Semiconductors have already revolutionised the world around us through the inventions of the transistor, diode lasers, integrated circuits and sensors. A new wave of semiconductor quantum technologies are finding commercial applications in ultra-secure communications, enhanced imaging, sensing, and possibly even computing and simulations. In all these cases the "quantum advantage" is theoretically proven and experimentally demonstrated, but there is a strong need for a scalable, practical and efficient source of quantum light. Naturally occurring point-like light sources called "colour centres" act as quantum light sources in wide bandgap semiconductors such as diamond, Silicon Carbide, Boron Nitride and Gallium Nitride (GaN) at room temperature. Commercially, GaN now dominates the market for solid state lighting, because it is an efficient and manufacturable material, leading to costs of less than one dollar per LED. However, Gallium-Nitride is also a promising material for quantum light sources as the colour centres within it emit from the ultra-violet to the near infra-red. This wide range means the emission overlaps with minima in the absorption curves of optical fibres (1310 and 1550 nm), transitions in the best atomic quantum memories (near 800nm) and low loss free-space communications in the blue. Furthermore, by engineering heterostructures within the semiconductor it is possible to electrically drive the emitter, rapidly switch the device and design efficiency-enhancing structures. This fellowship will apply manufacturing techniques widespread in the compound semiconductor manufacturing field, such as large area epitaxy and wafer scale processing, to deliver a bright and room temperature quantum light-emitting diode based on GaN. I will use laser lithography, standardised packaging and quality control to ensure the end device is produced in a manner that enables scale up to mass-production, with the full supply chain within the UK. Collaboration with the UK semiconductor industry for growth and packaging of devices, and use of processing facilities installed at Cardiff University, will foster two-way knowledge exchange between industry and academia. My experience of this type of collaboration at Imperial-Agilent and at Toshiba-Cambridge, makes me uniquely well-qualified to manage this interaction. By funding me to devote a significant amount of my time to research for the next 5 years this project will deliver high impact research and build a platform for future UK prosperity and technological know-how.

New higher temperature, high-voltage multilayer ceramic capacitors (MLCCs) are required to advance power electronics - an important technology in the energy transition to net zero CO2 emissions by 2050. Wide-bandgap semiconductor technologies for power electronic equipment already provide active components that can operate at 250-300C, allowing reductions in heatsink size and equipment weight. However due to the high switching speeds of wide-bandgap devices, passive and active components must be in close proximity, demanding high temperature operation of the (passive) capacitors. In addition to applications in renewable energy distribution, there are demands for higher temperature capacitors in transport electrification where electronic equipment needs to operate at high ambient temperatures. Unfortunately existing Class II capacitors, which are all based on the perovskite crystal structure, can only operate to 125-175 C. Global research into new higher temperature capacitor materials over the past decade has failed to produce any dielectric material suitable for mass market MLCCs, now manufactured using inexpensive nickel metal internal electrodes. The obstacle has been the presence of bismuth or lead oxide in the ceramic formulation. This would cause the dielectric materials and electrodes to degrade in the high temperature, chemically reducing atmospheres used to manufacture modern MLCCs. In a shift of research direction, we have recently obtained proof-of-principle that a new type of dielectric based on the tungsten bronze crystal structure offers uniformly high permittivity (>1300 +/- 15%) over the requisite -55 to 300 C temperature range. The material is based on strontium sodium niobate (SNN) co-doped with only 1-2.5 at.% calcium, yttrium and zirconium. Although promising, the dielectric properties fall short of the exceptional performance levels required of a next generation capacitor material. For example, dielectric losses (currently 4%) exceed industrial specifications (2.5%). Unlocking the true potential of the new tungsten bronze approach is severely hindered by a lack of knowledge as to underpinning mechanisms. For example, why low levels of dopants create extremely diffuse twin temperature-dependent dielectric anomalies. In preliminary work we have demonstrated that composition-structure-property relationships for existing temperature stable dielectrics based on titanate perovskites do not apply to this new type of high-temperature dielectric. We propose to unlock the true potential of tungsten bronzes by application of new scientific understanding to overcome existing limitations. We will discover how to raise permittivity and reduce dielectric losses in doped SNN ceramics across the challenging temperature range -55 to 300 C by studying how structure (crystal, nano, micro, defect) is modified by specific dopant systems, using a combination of: electron, neutron and synchrotron diffraction; atom column resolution electron microscopy; electrochemical impedance spectroscopy. First principles simulations will also assist us in interpreting experimental findings and developing structure-property models. From this framework of understanding, new compositions will be designed. Final materials selection criteria will include a range of other dielectric parameters, including dielectric breakdown strength and energy storage density. Our capacitor industry partner KEMET will help evaluate materials and conduct highly accelerated lifetime testing. The best material will be demonstrated within the project in a wide-bandgap switching cell with integrated high-voltage DC-link MLCCs. Alongside direct engagement with established company collaborations, wider benefits will be maximized by developing new activities with industry. This will be achieved in part using the resources of the University of Leeds's Research and Innovation Service, and the new Innovation and Enterprise Centre, Nexus.

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.

