Christos Papavassiliou

A Versatile Memristor Model With Nonlinear Dopant Kinetics

The need for reliable models that take into account the nonlinear kinetics of dopants is nowadays of paramount importance, particularly with the physical dimensions of electron devices shrinking to the deep nanoscale range and the development of emerging nanoionic systems such as the memristor. In this paper, we present a novel nonlinear dopant drift model that resolves the boundary issues existing in previously reported models that can be easily adjusted to match the dynamics of distinct memristive elements. With the aid of this model, we examine switching mechanisms, current-voltage characteristics, and the collective ion transport in two terminal memristive devices, providing new insights on memristive behavior.

Memory Impedance in TiO2 based Metal-Insulator-Metal Devices

Large attention has recently been given to a novel technology named memristor, for having the potential of becoming the new electronic device standard. Yet, its manifestation as the fourth missing element is rather controversial among scientists. Here we demonstrate that TiO2-based metal-insulator-metal devices are more than just a memory-resistor. They possess resistive, capacitive and inductive components that can concurrently be programmed; essentially exhibiting a convolution of memristive, memcapacitive and meminductive effects. We show how non-zero crossing current-voltage hysteresis loops can appear and we experimentally demonstrate their frequency response as memcapacitive and meminductive effects become dominant.

A

Selectorless crossbar arrays of resistive randomaccess memory (RRAM), also known as memristors, conduct large sneak currents during operation, which can significantly corrupt the accuracy of cross-point analog resistance (Mt) measurements. In order to mitigate this issue, we have designed, built, and tested a memristor characterization and testing (mCAT) instrument that forces redistribution of sneak currents within the crossbar array, dramatically increasing Mt measurement accuracy. We calibrated the mCAT using a custom-made 32 × 32 discrete resistive crossbar array, and subsequently demonstrated its functionality on solid-state TiO2-x RRAM arrays, on wafer and packaged, of the same size. Our platform can measure standalone Mt in the range of 1 kΩ to 1 MΩ with <;1% error. For our custom resistive crossbar, 90% of devices of the same resistance range were measured with <;10% error. The platforms limitations have been quantified using large-scale nonideal crossbar simulations.

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Realizing large-scale circuits utilizing emerging nanoionic devices known as memristors depends on the accurate modeling of their behavior under a wide range of biasing conditions. Currently, no available SPICE memristor model accounts for both nonvolatile and volatile resistive switching characteristics, the coexistence of which has been recently demonstrated to manifest on practical ReRAM. In this letter, we present a new memristor SPICE model that introduces volatile effects, which can render a rate-dependent bipolar nonvolatile switching operation. The model is demonstrated via a number of simulation cases and is benchmarked against measured results acquired by solid-state TiO2 ReRAM.

-Controller-Based System for Interfacing Selectorless RRAM Crossbar Arrays

This work exploits the switching dynamics of nanoscale resistive random access memory (ReRAM) cells with particular emphasis on the origin of the observed variability when cells are consecutively cycled/programmed at distinct memory states. It is demonstrated that this variance is a common feature of all ReRAM elements and is ascribed to the formation and rupture of conductive filaments that expand across the active core, independently of the material employed as the active switching core, the causal physical switching mechanism, the switching mode (bipolar/unipolar) or even the unit cells’ dimensions. Our hypothesis is supported through both experimental and theoretical studies on TiO2 and In2O3 : SnO2 (ITO) based ReRAM cells programmed at three distinct resistive states. Our prototypes employed TiO2 or ITO active cores over 5 × 5 µm 2 and 100 × 100 µm 2 cell areas, with all tested devices demonstrating both unipolar and bipolar switching modalities. In the case of TiO2-based cells, the underlying switching mechanism is based on the non-uniform displacement of ionic species that foster the formation of conductive filaments. On the other hand, the resistive switching observed in the ITO-based devices is considered to be due to a phase change mechanism. The selected experimental parameters allowed us to demonstrate that the observed programming variance is a common feature of all ReRAM devices, proving that its origin is dependent upon randomly oriented local disorders within the active core that have a substantial impact on the overall state variance, particularly for high-resistive states.

