Reliable synaptic plasticity of InGaZnO transistor with TiO2interlayer.

We demonstrate an InGaZnO (IGZO)-based synaptic transistor with a TiO2buffer layer. The structure of the synaptic transistor with TiO2inserted between the Ti metal electrode and an IGZO semiconductor channel O2trapping layer produces a large hysteresis window, which is crucial for achieving synaptic functionality. The Ti/TiO2/IGZO synaptic transistor exhibits reliable synaptic plasticity features such as excitatory post-synaptic current, paired-pulse facilitation, and potentiation and depression, originating from the reversible charge trapping and detrapping in the TiO2layer. Finally, the pattern recognition accuracy of Modified National Institute of Standards and Technology handwritten digit images was modeled using CrossSim simulation software. The simulation results present a high image recognition accuracy of ∼89%. Therefore, this simple approach using an oxide buffer layer can aid the implementation of high-performance synaptic devices for neuromorphic computing systems.

A Comprehensive Large Signal, Small Signal, and Noise Model for IGZO Thin Film Transistor Circuits.

We report a new physics-based model for dual-gate amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) which we developed and fine-tuned through experimental implementation and benchtop characterization. We fabricated and characterized a variety of test patterns, including a-IGZO TFTs with varying gate widths (100-1000 µm) and channel lengths (5-50 µm), transmission-line-measurement patterns and ground-signal-ground (GSG) radio frequency (RF) patterns. We modeled the contact resistance as a function of bias, channel area, and temperature, and captured all operating regimes, used physics-based modeling adjusted for empirical data to capture the TFT characteristics including ambipolar subthreshold currents, graded interbias-regime current changes, threshold and flat-band voltages, the interface trap density, the gate leakage currents, the noise, and the relevant small signal parameters. To design high-precision circuits for biosensing, we validated the dc, small signal, and noise characteristics of the model. We simulated and fabricated a two-stage common source amplifier circuit with a common drain output buffer and compared the measured and simulated gain and phase performance, finding an excellent fit over a frequency range spanning 10 kHz-10 MHz.

High-Performance Potassium-Selective Biosensor Platform Based on Resistive Coupling of a-IGZO Coplanar-Gate Thin-Film Transistor.

The potassium (K+) ion is an essential mineral for balancing body fluids and electrolytes in biological systems and regulating bodily function. It is associated with various disorders. Given that it exists at a low concentration in the human body and should be maintained at a precisely stable level, the development of highly efficient potassium-selective sensors is attracting considerable interest in the healthcare field. Herein, we developed a high-performance, potassium-selective field-effect transistor-type biosensor platform based on an amorphous indium gallium zinc oxide coplanar-gate thin-film transistor using a resistive coupling effect with an extended gate containing a potassium-selective membrane. The proposed sensor can detect potassium in KCl solutions with a high sensitivity of 51.9 mV/dec while showing a low sensitivity of <6.6 mV/dec for NaCl, CaCl2, and pH buffer solutions, indicating its high selectivity to potassium. Self-amplification through the resistive-coupling effect enabled an even greater potassium sensitivity of 597.1 mV/dec. Additionally, we ensured the stability and reliability of short- and long-term detection through the assessment of non-ideal behaviors, including hysteresis and drift effects. Therefore, the proposed potassium-sensitive biosensor platform is applicable to high-performance detection in a living body, with high sensitivity and selectivity for potassium.

Effect of oxygen partial pressure on the performance of homojunction amorphous In-Ga-Zn-O thin-film transistors.

In this study, the homojunction thin-film transistors (TFTs) with amorphous indium gallium zinc oxide (a-IGZO) as active channel layers and source/drain electrodes were fabricated by RF magnetron sputtering. The effect of oxygen partial pressure on the phase, microstructure, optical and electrical properties of IGZO thin films was investigated. The results showed that amorphous IGZO thin films always exhibit a high transmittance above 90% and wide band gaps of around 3.9 eV. The resistivity increases as the IGZO thin films are deposited at a higher oxygen partial pressure due to the depletion of oxygen vacancies. In addition, the electrical behaviors in homojunction IGZO TFTs were analyzed. When the active channel layers were deposited with an oxygen partial pressure of 1.96%, the homojunction IGZO TFTs exhibited optimal transfer and output characteristics with a field-effect mobility of 13.68 cm2V-1s-1. Its sub-threshold swing, threshold voltage and on/off ratio are 0.6 V/decade, 0.61 V and 107, respectively.

