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Recently, three-dimensional FLASH memory with multi-level cell characteristics has attracted increasing attention to enhance the capabilities of artificial intelligence (AI) by leveraging computingin-memory (CIM) systems. The focus is to maximize the computing performance and design FLASH memory suitable for various AI algorithms, where the memory must achieve a highly controllable multi-level threshold voltage (VT). Therefore, we developed a SPICE compact model that can rapidly simulate charge trap FLASH cells for CIM to identify optimal programming conditions. SPICE simulation results of the transfer characteristics are in good agreement with the results of experimentally fabricated FLASH memory, showing a low error rate of 10%. The model was also validated against the results obtained from the TCAD tool, showing that a consistent VT change was computed in a shorter time than that required using TCAD. Then, the developed model was used to comprehensively investigate how single or multiple gate voltage (VG) pulses affect VT. Moreover, considering recent FLASH memory fabrication processes, we found that grain boundaries in polycrystalline silicon channel materials can be involved in deteriorating gate controllability. Therefore, optimizing the pulse scheme by correcting potential errors identified in advance through fast SPICE simulation can enable the accurate achievement of the specific analog states of the FLASH cells of the CIM architecture, boosting computing performance.

Abstract This study optimizes VT and ΔVT in amorphous indium-gallium-zinc-oxide (a-IGZO) field-effect transistors (FETs) by examining the influence of both channel length (L) and Ga composition. It was observed that as the ratio of In: Ga: Zn changed from 1:1:1 to 0.307:0.39:0.303 in the IGZO film, both VT and ΔVT decreased by 0.1 V at the shortest channel length (L = 0.5 μm). This reduction was attributed to the change in the oxygen concentration in IGZO due to the variation in Ga composition. In addition, as the channel length decreased from 10 μm to 0.5 μm, VT decreased by up to 0.7 V, and ΔVT decreased by up to 0.4 V. This observation was due to diffusion of oxygen vacancies (VO) from the source and drain into the main channel. To provide a comprehensive understanding, we quantitatively modeled the doping concentration of IGZO and the trap density of gate insulator (GI) traps using TCAD simulation based on Ga composition and diffusion of VO. Using this approach, we propose a method to optimize the design of a-IGZO FETs with high VT and low ΔVT in short-channel devices by adjusting the Ga composition.

Abstract In this paper, the floating body effect (FBE) in indium-gallium-zinc-oxide (IGZO) thin-film transistor (TFT) and the mechanism of device failure caused by that are reported for the first time. If the toggle AC pulses are applied to the gate and drain simultaneously for the switching operation, the drain current of IGZO TFT increases dramatically and cannot show the on/off switching characteristics. This phenomenon was not reported before, and our study reveals that the main cause is the formation of a conductive path between the source and drain: short failure. It is attributed in part to the donor creation at the drain region during the high voltage (V high) condition and in part to the donor creation at the source region during the falling edge and low voltage (V low) conditions. Donor creation is attributed to the peroxide formation in the IGZO layer induced by the electrons under the high lateral field. Because the donor creation features positive charges, it lowers the threshold voltage of IGZO TFT. In detail, during the V high condition, the donor creation is generated by accumulated electrons with a high lateral field at the drain region. On the other hand, the floating electrons remaining at the short falling edge (i.e., FBE of the IGZO TFT) are affected by the high lateral field at the source region during the V low condition. As a result, the donor creation is generated at the source region. Therefore, the short failure occurs because the donor creations are generated and expanded to channel from the drain and source region as the AC stress accumulates. In summary, the FBE in IGZO TFT is reported, and its effect on the electrical characteristics of IGZO TFT (i.e., the short failure) is rigorously analyzed for the first time.

Obtaining symmetrical and highly linear synapse weight update characteristics of analog resistive switching devices is critical for attaining high performance and energy efficiency of the neural network system. In this work, based on the two-terminal one transistor-one memristor (1T1M) block, the improvement of the symmetry and linearity of synaptic weight update is demonstrated by combining the InGaZnO synaptic transistor and memristor. Due to the symmetric and linear weight update characteristic, a pattern recognition accuracy of 88% is achieved after 50 epochs in the on-chip learning simulation of the hand-written digit images (MNIST) data set. The proposed 1T1M device saves the hardware burden and additional power consumption required to implement non-identical programming pulses.

Abstract Oxide semiconductor thin-film transistors (TFTs) are used in the pixel array and gate driver circuits of organic light emitting diode (OLED) display panels. Long-term reliability characteristics of the TFTs are a barometer of the lifetime of OLED display panels. The long-term reliability of the driver TFTs is evaluated in a short time under high voltages and high temperature for an accelerated degradation test. If reliability parameters from the power law or stretched-exponential functions are the same for individual devices and devices in an operating panel, the lifetime of the panel can be accurately estimated. However, since compensation circuits are designed into operating panels, an environmental discrepancy exists between the accelerated test of single devices and the operation of devices in the panel. Herein, we propose a novel compensation stretched-exponential function (CSEF) model which captures the effect of the threshold voltage compensation circuit in the panel. The CSEF model not only bridges the discrepancy between individual devices and panel devices, but also provides a method to accurately and efficiently estimate the long-term lifetime of all display panels that utilize compensation circuits.