A Memristor SPICE Model Accounting for Volatile Characteristics of Practical ReRAM

The effects of the substrate temperature in the molecular beam epitaxy growth of on have been investigated. A strong dependence of the structural, electrical, and optical properties of films on the growth temperature has been found and optimized material can be grown at 530°C. The low substrate temperatures deteriorate the material quality due to insufficient growth kinetics, while the higher temperatures allow the formation of composition inhomogeneities which also deteriorate the structural, optical, and electrical characteristics of . Using buffers grown at 530°C, state‐of‐the‐art high electron mobility transistors were fabricated and showed reduced output conductance and no kink effect in the I(V) characteristics.

Origin of the OFF state variability in ReRAM cells

A novel physiological sensor which combines electrical and mechanical modalities is introduced. The electrical component behaves as a standard electrode and detects changes in bioelectrical potential, whereas the mechanical component comprises an electret condenser microphone with a thin and light diaphragm, making it sensitive to local mechanical activity but immune to global body movements. A key feature of the proposed sensor is that the microphone is positioned directly on top of the electrode component (co-location). In conjunction with co-located electromechanical sensing, the ability of the electrode to flex allows for motion to be detected at the same location where it corrupts the electrical physiological response. Thus, the output of the mechanical sensor can be used to reject motion-induced artifacts in physiological signals, offering improved recording quality in wearable health applications. We also show that the co-located electrical and mechanical modalities provide derived information beyond unimodal sensing, such as pulse arrival time and breathing, thus enhancing the utility of the proposed device and highlighting its potential as a diagnostic tool.

A Comprehensive Optimization of InAlAs Molecular Beam Epitaxy for InGaAs / InAlAs HEMT Technology

Future health systems require the means to assess and track the neural and physiological function of a user over long periods of time, and in the community. Human body responses are manifested through multiple, interacting modalities – the mechanical, electrical and chemical; yet, current physiological monitors (e.g. actigraphy, heart rate) largely lack in cross-modal ability, are inconvenient and/or stigmatizing. We address these challenges through an inconspicuous earpiece, which benefits from the relatively stable position of the ear canal with respect to vital organs. Equipped with miniature multimodal sensors, it robustly measures the brain, cardiac and respiratory functions. Comprehensive experiments validate each modality within the proposed earpiece, while its potential in wearable health monitoring is illustrated through case studies spanning these three functions. We further demonstrate how combining data from multiple sensors within such an integrated wearable device improves both the accuracy of measurements and the ability to deal with artifacts in real-world scenarios.

Co-Located Multimodal Sensing: A Next Generation Solution for Wearable Health

Novel zipper varactors with the potential for achieving large tuning ranges have been fabricated and characterized. These varactors have a curved cantilever electrode that is actuated by a single pull-down electrode. The shape of the cantilever is designed such that its local stiffness is tailored to enable extended stable zipping. In a series-mounted varactor, the measured capacitance ratio was 16.5 for actuation voltages between 0 and 46 V. However, the presence of an unexpected tuning instability at 32 V bias limited the practical tuning range in this varactor. This behaviour was attributed to fabrication imperfections. The first electrical self-resonant frequency of the same varactor was extrapolated to be 72.6 and 17.2 GHz at 20 and 329 fF, respectively. In a different shunt-mounted varactor, the quality factor (Q) was measured to be 91 (60 fF) and 176 (600 fF) at 2 GHz. Including the anchor, the varactors have a small device footprint and fit within an area of 500 by 100 µm.

Hearables: Multimodal physiological in-ear sensing

The incorporation of silicon-buffer layers is shown to be critical in attaining optimized microwave performance for GaAs on silicon MESFETs. A current gain cutoff frequency (f/sub t/) of 18 GHz and maximum power cutoff frequency (f/sub max/) of 30 GHz is reported for relaxed geometry devices. The low parasitic capacitance and excellent device isolation make this structure suitable for monolithic integration. >