Smart pH Sensing: A Self-Sensitivity Programmable Platform with Multi-Functional Charge-Trap-Flash ISFET Technology.

This study presents a novel pH sensor platform utilizing charge-trap-flash-type metal oxide semiconductor field-effect transistors (CTF-type MOSFETs) for enhanced sensitivity and self-amplification. Traditional ion-sensitive field-effect transistors (ISFETs) face challenges in commercialization due to low sensitivity at room temperature, known as the Nernst limit. To overcome this limitation, we explore resistive coupling effects and CTF-type MOSFETs, allowing for flexible adjustment of the amplification ratio. The platform adopts a unique approach, employing CTF-type MOSFETs as both transducers and resistors, ensuring efficient sensitivity control. An extended-gate (EG) structure is implemented to enhance cost-effectiveness and increase the overall lifespan of the sensor platform by preventing direct contact between analytes and the transducer. The proposed pH sensor platform demonstrates effective sensitivity control at various amplification ratios. Stability and reliability are validated by investigating non-ideal effects, including hysteresis and drift. The CTF-type MOSFETs' electrical characteristics, energy band diagrams, and programmable resistance modulation are thoroughly characterized. The results showcase remarkable stability, even under prolonged and repetitive operations, indicating the platform's potential for accurate pH detection in diverse environments. This study contributes a robust and stable alternative for detecting micro-potential analytes, with promising applications in health management and point-of-care settings.

Pushing the Limits of Biosensing: Selective Calcium Ion Detection with High Sensitivity via High-k Gate Dielectric Engineered Si Nanowire Random Network Channel Dual-Gate Field-Effect Transistors.

Calcium ions (Ca2+) are abundantly present in the human body; they perform essential roles in various biological functions. In this study, we propose a highly sensitive and selective biosensor platform for Ca2+ detection, which comprises a dual-gate (DG) field-effect transistor (FET) with a high-k engineered gate dielectric, silicon nanowire (SiNW) random network channel, and Ca2+-selective extended gate. The SiNW channel device, which was fabricated via the template transfer method, exhibits superior Ca2+ sensing characteristics compared to conventional film channel devices. An exceptionally high Ca2+ sensitivity of 208.25 mV/dec was achieved through the self-amplification of capacitively coupled DG operation and an enhanced amplification ratio resulting from the high surface-to-volume ratio of the SiNW channel. The SiNW channel device demonstrated stable and reliable sensing characteristics, as evidenced by minimal hysteresis and drift effects, with the hysteresis voltage and drift rate measuring less than 6.53% of the Ca2+ sensitivity. Furthermore, the Ca2+-selective characteristics of the biosensor platform were elucidated through experiments with pH buffer, NaCl, and KCl solutions, wherein the sensitivities of the interfering ions were below 7.82% compared to the Ca2+ sensitivity. The proposed Ca2+-selective biosensor platform exhibits exceptional performance and holds great potential in various biosensing fields.

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.

Implementation of Highly Stable Memristive Characteristics in an Organic-Inorganic Hybrid Resistive Switching Layer of Chitosan-Titanium Oxide with Microwave-Assisted Oxidation.

This study proposes a high-performance organic-inorganic hybrid memristor for the development of neuromorphic devices in the memristor-based artificial synapse. The memristor consists of a solid polymer electrolyte (SPE) chitosan layer and a titanium oxide (TiOx) layer grown with a low-thermal-budget, microwave-assisted oxidation. The fabricated Ti/SPE-chitosan/TiOx/Pt-structured memristor exhibited steady bipolar resistive switching (BRS) characteristics and demonstrated excellent endurance in 100-cycle repetition tests. Compared to SPE-chitosan memristors without a TiOx layer, the proposed organic-inorganic hybrid memristor demonstrated a higher dynamic range and a higher response to pre-synaptic stimuli such as short-term plasticity via paired-pulse facilitation. The effect of adding the TiOx layer on the BRS properties was examined, and the results showed that the TiOx layer improved the chemical and electrical superiority of the proposed memristor synaptic device. The proposed SPE-chitosan organic-inorganic hybrid memristor also exhibited a stable spike-timing-dependent plasticity, which closely mimics long-term plasticity. The potentiation and depression behaviors that modulate synaptic weights operated stably via repeated spike cycle tests. Therefore, the proposed SPE-chitosan organic-inorganic hybrid memristor is a promising candidate for the development of neuromorphic devices in memristor-based artificial synapses owing to its excellent stability, high dynamic range, and superior response to pre-synaptic stimuli.