The dual-detection mode and improved photoresponse of IGZO-based photodetectors by interfacing with water-soluble biomaterials.

The use of conventional fabrication methods rapidly developed the performance and notable enhancements of optoelectronic devices. However, it proved challenging to develop and demonstrate stable optoelectronic devices with biodegradability and biocompatibility properties towards sustainable development and extensive applications. This study incorporates a water-soluble Cr-phycoerythrin (Cr-PE) biomaterial to observe its optical and electronic properties effects on the pristine indium gallium zinc oxide (IGZO)-based photodetector. The fabricated photodetector demonstrates an extended absorption detection region, enhanced optoelectronic performance, and switchable function properties. The resulting photocurrent and responsivity of the IGZO/Cr-PE structure have increased by 5.7 and 7.1 times as compared to the pristine IGZO photodetector. It was also observed that the photodetector could operate in UV and UV-visible with enhanced optical properties by effectively adding the water-soluble Cr-PE. Also, the sensing region of IGZO photodetector becomes changeable. It exhibits switchable dual detection by alternatively dripping and removing the Cr-PE on the IGZO layer. Different measurement parameters such as detectivity, repeatability, and sensitivity are highlighted to effectively prove the advantage of including Cr-PE on the photodetector structure. This study contributes to understanding the potential functions in improving optoelectronic devices through an environmental-friendly method.

IGZO-decorated ZnO thin films and their application for gas sensing.

In this study, indium-gallium-zinc oxide (IGZO)-decorated ZnO thin films were investigated through the change in IGZO deposition time for the detection of NO2 gas. The atomic layer deposited ZnO on interdigitated Au electrode alumina substrates are decorated with IGZO by controlling the deposition time. The IGZO (ZnO:Ga2O3:In2O3 = 1:1:1 mol. %) polycrystalline target was used for deposition and effect of deposition time was investigated. The sensor responses (Rgas/Rair) of 20.6, 39.3, and 57.1 and 45.2, 102.5, and 243.5 were obtained at 150 °C, 200 °C, and 250 °C and 25-ppm NO2 concentration for ZnO (Z1) and IGZO-decorated ZnO (Z3) films, respectively. The sensor response (Rgas/Rair) increased from ∼27 to 243.5 by decorating the ZnO film with IGZO for a 60-s sputtering time. The sensor recovery and response times of the IGZO-decorated ZnO/ZnO sensor increased, and the sensor selectivity to different gases was also evaluated.

Flexible Light-to-Frequency Conversion Circuits Built with Si-Based Frequency-to-Digital Converters via Complementary Photosensitive Ring Oscillators with p-Type SWNT and n-Type a-IGZO Thin Film Transistors.

In this study, as system-level photodetectors, light-to-frequency conversion circuits (LFCs) are realized by i) photosensitive ring oscillators (ROs) composed of amorphous indium-gallium-zinc-oxide/single-walled carbon nanotube (a-IGZO/SWNT) thin film transistors (TFTs) and ii) phase-locked-loop Si circuits built with frequency-to-digital converters (PFDC). The 3-stage ROs and logic gates based on a-IGZO/SWNT TFTs successfully demonstrate its performance on flexible substrates. Herein, along with the advantage of scalability, a-IGZO films are used as photosensitive n-type TFTs and SWNTs are employed as photo-insensitive p-type TFTs for better photosensitivity in circuit level. Through the controlling a post-annealing condition of a-IGZO film, responsivities and detectivities of a-IGZO TFTs are obtained as 36 AW-1 and 0.3 × 1012 Jones for red, 93 AW-1 and 3.1 × 1012 Jones for green, and 194 AW-1 and 11.7 × 1012 Jones for blue. Furthermore, as an advanced demonstration for practical application of LFCs, a unique circuit (i.e., PFDC) is designed to analyze the generated oscillation frequency (fosc ) from the LFC device and convert it to a digital code. As a result, the designed PFDC can exactly count the generated fosc from the flexible a-IGZO/SWNT ROs under light illumination with an outstanding sensitivity and assign input frequencies to respective digital code.

