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Research

Our vision is to rejuvenate modern electronics by developing and enabling a new approach to electronic systems where reconfigurability, scalability, operational flexibility/resilience, power efficiency and cost-effectiveness are combined. This vision will be delivered by breaking out of the large, but comprehensively explored realm of CMOS technology upon which virtually all modern electronics are based; consumer and non-consumer alike.

Introducing novel nanoelectronic components never before used in the technology we all carry around in our phones will introduce new capabilities that have thus far been unattainable due to the limitations of current hardware technology. The resulting improved capability of engineers to squeeze more computational power in ever smaller areas at ever lower power costs will unlock possibilities such as: a) truly pervasive Internet-of-Things computing where minute sensors consuming nearly zero power monitor the world around us and inform our choices, b) truly smart implants that within extremely limited power and size budgets can not only interface with the brain, but also process that data in a meaningful way and send the results either onwards to e.g. a doctor, or even feed it back into the brain for further processing, c) radiation-resistant electronics to be deployed in satellites and aeroplanes, civilian and military and improve communication reliability while driving down maintenance costs.

In building this vision, our project will deliver a series of scientific and commercial objectives: i) Developing the foundations of nanoelectronic component (memristive) technologies to the point where it becomes a commercially available option for the general industrial designer. ii) Setting up a fully supported (models, tools, design rules etc.), end-to-end design infrastructure so that anyone with access to industry standard software used for electronics design today may utilise memristive technology in their design. iii) Introduce a new design paradigm where memristive technologies are intimately integrated with traditional analogue and digital circuitry in order to deliver performance unattainable by any in isolation. This includes designing primitive hardware modules that can act as building-blocks for higher level designs, allowing engineers to construct large-scale systems without worrying about the intricate details of memristor operation. iv) Actively foster a community of users, encouraged to explore potential commercial impact and further scientific development stemming from our work whilst feeding back into the project through e.g. collaborations. v) Start early by beginning to commercialise the most mature aspects of the proposed research as soon as possible in order to create jobs in the UK. 

Our team (academic and industry) is ideally placed for delivering this disruptive vision that will allow our society to efficiently expand the operational envelope of electronics, enabling its use in formidable environments as well as reuse or re-purpose electronics affordably.