Time-Dependent Sensitivity Tunable pH Sensors Based on the Organic-Inorganic Hybrid Electric-Double-Layer Transistor.

In this study, we propose tunable pH sensors based on the electric-double-layer transistor (EDLT) with time-dependent sensitivity characteristics. The EDLT is able to modulate the drain current by using the mobile ions inside the electrolytic gate dielectric. This property allows the implementation of a device with sensitivity characteristics that are simply adjusted according to the measurement time. An extended gate-type, ion-sensitive, field-effect transistor consisting of a chitosan/Ta2O5 hybrid dielectric EDLT transducer, and an SnO2 sensing membrane, were fabricated to evaluate the sensing behavior at different buffer pH levels. As a result, we were able to achieve tunable sensitivity by only adjusting the measurement time by using a single EDLT and without additional gate electrodes. In addition, to demonstrate the unique sensing behavior of the time-dependent tunable pH sensors based on organic−inorganic hybrid EDLT, comparative sensors consisting of a normal FET with a SiO2 gate dielectric were prepared. It was found that the proposed pH sensors exhibit repeatable and stable sensing operations with drain current deviations <1%. Therefore, pH sensors using a chitosan electrolytic EDLT are suitable for biosensor platforms, possessing tunable sensitivity and high-reliability characteristics.

Biocompatible Potato-Starch Electrolyte-Based Coplanar Gate-Type Artificial Synaptic Transistors on Paper Substrates.

In this study, we propose the use of artificial synaptic transistors with coplanar-gate structures fabricated on paper substrates comprising biocompatible and low-cost potato-starch electrolyte and indium-gallium-zinc oxide (IGZO) channels. The electrical double layer (EDL) gating effect of potato-starch electrolytes enabled the emulation of biological synaptic plasticity. Frequency dependence measurements of capacitance using a metal-insulator-metal capacitor configuration showed a 1.27 µF/cm2 at a frequency of 10 Hz. Therefore, strong capacitive coupling was confirmed within the potato-starch electrolyte/IGZO channel interface owing to EDL formation because of internal proton migration. An electrical characteristics evaluation of the potato-starch EDL transistors through transfer and output curve resulted in counterclockwise hysteresis caused by proton migration in the electrolyte; the hysteresis window linearly increased with maximum gate voltage. A synaptic functionality evaluation with single-spike excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), and multi-spike EPSC resulted in mimicking short-term synaptic plasticity and signal transmission in the biological neural network. Further, channel conductance modulation by repetitive presynaptic stimuli, comprising potentiation and depression pulses, enabled stable modulation of synaptic weights, thereby validating the long-term plasticity. Finally, recognition simulations on the Modified National Institute of Standards and Technology (MNIST) handwritten digit database yielded a 92% recognition rate, thereby demonstrating the applicability of the proposed synaptic device to the neuromorphic system.

Highly Sensitive and Transparent Urea-EnFET Based Point-of-Care Diagnostic Test Sensor with a Triple-Gate a-IGZO TFT.