Large-area, low-voltage, antiambipolar heterojunctions from solution-processed semiconductors.

The emergence of semiconducting materials with inert or dangling bond-free surfaces has created opportunities to form van der Waals heterostructures without the constraints of traditional epitaxial growth. For example, layered two-dimensional (2D) semiconductors have been incorporated into heterostructure devices with gate-tunable electronic and optical functionalities. However, 2D materials present processing challenges that have prevented these heterostructures from being produced with sufficient scalability and/or homogeneity to enable their incorporation into large-area integrated circuits. Here, we extend the concept of van der Waals heterojunctions to semiconducting p-type single-walled carbon nanotube (s-SWCNT) and n-type amorphous indium gallium zinc oxide (a-IGZO) thin films that can be solution-processed or sputtered with high spatial uniformity at the wafer scale. The resulting large-area, low-voltage p-n heterojunctions exhibit antiambipolar transfer characteristics with high on/off ratios that are well-suited for electronic, optoelectronic, and telecommunication technologies.

A Mixture of Negative-, Zero-, and Positive-Differential Transconductance Switching from Tellurium/Indium Gallium Zinc Oxide Heterostructures.

Conventional transistors have long emphasized signal modulation and amplification, often sidelining polarity considerations. However, the recent emergence of negative differential transconductance, characterized by a drain current decline during gate voltage sweeping, has illuminated an unconventional path in transistor technology. This phenomenon promises to simplify the implementation of ternary logic circuits and enhance energy efficiency, especially in multivalued logic applications. Our research has culminated in the development of a sophisticated mixed transconductance transistor (M-T device) founded on a precise Te and IGZO heterojunction. The M-T device exhibits a sequence of intriguing phenomena, zero differential transconductance (ZDT), positive differential transconductance (PDT), and negative differential transconductance (NDT) contingent on applied gate voltage. We clarify its operation using a three-segment equivalent circuit model and validate its viability with IGZO TFT, Te TFT, and Te/IGZO TFT components. In a concluding demonstration, the M-T device interconnected with Te TFT achieves a ternary inverter with an intermediate logic state. Remarkably, this configuration seamlessly transitions into a binary inverter when it is exposed to light.

A Physical TCAD Mobility Model of Amorphous In-Ga-Zn-O (a-IGZO) Devices with Spatially Varying Mobility Edges, Band-Tails, and Enhanced Low-Temperature Convergence.

Amorphous indium gallium zinc oxide (a-IGZO) is becoming an increasingly important technological material. Transport in this material is conceptualized as the heavy disorder of the material causing a conduction or mobility band-edge that randomly varies and undulates in space across the entire system. Thus, transport is envisioned as being dominated by percolation physics as carriers traverse this varying band-edge landscape of "hills" and "valleys". It is then something of a missed opportunity to model such a system using only a compact approach-despite this being the primary focus of the existing literature-as such a system can easily be faithfully reproduced as a true microscopic TCAD model with a real physically varying potential. Thus, in this work, we develop such a "microscopic" TCAD model of a-IGZO and detail a number of key aspects of its implementation. We then demonstrate that it can accurately reproduce experimental results and consider the issue of the addition of non-conducting band-tail states in a numerically efficient manner. Finally, two short studies of 3D effects are undertaken to illustrate the utility of the model: specifically, the cases of variation effects as a function of device size and as a function of surface roughness scattering.

Effect of interface defects on electrical characteristics of a-ITGZO TFTs under bottom, top, and dual gatings.