As smartphones become the dominant mechanism for information transfer and processing in modern society, our expectations on what we hope to achieve with them also increases proportionally. In particular, the smart phone has become our portal to the internet, replaced our television, radio and music devices, and also serves as our credit card and personal guide (GPS). We also expect our mobile phones to work seamlessly as we travel across international borders. All of this is enabled by the separation of the various functions into different wireless (RF) frequency bands, and the development of sophisticated analog and digital circuitry, that enables the phone to simultaneously carry out these communications. As we move towards 5G and other technologies that increase the data throughput available, these channels must increase. While on the digital signal processing side, the steady advance of Moore's law and microelectronic integration has enabled silicon technology to keep up with the demand, this is not the case for the RF front-end circuitry, which is primarily analog. The RF front-end circuit, receives the signal from the antenna and separates it into different channels (based on RF filters), amplifies it with a low noise amplifier (LNA) and then hands it over to the DSP for baseband signal processing. Currently, RF filters and LNAs are primarily discrete devices that are co-packaged together. While this hybrid approach has certain advantages (mainly the choice of piezoelectric materials for the filter), as demand for filters continuously rises, it is known that a co-packaging approach will not scale. The main reason is that the available smartphone footprint (in terms of chip area) for the RF front-end has remained roughly the same across generations, while the filtering demand has continuously increased. As the microelectronics industry has repeatedly taught us, monolithic integration is the only long-term solution to address these problems. In this project, we will demonstrate that gallium nitride (GaN) is the ideal platform for achieving monolithic integration by exploiting a key advantage that GaN provides over traditional solutions: acoustic waveguiding. GaN allows us to guide high-frequency sound on the surface of chip with low acoustic attenuation. By routing sound in nanoscale waveguides and localising it in micron-scale resonators, one can re-design RF system components from the ground up realizing a massive reduction in component footprint, which is key to enabling monolithic integration. By applying ideas from integrated photonics to high-frequency acoustics, we hope to realize for RF systems the same benefits (in terms of size, weight and performance) that silicon photonics has achieved for optical telecommunication systems. We will show that high quality RF passive devices (in particular, piezoelectric resonators and filters) can be built on the same GaN substrate as the active transistor devices. We will implement a process flow and design the associated process development kit to implement these ideas in commercial GaN RF foundries (for ex: the Newport wafer fab) in collaboration with our project partners.

Wireless communication systems require the translation of an information-bearing signal at higher frequencies (such as radio- or mm-wave frequencies) to allow propagation through the wireless medium (the channel). This translation is typically performed in transmitters and receivers that along with the channel form the communication system. In the transmitter, a power amplifier (PA) is used to boost the power of the signal to a level sufficient to overcome the channel's attenuation and arrive with sufficient signal strength at the receiver. Today's best PAs are capable of 60-70% efficiency when used at their maximum output power. This means that about 1/3 of the power is wasted into heat only for the purpose of amplifying at such higher frequencies. Efficiency decreases when reduced output powers are required. Modern communications standards such as 5G generate signals which present a large power variation over time (this is also described by the peak-to-average power ratio or PAPR) and this causes the PA to operate even more inefficiently with values down to 10-20%, instead of the aforementioned 60-70%. Wasting almost 90% of the DC power into heat causes additional demands on the energy supply network which may lead to an increase in carbon emissions. Higher DC power dissipations result in reduced transmitter performance (e.g. less output power and so less coverage), reduced battery lifetime, in additional weight, cost, and size because of the heatsinks and necessary cooling hardware. Heat dissipation causes the electronics within the PA to operate at higher temperatures which are known to degrade the component's reliability (ageing) and change their electrical behaviour. The goal of this project is to radically improve the RF PA efficiency by using a technique called supply modulation (SM). Unlike the 1952's envelope-tracking (ET) method, SM uses a very high-efficiency modulator to generate a number of voltage levels (Vmin, ..., Vmax) that are applied to the drain of the PA. When the RF output power in the PA is high, the PA is supplied with the maximum voltage level and so it operates at maximum efficiency. Vice-versa, when the PA output power is low, a lower voltage level is supplied to the PA drain. This change in the supply results in an efficiency improvement usually in the range of 20-30% (and so in a SM-PA efficiency of 30-50%), but most importantly, it typically reduces the DC power consumption by ~50% for the same output power. Achieving wider and wider bandwidths for high link capacities requires this SM-PA to commutate very rapidly as a consequence of a wideband signal. The current state-of-art bandwidth is ~100MHz for the SM-PA. Achieving 1GHz bandwidth, as required in multi-band and mm-wave PAs, is thus the target of this project. To achieve this, new circuit topologies combined with high figure-of-merit semiconductor technologies will be explored, with the unavoidable hardware imperfections compensated through signal processing techniques such as digital pre-distortion (DPD). The SMPA specifications and top-level design parameters will be agreed between the University of Bristol (UoB)'s team and the project's partners to ensure relevance for industrial applications. This SM-PA is firstly simulated in the SM part, then in the PA, and then co-simulated together as a complete sub-system. The fabricated prototype is then characterized in terms of linearity, efficiency, and power with the latest communication standards. The SM circuit can also be combined with existing PAs as an 'efficiency upgrade'. Results of this theoretical and experimental activity are presented at conferences and published in journals by the UoB team. Public engagement and industry impact is also ensured by the presence of an advisory board. In summary, this project is an adventurous research programme that will re-define next-generation RF transmitters amplifiers and so contribute to UK's leadership in wireless technologies.