In this study, we propose a highly sensitive transparent urea enzymatic field-effect transistor (EnFET) point-of-care (POC) diagnostic test sensor using a triple-gate amorphous indium gallium zinc oxide (a-IGZO) thin-film pH ion-sensitive field-effect transistor (ISFET). The EnFET sensor consists of a urease-immobilized tin-dioxide (SnO2) sensing membrane extended gate (EG) and an a-IGZO thin film transistor (TFT), which acts as the detector and transducer, respectively. To enhance the urea sensitivity, we designed a triple-gate a-IGZO TFT transducer with a top gate (TG) at the top of the channel, a bottom gate (BG) at the bottom of the channel, and a side gate (SG) on the side of the channel. By using capacitive coupling between these gates, an extremely high urea sensitivity of 3632.1 mV/pUrea was accomplished in the range of pUrea 2 to 3.5; this is 50 times greater than the sensitivities observed in prior works. High urea sensitivity and reliability were even obtained in the low pUrea (0.5 to 2) and high pUrea (3.5 to 5) ranges. The proposed urea-EnFET sensor with a triple-gate a-IGZO TFT is therefore expected to be useful for POC diagnostic tests that require high sensitivity and high reliability.

Highly Sensitive and Selective Sodium Ion Sensor Based on Silicon Nanowire Dual Gate Field-Effect Transistor.

In this study, a highly sensitive and selective sodium ion sensor consisting of a dual-gate (DG) structured silicon nanowire (SiNW) field-effect transistor (FET) as the transducer and a sodium-selective membrane extended gate (EG) as the sensing unit was developed. The SiNW channel DG FET was fabricated through the dry etching of the silicon-on-insulator substrate by using electrospun polyvinylpyrrolidone nanofibers as a template for the SiNW pattern transfer. The selectivity and sensitivity of sodium to other ions were verified by constructing a sodium ion sensor, wherein the EG was electrically connected to the SiNW channel DG FET with a sodium-selective membrane. An extremely high sensitivity of 1464.66 mV/dec was obtained for a NaCl solution. The low sensitivities of the SiNW channel FET-based sodium ion sensor to CaCl2, KCl, and pH buffer solutions demonstrated its excellent selectivity. The reliability and stability of the sodium ion sensor were verified under non-ideal behaviors by analyzing the hysteresis and drift. Therefore, the SiNW channel DG FET-based sodium ion sensor, which comprises a sodium-selective membrane EG, can be applied to accurately detect sodium ions in the analyses of sweat or blood.

Fully Transparent and Sensitivity-Programmable Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistor-Based Biosensor Platforms with Resistive Switching Memories.

This paper presents a fully transparent and sensitivity-programmable biosensor based on an amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) with embedded resistive switching memories (ReRAMs). The sensor comprises a control gate (CG) and a sensing gate (SG), each with a resistive switching (RS) memory connected, and a floating gate (FG) that modulates the channel conductance of the a-IGZO TFT. The resistive coupling between the RS memories connected to the CG and SG produces sensitivity properties that considerably exceed the limit of conventional ion-sensitive field-effect transistor (ISFET)-based sensors. The resistances of the embedded RS memories were determined by applying a voltage to the CG-FG and SG-FG structures independently and adjusting the compliance current. Sensors constructed using RS memories with different resistance ratios yielded a pH sensitivity of 50.5 mV/pH (RCG:RSG = 1:1), 105.2 mV/pH (RCG:RSG = 2:1), and 161.9 mV/pH (RCG:RSG = 3:1). Moreover, when the RCG:RSG = 3:1, the hysteresis voltage width (VH) and drift rate were 54.4 mV and 32.9 mV/h, respectively. As the increases in VH and drift rate are lower than the amplified sensitivity, the sensor performs capably. The proposed device is viable as a versatile sensing device capable of detecting various substances, such as cells, antigens, DNA, and gases.

Memristive Switching Characteristics in Biomaterial Chitosan-Based Solid Polymer Electrolyte for Artificial Synapse.