Here, we investigate the effects of interface defects on the electrical characteristics of amorphous indium-tin-gallium-zinc oxide (a-ITGZO) thin-film transistors (TFTs) utilizing bottom, top, and dual gatings. The field-effect mobility (27.3 cm2/V∙s) and subthreshold swing (222 mV/decade) under a dual gating is substantially better than those under top (12.6 cm2/V∙s, 301 mV/decade) and bottom (11.1 cm2/V∙s, 487 mV/decade) gatings. For an a-ITGZO TFT, oxygen deficiencies are more prevalent in the bottom-gate dielectric interface than in the top-gate dielectric interface, and they are less prevalent inside the channel layer than at the interfaces, indicating that the presence of oxygen deficiencies significantly affects the field-effect mobility and subthreshold swing. Moreover, the variation in the electrical characteristics due to the positive bias stress is discussed here.

Heterogeneous Integration of Complementary Field-Effect Transistors for High-Performance Micro LED Displays.

This study investigates a micro light-emitting diode (µLED) pixel circuit using the heterogeneous integration of complementary field-effect transistors (CFETs). The CFETs are fabricated using a semiconductor layer composed of tellurium (Te) and indium-gallium-zinc oxide (IGZO) layers. Te and IGZO layers in the heterostructure IGZO/Te film exhibit hexagonal and amorphous phases, respectively, indicating that each layer maintains independent material characteristics. The fabricated IGZO/Te CFETs exhibit ambipolar behavior with a turn-on voltage of 2.0 V and field-effect mobility of 0.74 and 1.42 cm2 V-1 s-1 for p-type and n-type channels, respectively. Inverters comprising IGZO/Te CFET and IGZO TFT exhibit inverting behavior. A µLED pixel circuit is designed using IGZO/Te CFETs based on pulse width modulation (PWM). The proposed circuit uses an inverter structure with IGZO/Te and IGZO to control the emission time, suppressing the wavelength shift of µLEDs depending on the µLED current levels. The operation of the proposed pixel circuit is investigated through simulation and measurement of the fabricated circuit. The fabricated µLED pixel circuit successfully exhibits PWM operation, controlling the emission time and luminance. Consequently, IGZO/Te CFETs show promise as devices for high-quality µLED displays.

Tailoring Subthreshold Swing in A-IGZO Thin-Film Transistors for Amoled Displays: Impact of Conversion Mechanism on Peald Deposition Sequences.

Amorphous IGZO (a-IGZO) thin-film transistors (TFTs) are standard backplane electronics to power active-matrix organic light-emitting diode (AMOLED) televisions due to their high carrier mobility and negligible low leakage characteristics. Despite their advantages, limitations in color depth arise from a steep subthreshold swing (SS) (≤ 0.1 V/decade), necessitating costly external compensation for IGZO transistors. For mid-size mobile applications such as OLED tablets and notebooks, it is important to ensure controllable SS value (≥ 0.3 V/decade). In this study, a conversion mechanism during plasma-enhanced atomic layer deposition (PEALD) is proposed as a feasible route to control the SS. When a pulse of a diethylzinc (DEZn) precursor is exposed to the M2O3 (M = In or Ga) surface layer, partial conversion of the underlying M2O3 to ZnO is predicted on the basis of density function theory calculations. Notably, significant distinctions between In-Ga-Zn (Case I) and In-Zn-Ga (Case II) films are observed: Case II exhibits a lower growth rate and larger Ga/In ratio. Case II TFTs with a-IGZO (subcycle ratio of In:Ga:Zn = 3:1:1) show reasonable SS values (313 mV decade-1) and high mobility (µFE) of 29.3 cm2 Vs-1 (Case I: 84 mV decade-1 and 33.4 cm2 Vs-1). The rationale for Case II's reasonable SS values is discussed, attributing it to the plausible formation of In-Zn defects, supported by technology computer-aided design (TCAD) simulations.

Effects of Post-Annealing on the Properties of ZnO:Ga Films with High Transparency (94%) and Low Sheet Resistance (29 Ω/square).