Short- and long-term neuroplasticity behaviors are key mechanisms for various activities. In this paper, we propose a synaptic transistor with a floating-gate (FG) node and an amorphous InGaZnO (IGZO) channel layer. The proposed device emulates the neuroplasticity functions of both short-term memory (STM) and long-term memory (LTM) through the control of the amplitude and the number of input pulses. The STM operated by ion movement in the gate dielectrics occurs when the input amplitude is relatively small (< 9 V). The LTM operated through the storage of electrons in the FG occurs when the input amplitude is relatively large (>10 V). In addition, as the number of input pulses increases, information is stored for a longer time. Our FG IGZO synaptic transistor could be a promising device solution for brain-inspired computing systems.

Abstract Oxide semiconductor transistors control the brightness and color of organic light‐emitting diode (OLED) displays in large‐screen televisions to portable telecommunications devices. Oxide semiconductor thin‐film transistors under driving conditions are required to maintain a steady current through the OLED for constant illuminance. Interestingly, for driving conditions under strong saturation where both gate and drain bias are high, a boosting phenomenon of the drain current is discovered, even with compensation of the threshold voltage. In this paper, the current boosting effect of self‐aligned InGaZnO transistors under driving conditions is comprehensively investigated. Based on experimental extraction methods, two distinct regions within the device are identified: an electron‐capture‐dominant region including electron trapping in the gate insulator and O–O dimer bond‐breaking, and an electron‐emission‐dominant region caused by peroxide formation. A dual‐transistor‐in‐series model is proposed, where each region is modeled as a local transistor. The current boosting phenomena as a function of time are well‐reproduced for various channel length devices, which validate the accuracy of the model. Better understanding of the underlying mechanisms enables increased effectiveness of compensation schemes for transistors under long‐term current‐driving conditions.

Hardware-oriented neuromorphic computing is gaining great deal of interest for highly parallel data processing and superb energy efficiency, as the candidate for replacement of conventional von Neumann computing. In this work, a novel synaptic transistor constructing the neuromorphic system is proposed, fabricated, and characterized. Amorphous indium-gallium-zinc-oxide ( $\alpha $ -IGZO) and Al2O3 are introduced as the channel and gate dielectric materials, respectively. Along with the high functionality and low-temperature processing viability, geometric peculiarity featuring extended gate structure improves the performances of the proposed transistor as synaptic component in the neuromorphic system. The insight into the substantial effect of optimal device structure design on energy efficiency is highlighted.

In this article, we study the post-annealing effect on the synaptic characteristics in Pd/IGZO/SiO2/p+-Si memristor devices. The O-H bond in IGZO films affects the switching characteristics that can be controlled by the annealing process. We propose a switching model based on using a native oxide as the Schottky barrier. The barrier height is extracted by the conduction mechanism of thermionic emission in samples with different annealing temperatures. Additionally, the change in conductance is explained by an energy band diagram including trap models. The activation energy is obtained by the depression curve of the samples with different annealing temperatures to better understand the switching mechanism. Moreover, our results reveal that the annealing temperature and retention can affect the linearity of potentiation and depression. Finally, we investigate the effect of the annealing temperature on the recognition rate of MNIST in the proposed neural network.

This paper introduces a compact SPICE model of a two-terminal memory with a Pd/Ti/IGZO/p+-Si structure. In this paper, short- and long-term components are systematically separated and applied in each model. Such separations are conducted by the applied bias and oxygen flow rate (OFR) during indium gallium zinc oxide (IGZO) deposition. The short- and long-term components in the potentiation and depression curves are modeled by considering the process (OFR of IGZO) and bias conditions. The compact SPICE model with the physical mechanism of SiO2 modulation is introduced, which can be useful for optimizing the specification of memristor devices.

Here, highly transparent nanocomposite films with an adjustable refractive index were fabricated through stable dispersion of ZrO2 (n = 2.16) nanoparticles (NPs) subjected to surface modification with SiO2 (n = 1.46) in polydimethylsiloxane (PDMS) (n = 1.42) using the Stöber method. ZrO2 NPs (13.7 nm) were synthesized using conventional hydrothermal synthesis, and their surface modification with SiO2 (ZrO2@SiO2 NPs) was controlled by varying the reaction time (3–54 h). The surface modification of the NPs was characterized using Fourier-transform infrared spectroscopy, dynamic light scattering, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and ellipsometry. The surface modification was monitored, and the effective layer thickness of SiO2 varied from 0.1 nm to 4.2 nm. The effective refractive index of the ZrO2@SiO2 NPs at λ = 633 nm was gradually reduced from 2.16 to 1.63. The 100 nm nanocomposite film was prepared by spin-coating the dispersion of ZrO2@SiO2 NPs in PDMS on the coverslip. The nanocomposite film prepared using ZrO2@SiO2 NPs with a reaction time of 18 h (ZrO2@SiO2-18h-PDMS) exhibited excellent optical transparency (Taverage = 91.1%), close to the transparency of the coverslip (Taverage = 91.4%) in the visible range, and an adjustable refractive index (n = 1.42–1.60) as the NP content in the film increased from 0 to 50.0 wt%.