This study evaluated the memristive switching characteristics of a biomaterial solid polymer electrolyte (SPE) chitosan-based memristor and confirmed its artificial synaptic behavior with analog switching. Despite the potential advantages of organic memristors for high-end electronics, the unstable multilevel states and poor reliability of organic devices must be overcome. The fabricated Ti/SPE-chitosan/Pt-structured memristor has stable bipolar resistive switching (BRS) behavior due to a cation-based electrochemical reaction between a polymeric electrolyte and metal ions and exhibits excellent endurance in 5 × 102 DC cycles. In addition, we achieved multilevel per cell (MLC) BRS I-V characteristics by adjusting the set compliance current (Icc) for analog switching. The multilevel states demonstrated uniform resistance distributions and nonvolatile retention characteristics over 104 s. These stable MLC properties are explained by the laterally intensified conductive filaments in SPE-chitosan, based on the linear relationship between operating voltage margin (ΔVswitching) and Icc. In addition, the multilevel resistance dependence on Icc suggests the capability of continuous analog resistance switching. Chitosan-based SPE artificial synapses ensure the emulation of short- and long-term plasticity of biological synapses, including excitatory postsynaptic current, inhibitory postsynaptic current, paired-pulse facilitation, and paired-pulse depression. Furthermore, the gradual conductance modulations upon repeated stimulation by 104 electric pulses were evaluated in high stability.

Resistive Switching Characteristic Improvement in a Single-Walled Carbon Nanotube Random Network Embedded Hydrogen Silsesquioxane Thin Films for Flexible Memristors.

In this study, we evaluated the improved memristive switching characteristics of hydrogen silsesquioxane (HSQ) nanocomposites embedded with a single-walled carbon nanotube (SWCNT) random network. A low-temperature solution process was implemented using a flexible memristor device on a polyethylene naphthalate (PEN) substrate. The difference in the resistive switching (RS) behavior due to the presence of the SWCNT random network was analyzed by the current transport mechanism. Such a random network not only improves the RS operation but also facilitates a stable multilevel RS performance. The multiple-resistance states exhibited highly reliable nonvolatile retention properties over 104 s at room temperature (25 °C) and at a high temperature (85 °C), showing the possibility of an analog synaptic weight modulation. Consequently, the gradual weight potentiation/depression was realized through 3 × 102 synaptic stimulation pulses. These findings suggest that the embedded SWCNT random network can improve the synaptic weight modulation characteristics with high stability for an artificial synapse and hence can be used in future neuromorphic circuits.

Lithography Processable Ta2O5 Barrier-Layered Chitosan Electric Double Layer Synaptic Transistors.

We proposed a synaptic transistor gated using a Ta2O5 barrier-layered organic chitosan electric double layer (EDL) applicable to a micro-neural architecture system. In most of the previous studies, a single layer of chitosan electrolyte was unable to perform lithography processes due to poor mechanical/chemical resistance. To overcome this limitation, we laminated a high-k Ta2O5 thin film on chitosan electrolyte to ensure high mechanical/chemical stability to perform a lithographic process for micropattern formation. Artificial synaptic behaviors were realized by protonic mobile ion polarization in chitosan electrolytes. In addition, neuroplasticity modulation in the amorphous In-Ga-Zn-oxide (a-IGZO) channel was implemented by presynaptic stimulation. We also demonstrated synaptic weight changes through proton polarization, excitatory postsynaptic current modulations, and paired-pulse facilitation. According to the presynaptic stimulations, the magnitude of mobile proton polarization and the amount of weight change were quantified. Subsequently, the stable conductance modulation through repetitive potential and depression pulse was confirmed. Finally, we consider that proposed synaptic transistor is suitable for advanced micro-neural architecture because it overcomes the instability caused when using a single organic chitosan layer.

Sol-Gel Composites-Based Flexible and Transparent Amorphous Indium Gallium Zinc Oxide Thin-Film Synaptic Transistors for Wearable Intelligent Electronics.

