Jiadi Zhu

Ion Gated Synaptic Transistors Based on 2D van der Waals Crystals with Tunable Diffusive Dynamics

Neuromorphic computing represents an innovative technology that can perform intelligent and energy-efficient computation, whereas construction of neuromorphic systems requires biorealistic synaptic elements with rich dynamics that can be tuned based on a robust mechanism. Here, an ionic-gating-modulated synaptic transistor based on layered crystals of transitional metal dichalcogenides and phosphorus trichalcogenides is demonstrated, which produce a diversity of short-term and long-term plasticity including excitatory postsynaptic current, paired pulse facilitation, spiking-rate-dependent plasticity, dynamic filtering, etc., with remarkable linearity and ultralow energy consumption of ≈30 fJ per spike. Detailed transmission electron microscopy characterization and ab initio calculation reveal two-stage ionic gating effects, namely, surface adsorption and internal intercalation in the channel medium, causing different poststimulation diffusive dynamics and thus accounting for the observed short-term and long-term plasticity, respectively. The synaptic activity can therefore be effectively manipulated by tailoring the ionic gating and consequent diffusion dynamics with varied thickness and structure of the van der Waals material as well as the number, duration, rate, and polarity of gate stimulations, making the present synaptic transistors intriguing candidates for low-power neuromorphic systems.

Voltage-Controlled Magnetoresistance in Silicon Nanowire Transistors

Magneto-electronic logic is an innovative approach to performing high-efficiency computations. Additionally, the ultra-large scale integration requirement for computation strongly suggests exploiting magnetoresistance effects in non-magnetic semiconductor materials. Here, we demonstrate the magnetoresistance effect in a silicon nanowire field effect transistor (SNWT) fabricated by complementary metal-oxide-semiconductor (CMOS)-compatible technology. Our experimental results show that the sign and the magnitude of the magnetoresistance in SNWTs can be effectively controlled by the drain-source voltage and the gate-source voltage, respectively, playing the role of a multi-terminal tunable magnetoresistance device. Various current models are established and in good agreement with the experimental results that describe the impact of electrical voltage and magnetic field on magnetoresistance, which provides design feasibility for the high-density magneto-electronic circuit. Such findings will further pave the way for nanoscale silicon-based magneto-electronics logic devices and show a possible path beyond the developmental limits of CMOS logic.

Bipolar to unipolar mode transition and imitation of metaplasticity in oxide based memristors with enhanced ionic conductivity

Neuromorphic engineering offers a promising route toward intelligent and low power computing systems that may find applications in artificial intelligence and the Internet. Construction of neuromorphic systems, however, requires scalable nanodevices that could implement the key functionalities of biological synapses. Here, we demonstrate an artificial synaptic device consisting of a Ti/yttria-stabilized-zirconia (ZrO2:Y)/Pt memristive structure, where the loss microstructure, high oxygen vacancy concentration, and resultant high ionic conductivity in ZrO2:Y facilitate the oxygen vacancy migration and filament evolution in the devices, leading to a bipolar artificial synapse with low forming and operation voltages. As the thickness of ZrO2:Y film increases, a transition from bipolar to unipolar resistive switching was observed, which can be ascribed to the competing vertical and radial ion transport dynamics. The emergence of unipolar switching has in turn allowed the device to exhibit metaplasticity, a history dependent plasticity that is important for memory and learning functions. This work thus demonstrates on-demand manipulation of ionic transport properties for building synaptic elements with rich functionalities.Neuromorphic engineering offers a promising route toward intelligent and low power computing systems that may find applications in artificial intelligence and the Internet. Construction of neuromorphic systems, however, requires scalable nanodevices that could implement the key functionalities of biological synapses. Here, we demonstrate an artificial synaptic device consisting of a Ti/yttria-stabilized-zirconia (ZrO2:Y)/Pt memristive structure, where the loss microstructure, high oxygen vacancy concentration, and resultant high ionic conductivity in ZrO2:Y facilitate the oxygen vacancy migration and filament evolution in the devices, leading to a bipolar artificial synapse with low forming and operation voltages. As the thickness of ZrO2:Y film increases, a transition from bipolar to unipolar resistive switching was observed, which can be ascribed to the competing vertical and radial ion transport dynamics. The emergence of unipolar switching has in turn allowed the device to exhibit metaplasticity, a hi...