Research Papers
Seamlessly fused digital-analogue reconfigurable computing using memristors - Nature Communicationsvolume 9, Article number: 2170 (2018)
Alexantrou Serb, Ali Khiat & Themistoklis Prodromakis
As the world enters the age of ubiquitous computing, the need for reconfigurable hardware operating close to the fundamental limits of energy consumption becomes increasingly pressing. Simultaneously, scaling-driven performance improvements within the framework of traditional analogue and digital design become progressively more restricted by fundamental physical constraints. Emerging nanoelectronics technologies bring forth new prospects yet a significant rethink of electronics design is required for realising their full potential. Here we lay the foundations of a design approach that fuses analogue and digital thinking by combining digital electronics with analogue memristive devices for achieving charge-based computation; information processing where every dissipated charge counts. This is realised by introducing memristive devices into standard logic gates, thus rendering them reconfigurable and capable of performing analogue computation at a power cost close to digital. The versatility and benefits of our approach are experimentally showcased through a hardware data clusterer and an analogue NAND gate.
A technology agnostic RRAM characterisation methodology protocol - Applied Physics 2018
Spyros Stathopoulos, Loukas Michalas, Ali Khiat, Alexantrou Serb and Themis Prodromakis
The emergence of memristor technologies brings new prospects for modern electronics via enabling novel in-memory computing solutions and affordable and scalable reconfigurable hardware implementations. Several competing memristor technologies have been presented with each bearing distinct performance metrics across multi-bit memory capacity, low-power operation, endurance, retention and stability. Application needs however are constantly driving the push towards higher performance, which necessitates the introduction of standard characterisation protocols for fair benchmarking. At the same time, opportunities for innovation are missed by focusing on excessively narrow performance aspects. To that end our work presents a complete, technology agnostic, characterisation methodology based on established techniques that are adapted to memristors/RRAM characterisation needs. Our approach is designed to extract information on all aspects of device behaviour, ranging from deciphering underlying physical mechanisms to benchmarking across a variety of electrical performance metrics that can in turn support the generation of device models
Electrical characteristics of interfacial barriers at metal—TiO2 contacts - Journal of Physics D: Applied Physics, Volume 51, Number 42
Loukas Michalas, Ali Khiat, Spyros Stathopoulos and Themis Prodromakis
The electrical properties of thin TiO2 films have recently been extensively exploited with the aim of enabling a variety of metal-oxide electron devices: unipolar and bipolar semiconductor devices and/or memristors. In these efforts, investigations into the role of TiO2 as active material were the main focus; however, electrode materials are equally important. In this work we address this point by presenting a systematic quantitative electrical characterization study on the interface characteristics of metal-TiO2-metal structures. Our study employs typical contact materials that are used both as top and bottom electrodes in a metal-TiO2-metal setting. This allows an investigation of the characteristics of the interfaces as well as holistically studying an electrode's influence on the opposite interface, referred to in this work as the top/bottom electrodes inter-relationship. Our methodology comprises the recording of current–voltage (I–V) characteristics from a variety of solid-state prototypes in the temperature range of 300 K –350 K, and their analysis through appropriate modelling. Clear field- and temperature-dependent signature plots were also obtained, so as to shine more light on the role of each material as top/bottom electrodes in metal-TiO2-metal configurations. Our results highlight that these are not conventional metal–semiconductor contacts, and that several parameters are involved in the formation of the interfacial barriers, such as the electrode's position (atop or below the film), the electronegativity, the interface states, and even the opposite interface electrode material. Overall, our study provides a useful database for selecting appropriate electrode materials in TiO2-based devices, offering new insights into the role of electrodes in metal-oxide electronics applications.ogical and workable to encircle one’s own banker militarily?
Conduction mechanisms at distinct resistive levels of Pt/TiO2-x/Pt memristors - Appl. Phys. Lett. 113, 143503 (2018
L. Michalas, S. Stathopoulos, A. Khiat, and T. Prodromakis
Resistive random access memories (RRAMs) are considered as key enabling components for a variety of emerging applications due to their capacity to support multiple resistive states. Deciphering the underlying mechanisms that support resistive switching remains to date a topic of debate, particularly for metal-oxide technologies, and is very much needed for optimizing their performance. This work aims to identify the dominant conduction mechanisms during switching operation of Pt/TiO2-x/Pt stacks, which is without a doubt one of the most celebrated ones. A number of identical devices were accordingly electroformed for acquiring distinct resistive levels through a pulsing-based and compliance-free protocol. For each obtained level, the switching current-voltage (I-V) characteristics were recorded and analyzed in the temperature range of 300 K–350 K. This allowed the extraction of the corresponding signature plots revealing the dominant transport mechanism for each of the I-V branches. Gradual (analogue) switching was obtained for all cases, and two major regimes were identified. For the higher resistance regime, the transport at both the high and low resistive states was found to be interface controlled due to Schottky emission. As the resistance of devices reduces to lower levels, the dominant conduction changes from an interface to the core-material controlled mechanism. This study overall supports that engineering the metal-oxide/metal electrode interface can lead to tailored barrier modifications for controlling the switching characteristics of TiO2 RRAM.
Challenges hindering memristive neuromorphic hardware from going mainstream. - Nature Communications volume 9, Article number: 5267 (2018)
Gina C. Adam, Ali Khiat & Themis Prodromakis
Memristive devices have elicited intense research in the past decade thanks to their inherent low voltage operation, multi-bit storage and cost-effective manufacturability. Nonetheless, several outstanding performance and manufacturability challenges have prevented the widespread industry adoption of redox-based memristive matrices. Here, we discuss these challenges in terms of key metrics and propose a roadmap towards realizing competitive memristive-based neuromorphic processing systems.