This study presents gallium-doped zinc oxide (ZnO:Ga, GZO) thin films. GZO thin films with both high transparency and low sheet resistance were prepared by RF sputtering and then post-annealed under nitrogen and hydrogen forming gas. With post-annealing at 450 °C, the proposed films with a film thickness of 100 nm showed high transparency (94%), while the sheet resistance of the films was reduced to 29 Ω/square, which was comparable with the performances of commercial indium tin oxide (ITO) samples. Post-annealing under nitrogen and hydrogen forming gas enhanced the films' conductivity while altering the thin-film composition and crystallinity. Nitrogen gas played a role in improving the crystallinity while maintaining the oxygen vacancy of the proposed films, whereas hydrogen did not dope into the thin film, thus maintaining its transparency. Furthermore, hydrogen lowered the resistance of GZO thin films during the annealing process. Then, the detailed mechanisms were discussed. Hydrogen post-annealing helped in the removal of oxygen, therefore increasing the Ga3+ content, which provided extra electrons to lower the resistivity of the films. After the preferable nitrogen/hydrogen forming gas treatment, our proposed films maintained high transparency and low sheet resistance, thus being highly useful for further opto-electronic applications.

Effect of Channel Shape on Performance of Printed Indium Gallium Zinc Oxide Thin-Film Transistors.

Printing technology will improve the complexity and material waste of traditional deposition and lithography processes in device fabrication. In particular, the printing process can effectively control the functional layer stacking and channel shape in thin-film transistor (TFT) devices. We prepared the patterning indium gallium zinc oxide (IGZO) semiconductor layer with Ga, In, and Zn molar ratios of 1:2:7 on Si/SiO2 substrates. And the patterning source and drain electrodes were printed on the surface of semiconductor layers to construct a TFT device with the top contact and bottom gate structures. To overcome the problem of uniform distribution of applied voltages between electrode centers and edges, we investigated whether the circular arc channel could improve the carrier regulation ability under the field effect in printed TFTs compared with a traditional structure of rectangular symmetry and a rectangular groove channel. The drain current value of the IGZO TFT with a circular arc channel pattern was significantly enhanced compared to that of a TFT with rectangular symmetric source/drain electrodes under the corresponding drain-source voltage and gate voltage. The field effect properties of the device were obviously improved by introducing the arc-shaped channel structure.

Light-Stimulated IGZO Transistors with Tunable Synaptic Plasticity Based on Casein Electrolyte Electric Double Layer for Neuromorphic Systems.

In this study, optoelectronic synaptic transistors based on indium-gallium-zinc oxide (IGZO) with a casein electrolyte-based electric double layer (EDL) were examined. The casein electrolyte played a crucial role in modulating synaptic plasticity through an internal proton-induced EDL effect. Thus, important synaptic behaviors, such as excitatory post-synaptic current, paired-pulse facilitation, and spike rate-dependent and spike number-dependent plasticity, were successfully implemented by utilizing the persistent photoconductivity effect of the IGZO channel stimulated by light. The synergy between the light stimulation and the EDL effect allowed the effective modulation of synaptic plasticity, enabling the control of memory levels, including the conversion of short-term memory to long-term memory. Furthermore, a Modified National Institute of Standards and Technology digit recognition simulation was performed using a three-layer artificial neural network model, achieving a high recognition rate of 90.5%. These results demonstrated a high application potential of the proposed optoelectronic synaptic transistors in neuromorphic visual systems.

Charge Trapping and Emission Properties in CAAC-IGZO Transistor: A First-Principles Calculations.

The c-axis aligned crystalline indium-gallium-zinc-oxide field-effect transistor (CAAC-IGZO FET), exhibiting an extremely low off-state leakage current (~10-22 A/µm), has promised to be an ideal candidate for Dynamic Random Access Memory (DRAM) applications. However, the instabilities leaded by the drift of the threshold voltage in various stress seriously affect the device application. To better develop high performance CAAC-IGZO FET for DRAM applications, it's essential to uncover the deep physical process of charge transport mechanism in CAAC-IGZO FET. In this work, by combining the first-principles calculations and nonradiative multiphonon theory, the charge trapping and emission properties in CAAC-IGZO FET have been systematically investigated. It is found that under positive bias stress, hydrogen interstitial in Al2O3 gate dielectric is probable effective electron trap center, which has the transition level (ε (+1/-1) = 0.52 eV) above Fermi level. But it has a high capture barrier about 1.4 eV and low capture rate. Under negative bias stress, oxygen vacancy in Al2O3 gate dielectric and CAAC-IGZO active layer are probable effective electron emission centers whose transition level ε (+2/0) distributed at -0.73~-0.98 eV and 0.69 eV below Fermi level. They have a relatively low emission barrier of about 0.5 eV and 0.25 eV and high emission rate. To overcome the instability in CAAC-IGZO FET, some approaches can be taken to control the hydrogen concentration in Al2O3 dielectric layer and the concentration of the oxygen vacancy. This work can help to understand the mechanisms of instability of CAAC-IGZO transistor caused by the charge capture/emission process.