Interfacial redox processes in memristive devices based on valence change and electrochemical metallization

Memristive devices based on electrochemical processes are promising candidates for next-generation memory and neuromorphic applications. The redox processes happening at the interfaces are crucial steps for the ionization as well as generation of counter charges, and are thus indispensable for successful resistive switching, but their detailed mechanism has not been fully clarified. Here, we study the interfacial redox reactions in the forming process of memristive devices based on valence change and electrochemical metallization, using high-resolution electron microscopy and electrostatic force microscopy observations. We show direct evidence for the anodic oxidation of oxygen ions and cathodic reduction of moisture in HfO2- and Ta2O5-based valence change cells, which could take place in different horizontal locations. We further found that the anodic reactions always led to more pronounced structural damage to the electrode, indicating the possibility of additional cathodic reactions without producing gaseous products. When an active electrode is present, oxidation of metal atoms takes place at the anodic interface instead. Further investigations on electrochemical metallization cells have identified Cu ionization and moisture reduction as the anodic and cathodic reactions, respectively, and formation of Cu nuclei at the cathodic interface was directly observed. These findings with microscopic evidence could facilitate future development of memristive devices.

New Understanding of Random Telegraph Noise Amplitude in Tunnel FETs

As one of the important sources of low-frequency noise, random telegraph noise (RTN) in tunnel FET (TFET) has attracted growing attention recently. However, there is still lack of explanation for the high-amplitude RTN, which may cause serious variability and reliability problems to TFET-based ultralow-power circuits. In this paper, we experimentally investigate the RTN amplitude characteristics in TFETs, revealing the mechanism of high-amplitude RTN. It is found that the nonuniform distribution of band-to-band tunneling (BTBT) generation rate along device width direction is responsible for the high-amplitude RTN. Locations with relatively higher BTBT generation rate may act as critical paths dominating the device current. “Lucky” trap located at a critical path can induce the high-amplitude RTN. With device width shrinking, the number of critical paths may be reduced, resulting in much higher maximum RTN amplitude. In addition, the impacts of process parameters on RTN amplitude are discussed. Both higher source doping concentration (

Deep insights into dielectric breakdown in tunnel FETs with awareness of reliability and performance co-optimization

\text{N}_{\text {S}})

Combinational Access Tunnel FET SRAM for Ultra-Low Power Applications

and higher thermal budget can effectively mitigate the nonuniformity of BTBT generation rate along device width direction, causing suppressed RTN amplitudes. Considering that the higher thermal budget may lead to degraded device performance, the annealing process should compromisingly be designed in terms of both variation and device performance.

Resistive switching and synaptic plasticity in HfO 2 -based memristors with single-layer and bilayer structures

The gate dielectrics reliability in Tunnel FETs (TFETs) has been thoroughly investigated for the first time, which is found to be the dominant device failure mechanism compared with bias temperature ins tability degradation, and is much worse than MOSFETs with the same gate stacks due to a new stronger localized dielectric field peak at gate/source overlap region. The non-uniform electric field of dielectric in TFET also leads to the different mechanisms between soft breakdown and hard breakdown failure. Moreover, dielectric-field-associated parameters are discussed in detail, showing an intrinsic trade-off between dielectrics reliability and device performance optimization caused by the positive correlation between dielectric field and source junction field. A new robust design consideration is further proposed for reliability and performance co-optimization, which is experimentally realized by a new TFET design with both dramatically improved performance and reliability, indicating its great potentials for ultralow-power applications.