Analogue-to-digital converters (ADCs) are the essential links between physical world in which all signals are 'analogue' (e.g., electric current generated by a microphone or a picture captured by a mobile phone camera) and the digital world of '0s' and '1s', where we store, transmit and process signals and information. ADCs enable (digital) computers to process signals from the (analogue) physical world. This capability has revolutionised our entire society, making computers (desk-tops, lap-tops, or smartphones) ubiquitous. In recent years, we have witnessed a dramatic increase of the amount of information that is generated, stored, transmitted, and processed, driven by increased demand of our society on data and information and newly emerging applications such as virtual and augmented reality. All this information needs to be processed by ADCs, which can address the abovementioned need only when performing with better accuracy, affordable power consumption, in real-time (with low latency), and for increasingly broader bandwidth (faster) signals. This is extremely challenging with currently-existing technologies and is being vigorously pursued by both academia and industry. Most of these approaches are based on strategies like the use of application-specific integrated circuits (ASICs), photonic time stretch, or time interleaving. Unfortunately, all of these approaches seem to have formidable challenges. A clearly realisable route to next-generation ADCs that could support information growth in the next decade and beyond is currently lacking. ORBITS aims to provide a radically novel and future-growth-proof solution to ADCs using optical assisted means. Specifically, it will exploit unique features of recently-emerged optical and photonics technologies, including optical frequency combs, coherent optical processing, and precise optical phase control. Optics offers three orders of magnitude larger bandwidth than microwave electronics used for ADCs today and has the advantages of ultrafast (femtosecond level) responses. The optical frequency comb technologies, in conjunction with coherent optical processing and phase control, enables dividing signal with high accuracy in the optical domain, which overcomes the fundamental limits such as timing jitter (time uncertainty) in conventional approaches, opening up a scalable and integratable technology for large bandwidth high resolution ADCs. For practical (low-cost when volume-manufactured, compact, and low-power-consuming) implementation, ORBITS will investigate optical and electronic integration, which permit to harness merits across different photonics integration platforms, through collaborations and open foundries. Besides next-generation ADCs, ORBITS will study applications in future-proof high capacity optical and wireless communications. It assembles complementary expertise from top research groups in Universities and companies, aiming for a wide academic impact and a straightforward knowledge transfer to industry.

AlGaN/GaN high electron mobility transistors (HEMT) are a key technology currently envisioned for future radar, satellite and communication applications, ranging from civilian to military use. Although the performance of AlGaN/GaN HEMTs presently reaches power levels up to 40W/mm at frequencies as high as 2-10 GHz, i.e., a spectacular performance enabling disruptive changes for many system applications, long-term reliability of AlGaN/GaN HEMTs is still a serious issue, not only in the UK and Europe, but also in the USA and Japan. There are several key factors affecting AlGaN/GaN HEMT reliability resulting in a variety of different failure mechanisms, including trap generation, metal migration and others. These are accelerated by: (i) device temperature, (ii) local stresses / strains (converse piezo-electric and thermal), (iii) high electric fields. Knowledge of these parameters is essential for reliability testing, in particular, for accelerated lifetime testing to predict mean time to failure (MTTF). The CDTR in Bristol developed and pioneered Raman thermography, to probe temperature and stress/strain with sub-micron spatial and nano-second time resolution in the only a few micron size active device area of AlGaN/GaN HEMTs, but there is presently no non-invasive probe available for experimentally quantifying electric field strength and its lateral distribution in particular when operating devices at high voltages. Therefore presently only simulation can be used to estimate electric field strength. The key aim of this research project is to develop, test and employ a non-invasive novel optical probe (E-probe) to quantify electric field strength and its lateral distribution in the device channel of AlGaN/GaN HEMTs, and to integrate it into Raman thermography, to enable simultaneous electric field, temperature and stress analysis of AlGaN/GaN HEMTs, to develop a unique and highly beneficial analysis technique for AlGaN/GaN HEMT reliability research. Experiments on degrading / stressing of devices to probe the resulting changes in the electric field strength and its distribution will be performed for state-of-the-art reliability research. Charge carrier traps generated during stressing change electric field strength which we expect to be able to probe directly here for the first time. Carrier trapping times range from milliseconds to seconds. We aim, a higher-risk component of this project, to also developing the ability to probing time-dependent changes in the electric field with carrier trapping / detrapping in the devices.

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.