It has been elucidated that the responses to light by the human eye influence not only color and brightness recognition but also physiological aspects such as hormones. Melatonin, which affects human circadian rhythms, is sensitive to the blue wavelength region of light. We have investigated the spectroscopic effects of light from virtual reality (VR) and augmented reality (AR) display devices on nonvisual characteristics by means of an analysis of the spectral power distribution (SPD). The circadian illuminance and melatonin suppression value were introduced as representative figures of merits for spectroscopically evaluating nonvisual characteristics. A VR/AR-like custom-made instrument was used to study how the SPD of the display light source affects nonvisual characteristics. Moreover, using multilayer thin-film filters, optimal conditions for minimizing the change of the visual color characteristics of the display while reducing the influence on the non-visual characteristics are also discussed. For the AR device, we found that its spectrum was mostly affected by external light rather than by the spectrum of the AR display itself.

Given that light is known to function as a zeitgeber, having the greatest influence on the human circadian rhythm, it is necessary to assess the effects of light on humans with the goal of maintaining the circadian rhythm. Herein, we fabricated a simple circadian light meter that directly measures the non-visual effects of light using optical filters that mimic the non-visual action spectrum. The fabricated light meter was calibrated and verified through the values obtained from a conventional illuminance spectrophotometer. Furthermore, during 24 h of everyday life, 11 participants wore hats equipped with the developed light meter so that we could investigate the effects of the light environment to which they were exposed to, both indoors and outdoors. For comparison, natural outdoor illumination was also measured with the same light meter. Based on the considerable difference between the light exposure levels during the daytime and nighttime, it is possible that the participant’s melatonin levels would be impacted by the light exposure measured by the light meter. Consequently, based on the light exposure measurements made in this study, the proposed circadian light meter would be a valuable tool for real world circadian lighting studies that require actual light dose to the eyes of the test subjects.

Large-scale graded-index metamaterial devices are difficult to fabricate owing to limitations of typical micro- and nanofabrication approaches. Here, Shinet al. demonstrate millimetre-scale transformation elements based on elasto-optic metamaterials made from aerogels.

Accurate circuit simulation reflecting physical and electrical stress is of importance in indium gallium zinc oxide (IGZO)-based flexible electronics. In particular, appropriate modeling of threshold voltage (VT) changes in different bias and bending conditions is required for reliability-aware simulation in both device and circuit levels. Here, we present SPICE compatible compact modeling of IGZO transistors and inverters having an atomic layer deposition (ALD) Al2O3 gate insulator on a polyethylene terephthalate (PET) substrate. Specifically, the modeling was performed to predict the behavior of the circuit using stretched exponential function (SEF) in a bending radius of 10 mm and operating voltages ranging between 4 and 8 V. The simulation results of the IGZO circuits matched well with the measured values in various operating conditions. It is expected that the proposed method can be applied to process improvement or circuit design by predicting the direct current (DC) and alternating current (AC) responses of flexible IGZO circuits.

In this study, we analyzed the threshold voltage shift characteristics of bottom-gate amorphous indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) under a wide range of positive stress voltages. We investigated four mechanisms: electron trapping at the gate insulator layer by a vertical electric field, electron trapping at the drain-side GI layer by hot-carrier injection, hole trapping at the source-side etch-stop layer by impact ionization, and donor-like state creation in the drain-side IGZO layer by a lateral electric field. To accurately analyze each mechanism, the local threshold voltages of the source and drain sides were measured by forward and reverse read-out. By using contour maps of the threshold voltage shift, we investigated which mechanism was dominant in various gate and drain stress voltage pairs. In addition, we investigated the effect of the oxygen content of the IGZO layer on the positive stress-induced threshold voltage shift. For oxygen-rich devices and oxygen-poor devices, the threshold voltage shift as well as the change in the density of states were analyzed.

Background: The examination of the longitudinal effect of leisure-time physical activity (LTPA) on mental well-being is important, but previous studies have typically been limited by their use of a cross-sectional approach. This study empirically examined how LTPA intensity was associated with changes in distinct functions of mental well-being (eg, emotional, psychological, social) over time, and vice versa. Methods: Parallel latent growth curve modeling in combination with propensity score matching analysis was conducted. Data were derived from a sample of adults from the Midlife in the United States (MIDUS) study. Results: The results showed that the initial level of moderate LTPA at the baseline was associated with growth in psychological and social functioning over time, and vice versa. However, vigorous LTPA at the baseline was related only to growth in emotional functioning over time. Conclusion: The longitudinal association between LTPA and mental well-being had different matching mechanisms for LTPA intensities and their relation to distinct functioning for mental well-being. The findings contribute to an enhanced understanding of LTPA's longitudinal effect on mental well-being. [ABSTRACT FROM AUTHOR]