegetation results in carbon losses to the atmosphere from combustion and post-fire decomposition. The net climate impact of RF is presently unknown. Albedo is initially low following a fire and then may become higher due to the higher albedo of replacement vegetation. Changes in evapotranspiration rates affect latent and sensible heat fluxes. The area of RF is likely to grow in response to increasing fire frequency and severity, but the dynamics of recovery from wildfire and RF have not been incorporated into any coupled climate-vegetation models.

Global demand for high power microwave electronic devices that can deliver power densities well exceeding current technology is increasing. In particular Gallium Nitride (GaN) based high electron mobility transistors (HEMTs) are a key enabling technology for high-efficiency military and civilian microwave systems, and increasingly for power conditioning applications in the low carbon economy. This material and device system well exceeds the performance permitted by the existing Si LDMOS, GaAs PHEMT or HBT technologies. GaN-based HEMTs have reached RF power levels up to 40 W/mm, and at frequencies exceeding 300 GHz, i.e., a spectacular performance enabling disruptive changes for many system applications. However, transistor reliability is driven by electric field and channel temperature, so self-heating means in practice that reliable devices can only be operated up to RF power densities of 10 W/mm in contrast to the 40 W/mm hero data published in the literature. Considerable concern also exists in the UK and across Europe that access to state-of-the-art GaN microwave technology is limited by US ITAR (International Traffic in Arms Regulation) restrictions. The most advanced capabilities for all elements of GaN HEMT technology, using traditional SiC substrates, epitaxy and device processing currently reside in the US, with restricted access by UK industry. The vision of Integrated GaN-Diamond Microwave Electronics: From Materials, Transistors to MMICs (GaN-DaME) is to develop transformative GaN-on-Diamond HEMTs and MMICs, the technology step beyond GaN-on-SiC, which will revolutionize the thermal management which presently limits GaN electronics. Challenges occur in terms of how to integrate such dissimilar materials into a reliable device technology. The outcome will be devices with a >5x increase in RF power compared to GaN-on-SiC, or alternatively and equally valuably, a dramatic 'step-change' shrinkage in MMIC or PA size, and hence an increase in efficiency through the removal of lossy combining networks as well as a reduction in power amplifier (PA) cost. This represents a disruptive change in capability that will allow the realisation of new system architectures e.g. for RF seekers and medical applications, and enable the bandwidths needed to deliver 5G and beyond. Reduced requirements for cooling / increased reliability will result in major cost savings at the system level. To enable our vision to become reality, we will develop new diamond growth approaches that maximize diamond thermal conductivity close to the active GaN device area. In present GaN-on-Diamond devices a thin dielectric layer is required on the GaN surface to enable seeding and successful deposition of diamond onto the GaN. Unfortunately, most of the thermal barrier in these devices then exists at this GaN-dielectric-diamond interface, which has much poorer thermal conductivity than desired. Any reduction in this thermal resistance, either by removing the need for a dielectric seeding layer for diamond growth, or by optimizing the grain structure of the diamond near the seeding, would be of huge benefit. Novel diamond growth will be combined with innovative micro-fluidics using phase-change materials, a dramatically more powerful approach than conventional micro-fluidics, to further aid heat extraction. An undiscussed consequence of using diamond, its low dielectric constant, which poses challenges and opportunities for microwave design will be exploited. At the most basic level, the reliability of this technology is not known. For instance, at the materials level the diamond and GaN have very different coefficients of thermal expansion (CTE). Mechanically rigid interfaces will need to be developed including interdigitated GaN-diamond interfaces.

Future generation (5G) mobile phones and other portable devices will need to transfer data at a much higher rate than at present in order to accommodate an increase in the number of users, the employment of multi-band and multi-channel operation, the projected dramatic increase in wireless information exchange such as with high definition video and the large increase in connectivity where many devices will be connected to other devices (called "The Internet of Things"). This places big challenges on the performance of base stations in terms of fidelity of the signal and improved energy efficiency since energy usage could increase in line with the amount of data transfer. To meet the predicted massive increase in capacity there will be a reduced reliance on large coverage base-stations, with small-cell base-stations (operating at lower power levels) becoming much more common. In addition to the challenges mentioned above, small cells will demand a larger number of low cost systems. To meet these challenges this proposal aims to use electronic devices made from gallium nitride (GaN) which has the desirable property of being able to operate at very high frequencies (for high data transfer rates) and in a very efficient manner to reduce the projected energy usage. To maintain the high frequency capability of these devices, circuits will be integrated into a single circuit to reduce the slowing effects of stray inductances and capacitances. Additionally these integrated circuits will be manufactured on large area silicon substrates which will reduce the system unit cost significantly. The proposed high levels of integration using GaN devices as the basic building block and combining microwave and switching technologies have never been attempted before and requires a multi-disciplinary team with complementary specialist expertise. The proposed consortium brings together the leading UK groups with expertise in GaN crystal growth (Cambridge), device design and fabrication (Sheffield), high frequency circuit design and fabrication (Glasgow), variable power supply design (Manchester) and high frequency characterisation and power amplifier design (Cardiff). Before designing and developing the technology for fabricating the integrated systems to demonstrate the viability of the proposed solutions, a deep scientific understanding is required into how the quality of the GaN crystals on silicon substrates affect the operation of the devices. In summary, the powerful grouping within the project will bring together the expertise to design and produce the novel integrated circuits and systems to meet the demanding objectives of this research proposal.