Approaches for 3D Integration Using Plasma-Enhanced Atomic-Layer-Deposited Atomically-Ordered InGaZnO Transistors with Ultra-High Mobility.

As the scale-down and power-saving of silicon-based channel materials approach the limit, oxide semiconductors are being actively researched for applications in 3D back-end-of-line integration. For these applications, it is necessary to develop stable oxide semiconductors with electrical properties similar to those of Si. Herein, a single-crystal-like indium-gallium-zinc-oxide (IGZO) layer (referred to as a pseudo-single-crystal) is synthesized using plasma-enhanced atomic layer deposition and fabricated stable IGZO transistors with an ultra-high mobility of over 100 cm2  Vs-1 . To acquire high-quality atomic layer deposition-processed IGZO layers, the plasma power of the reactant is controlled as an effective processing parameter by evaluating and understanding the effect of the chemical reaction of the precursors on the behavior of the residual hydrogen, carbon, and oxygen in the as-deposited films. Based on these insights, this study found that there is a critical relationship between the optimal plasma reaction energy, superior electrical performance, and device stability.

Device-Algorithm Co-Optimization for an On-Chip Trainable Capacitor-Based Synaptic Device with IGZO TFT and Retention-Centric Tiki-Taka Algorithm.

Analog in-memory computing synaptic devices are widely studied for efficient implementation of deep learning. However, synaptic devices based on resistive memory have difficulties implementing on-chip training due to the lack of means to control the amount of resistance change and large device variations. To overcome these shortcomings, silicon complementary metal-oxide semiconductor (Si-CMOS) and capacitor-based charge storage synapses are proposed, but it is difficult to obtain sufficient retention time due to Si-CMOS leakage currents, resulting in a deterioration of training accuracy. Here, a novel 6T1C synaptic device using only n-type indium gaIlium zinc oxide thin film transistor (IGZO TFT) with low leakage current and a capacitor is proposed, allowing not only linear and symmetric weight update but also sufficient retention time and parallel on-chip training operations. In addition, an efficient and realistic training algorithm to compensate for any remaining device non-idealities such as drifting references and long-term retention loss is proposed, demonstrating the importance of device-algorithm co-optimization.

Nanowire-Enhanced Fully Transparent and Flexible Indium Gallium Zinc Oxide Transistors with Chitosan Hydrogel Gate Dielectric: A Pathway to Improved Synaptic Properties.

In this study, a transparent and flexible synaptic transistor was fabricated based on a random-network nanowire (NW) channel made of indium gallium zinc oxide. This device employs a biocompatible chitosan-based hydrogel as an electrolytic gate dielectric. The NW structure, with its high surface-to-volume ratio, facilitated a more effective modulation of the channel conductance induced by protonic-ion polarization. A comparative analysis of the synaptic properties of NW- and film-type devices revealed the distinctive features of the NW-type configuration. In particular, the NW-type synaptic transistors exhibited a significantly larger hysteresis window under identical gate-bias conditions. Notably, these transistors demonstrated enhanced paired-pulse facilitation properties, synaptic weight modulation, and transition from short- to long-term memory. The NW-type devices displayed gradual potentiation and depression of the channel conductance and thus achieved a broader dynamic range, improved linearity, and reduced power consumption compared with their film-type counterparts. Remarkably, the NW-type synaptic transistors exhibited impressive recognition accuracy outcomes in Modified National Institute of Standards and Technology pattern-recognition simulations. This characteristic enhances the efficiency of practical artificial intelligence (AI) processes. Consequently, the proposed NW-type synaptic transistor is expected to emerge as a superior candidate for use in high-efficiency artificial neural network systems, thus making it a promising technology for next-generation AI semiconductor applications.