Spike sorting using non-volatile metal-oxide memristors - Faraday discussions 213 511-520
Isha Gupta, Alexantrou Serb, Ali Khiat, Maria Trapatselia and Themistoklis Prodromakisa
efficient computation paradigms for handling neural data in situ; in particular the computationally heavy task of events classification. Here, we demonstrate how the intrinsic analogue programmability of memristive devices can be exploited to perform spike-sorting on single devices. Leveraging the physical properties of nanoscale memristors allows us to demonstrate that these devices can capture enough information in neural signal for performing spike detection (shown previously) and spike sorting at no additional power cost.
An Analogue-Domain, Switch-Capacitor-Based Arithmetic-Logic Unit - 2019 IEEE International Symposium on Circuits and Systems (ISCAS)
Alexander Serb and Themis Prodromakis
The continuous maturation of novel nanoelectronic devices exhibiting finely tuneable resistive switching is rekindling interest in analogue-domain computation. Regardless of domain, a useful computational module is the arithmetic-logic unit (ALU), which is capable of performing one or more fundamental mathematical operations (typical example: addition and subtraction). In this work we report on a design for an analogue ALU (aALU) capable of performing barrel addition and subtraction (i.e. ADD/SUB in modular arithmetic). The circuit only requires 5 minimum-size transistors and 1 capacitor. We show that our aALU is in principle capable of handling 5 bits of information using a single input/output wire. Core power dissipation per operation is estimated to peak at ≈ 59 f J (input operand-dependent) in TSMC's 65 nm technology.
A 68μW 31kS/s Fully-Capacitive Noise-Shaping SAR ADC with 102 dB SNDR - 2019 IEEE International Symposium on Circuits and Systems (ISCAS)
Lieuwe B. Leene ; Shiva Letchumanan ; Timothy G. Constandinou
This paper presents a 17 bit analogue-to-digital converter that incorporates mismatch and quantisation noise-shaping techniques into an energy-saving 10 bit successive approximation quantiser to increase the dynamic range by another 42 dB. We propose a novel fully-capacitive topology which allows for high-speed asynchronous conversion together with a background calibration scheme to reduce the oversampling requirement by 10× compared to prior-art. A 0.18μm CMOS technology is used to demonstrate preliminary simulation results together with analytic measures that optimise parameter and topology selection. The proposed system is able to achieve a FoM S of 183 dB for a maximum signal bandwidth of 15.6 kHz while dissipating 68 μW from a 1.8 V supply. A peak SNDR of 102 dB is demonstrated for this rate with a 0.201 mm 2 area requirement.
An electrical characterisation methodology for identifying the switching mechanism in TiO2 memristive stacks - Scientific Reports volume 9, Article number: 8168 (2019)
L. Michalas, S. Stathopoulos, A. Khiat & T. Prodromakis
Resistive random access memories (RRAMs) can be programmed to discrete resistive levels on demand via voltage pulses with appropriate amplitude and widths. This tuneability enables the design of various emerging concepts, to name a few: neuromorphic applications and reconfigurable circuits. Despite the wide interest in RRAM technologies there is still room for improvement and the key lies with understanding better the underpinning mechanism responsible for resistive switching. This work presents a methodology that aids such efforts, by revealing the nature of the resistive switching through assessing the transport properties in the non-switching operation regimes, before and after switching occurs. Variation in the transport properties obtained by analysing the current-voltage characteristics at distinct temperatures provides experimental evidence for understanding the nature of the responsible mechanism. This study is performed on prototyped device stacks that possess common Au bottom electrodes, identical TiO2 active layers while employing three different top electrodes, Au, Ni and Pt. Our results support in all cases an interface controlled transport due to Schottky emission and suggest that the acquired gradual switching originates by the bias induced modification of the interfacial barrier. Throughout this study, the top electrode material was found to play a role in determining the electroforming requirements and thus indirectly the devices’ memristive characteristics whilst both the top and bottom metal/oxide interfaces are found to be modified as result of this process.
A 3rd Order Time Domain Delta Sigma Modulator with Extended-Phase Detection - 2019 IEEE International Symposium on Circuits and Systems (ISCAS)
Lieuwe B Leene and Timothy G. Constandinou
This paper presents a novel analogue to digital converter using an oscillator-based loop filter for high-dynamic range bio-sensing applications. This is the first third-order feedforward ΔΣ modulator that strictly uses time domain integration for quantisation noise shaping. Furthermore we propose a new asynchronous extended-phase detection technique that increases the resolution of the 4 bit phase quantiser by another 5 bits to significantly improve both dynamic range and reduce the noise-shaping requirements. Preliminary simulation results show that this type of loop-filter can virtually prevent integrator saturation and achieves a peak 88 dB SNDR for kHz signals. The proposed system has been implemented using a 180 nm CMOS technology occupying 0.102 mm 2 and consumes 13.7 μW of power to digitise the 15 kHz signal bandwidth using a 2 MHz sampling clock.
A Memristive Switching Uncertainty Model - IEEE Transactions on Electron Devices (Volume: 66, Issue: 7, July 2019)
Spyros Stathopoulos ; Alexantrou Serb ; Ali Khiat ; Maciej Ogorzałek ; Themis Prodromakis
In this paper, we endeavor to evaluate and model switching noise in resistive random access memory (RRAM) devices. Although noise is always present in physical systems, the sources of which can be attributed to many different effects, in this paper, we are focusing our attention on a specific type-switching noise. Using alternating pulse programming and read trains across different voltages, we acquire a large data set below and above the switching threshold and construct what we define as increment plots, ΔR versus R. Then, through a detailed statistical analysis, we quantify the localized uncertainty among consecutive points using a sliding window of up to N points accounting for any statistical artifacts that arise. By separating the data accumulated from programming and read-out and analyzing them individually, we can subtract a baseline noise floor from the overall switching uncertainty. In this way, we effectively decouple it from other noise sources that affect the device at rest. In the end, an F(R, V) surface can be extracted that closely follows the behavior of uncertainty of the device during programming. This modeled surface can be used as an approximation of the noise behavior of the device or it can be readily incorporated as an additional component to existing switching models.
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