The rapid increase in information in the big-data era calls for changes to information-processing paradigms, which, in turn, demand new circuit-building blocks to overcome the decreasing cost-effectiveness of transistor scaling and the intrinsic inefficiency of using transistors in non-von Neumann computing architectures. Accordingly, resistive switching materials (RSMs) based on different physical principles have emerged for memories that could enable energy-efficient and area-efficient in-memory computing. In this Review, we survey the four physical mechanisms that lead to such resistive switching: redox reactions, phase transitions, spin-polarized tunnelling and ferroelectric polarization. We discuss how these mechanisms equip RSMs with desirable properties for representation capability, switching speed and energy, reliability and device density. These properties are the key enablers of processing-in-memory platforms, with applications ranging from neuromorphic computing and general-purpose memcomputing to cybersecurity. Finally, we examine the device requirements for such systems based on RSMs and provide suggestions to address challenges in materials engineering, device optimization, system integration and algorithm design. Resistive switching materials enable novel, in-memory information processing, which may resolve the von Neumann bottleneck. This Review focuses on how the switching mechanisms and the resultant electrical properties lead to various computing applications.

Resistive switching materials for information processing

Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones wi...

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Heterostructured materials: superior properties from hetero-zone interaction

In this manuscript, recent progress in the area of resistive random access memory (RRAM) technology which is considered one of the most standout emerging memory technologies owing to its high speed, low cost, enhanced storage density, potential applications in various fields, and excellent scalability is comprehensively reviewed. First, a brief overview of the field of emerging memory technologies is provided. The material properties, resistance switching mechanism, and electrical characteristics of RRAM are discussed. Also, various issues such as endurance, retention, uniformity, and the effect of operating temperature and random telegraph noise (RTN) are elaborated. A discussion on multilevel cell (MLC) storage capability of RRAM, which is attractive for achieving increased storage density and low cost is presented. Different operation schemes to achieve reliable MLC operation along with their physical mechanisms have been provided. In addition, an elaborate description of switching methodologies and current voltage relationships for various popular RRAM models is covered in this work. The prospective applications of RRAM to various fields such as security, neuromorphic computing, and non-volatile logic systems are addressed briefly. The present review article concludes with the discussion on the challenges and future prospects of the RRAM.

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Resistive Random Access Memory (RRAM): an Overview of Materials, Switching Mechanism, Performance, Multilevel Cell (mlc) Storage, Modeling, and Applications

As the research on artificial intelligence booms, there is broad interest in brain-inspired computing using novel neuromorphic devices. The potential of various emerging materials and devices for neuromorphic computing has attracted extensive research efforts, leading to a large number of publications. Going forward, in order to better emulate the brain's functions, its relevant fundamentals, working mechanisms, and resultant behaviors need to be re-visited, better understood, and connected to electronics. A systematic overview of biological and artificial neural systems is given, along with their related critical mechanisms. Recent progress in neuromorphic devices is reviewed and, more importantly, the existing challenges are highlighted to hopefully shed light on future research directions.

Bridging Biological and Artificial Neural Networks with Emerging Neuromorphic Devices: Fundamentals, Progress, and Challenges.

The intrinsic variability of switching behavior in memristors has been a major obstacle to their adoption as the next generation of universal memory. On the other hand, this natural stochasticity can be valuable for hardware security applications. Here we propose and demonstrate a novel true random number generator utilizing the stochastic delay time of threshold switching in a Ag:SiO2 diffusive memristor, which exhibits evident advantages in scalability, circuit complexity, and power consumption. The random bits generated by the diffusive memristor true random number generator pass all 15 NIST randomness tests without any post-processing, a first for memristive-switching true random number generators. Based on nanoparticle dynamic simulation and analytical estimates, we attribute the stochasticity in delay time to the probabilistic process by which Ag particles detach from a Ag reservoir. This work paves the way for memristors in hardware security applications for the era of the Internet of Things. Memristors can switch between high and low electrical-resistance states, but the switching behaviour can be unpredictable. Here, the authors harness this unpredictability to develop a memristor-based true random number generator that uses the stochastic delay time of threshold switching

/pdf/a-novel-true-random-number-generator-based-on-a-stochastic-4euhebzake.pdf

A novel true random number generator based on a stochastic diffusive memristor

Deformation behavior of a harmonic structured material (HSM), core–shell 304L stainless steel, is investigated using micro-digital image correlation (micro-DIC). High strain-partitioning between co...

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2018.1439115

Additional hardening in harmonic structured materials by strain partitioning and back stress

Various Threshold Switching Devices for Integrate and Fire Neuron Applications

In this research, we investigate electrically driven threshold switching (TS) characteristics in electrochemical metallization cells by adopting the field-induced nucleation theory. For this aim, Ag/HfO2 and Ag/TiO2 based TS devices are prepared and examined. First, we carry out the field driven turn-on process to form Ag filaments created as a consequence of sequential nucleation of Ag ions from the bottom electrode. During the filament formation process, it is observed that the prepared devices show switching time exponential in voltage and temperature with different nucleation barrier energies (W0), which confirms the field-induced nucleation theory. Furthermore, we find that the device with higher W0 shows faster dissolution speed. This implies that the slow turn-off speed of the TS device can be improved by finding a material system with a higher W0 value.

Field-induced nucleation in threshold switching characteristics of electrochemical metallization devices

The effect of hydrogen treatment on the threshold switching property in a Ag/amorphous Si based programmable metallization cells was investigated for selector device applications. Using the Ag filament formed during motion of Ag ions, a steep-slope (5 mV/dec.) for threshold switching with higher selectivity (∼105) could be achieved. Because of the faster diffusivity of Ag atoms, which are inside solid-electrolytes, the resulting Ag filament could easily be dissolved under low current regime, where the Ag filament possesses weak stability. We found that the dissolution process could be further enhanced by hydrogen treatment that facilitated the movement of the Ag atoms.

/pdf/threshold-switching-behavior-of-ag-si-based-selector-device-41ecikgvxj.pdf

Threshold switching behavior of Ag-Si based selector device and hydrogen doping effect on its characteristics

In this letter, we demonstrate a steep slope field-effect transistor (FET) using a threshold switching (TS) device. The Ag/TiO2-based TS device reported in our previous work was implemented in series with the drain region of a transistor. Since the TS device has an abrupt transition between the OFF- and ON-states and vice versa, the transistor has a 5-mV/decade subthreshold slope and a high ON/OFF-current ratio ( $I_{\mathrm{\scriptscriptstyle ON}}/I_{\mathrm{\scriptscriptstyle OFF}}$ ) of $> 10^{7}$ with a low drain voltage (0.3 V). Furthermore, the threshold voltage ( $V_{\mathrm {th, FET}})$ of the transistor can be tuned by controlling the thickness of the TS device.

Jongmyung Yoo

Papers

Additional hardening in harmonic structured materials by strain partitioning and back stress

Various Threshold Switching Devices for Integrate and Fire Neuron Applications

Field-induced nucleation in threshold switching characteristics of electrochemical metallization devices

Threshold switching behavior of Ag-Si based selector device and hydrogen doping effect on its characteristics

Steep Slope Field-Effect Transistors With Ag/TiO 2 -Based Threshold Switching Device