In this study, we propose the fabrication of sol-gel composite-based flexible and transparent synaptic transistors on polyimide (PI) substrates. Because a low thermal budget process is essential for the implementation of high-performance synaptic transistors on flexible PI substrates, microwave annealing (MWA) as a heat treatment process suitable for thermally vulnerable substrates was employed and compared to conventional thermal annealing (CTA). In addition, a solution-processed wide-bandgap amorphous In-Ga-Zn (2:1:1) oxide (a-IGZO) channel, an organic polymer chitosan electrolyte-based electric double layer (EDL), and a high-k Ta2O5 thin-film dielectric layer were applied to achieve high flexibility and transparency. The essential synaptic plasticity of the flexible and transparent synaptic transistors fabricated with the MWA process was demonstrated by single spike, paired-pulse facilitation, multi-spike facilitation excitatory post-synaptic current (EPSC), and three-cycle evaluation of potentiation and depression behaviors. Furthermore, we verified the mechanical robustness of the fabricated device through repeated bending tests and demonstrated that the electrical properties were stably maintained. As a result, the proposed sol-gel composite-based synaptic transistors are expected to serve as transparent and flexible intelligent electronic devices capable of stable neural operation.

High-performance extended-gate ion-sensitive field-effect transistors with multi-gate structure for transparent, flexible, and wearable biosensors.

In this study, we developed a high-performance extended-gate ion-sensitive field-effect transistor (EG-ISFET) sensor on a flexible polyethylene naphthalate (PEN) substrate. The EG-ISFET sensor comprises a tin dioxide (SnO2) extended gate, which acts as a detector, and an amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) for a transducer. In order to self-amplify the sensitivity of the pH sensors, we designed a uniquely-structured a-IGZO TFT transducer with a high-k engineered top gate insulator consisting of a silicon dioxide/tantalum pentoxide (SiO2/Ta2O5) stack, a floating layer under the channel, and a control gate coplanar with the channel. The SiO2/Ta2O5 stacked top gate insulator and in-plane control gate significantly contribute to capacitive coupling, enabling the amplification of sensitivity to be enlarged compared to conventional dual-gate transducers. For a pH sensing method suitable for EG-ISFET sensors, we propose an in-plane control gate (IG) sensing mode, instead of conventional single-gate (SG) or dual-gate (DG) sensing modes. As a result, a pH sensitivity of 2364 mV/pH was achieved at room temperature - this is significantly superior to the Nernstian limit (59.15 mV/pH at room temperature). In addition, we found that non-ideal behavior was improved in hysteresis and drift measurements. Therefore, the proposed transparent EGISFFET sensor with an IG sensing mode is expected to become a promising platform for flexible and wearable biosensing applications.

Investigation of Multi-Level Cell Characteristic in Amorphous Indium-Gallium-Zinc Oxide Thin-Film-Transistor Based 1T-1R Non-Volatile Memory Device.

In this work, we have implemented nonvolatile resistive random access memory (ReRAM) cells consisting of one transistor (1T) and one resistor (1R) configuration by integrating a high-performance InGaZnO (IGZO) thin-film-transistor (TFT) and an HfO2 resistive switching component on a single substrate for system-on-panel (SoP) technology. ReRAM cells with a 1T-1R configuration can reduce undesired crosstalk caused by leakage current between adjacent memory cells. The a-IGZO TFTs used for 1T-1R integration showed excellent electrical characteristics with a high field-effect mobility of 10.8 cm²/V s, a low subthreshold swing of 226 mV/dec and a large on-off current ratio of 9.2 × 107. The 1T-1R integrated memory cells exhibited BRS behavior, uniform distribution of resistance states and operating voltage, stable DC endurance and reliable data retention characteristics. In addition, we obtained MLC characteristics in a 1T-1R integrated memory cell by controlling the driving current with the gate voltage of a-IGZO TFTs. In multi-level operation, it showed a low operating voltage of 1 V, stable endurance and reliable retention characteristics at 85 °C.

Effect of Microwave Annealing on the Interface Properties Between the Top Silicon and Buried Oxide Layers in Silicon-on-Insulator MOSFETs.

In this paper, a post-annealing process that uses microwaves to remove the back-interface thermal damage at the interface between the top silicon layer and the buried oxide layer of a silicon-on-insulator (SOI) device caused during rapid thermal annealing (RTA) was studied. RTA, which is widely used in highly integrated short-channel silicon device manufacturing processes, deteriorates the electrical characteristics of SOI pseudo metal-oxide-semiconductor field-effect transistors (MOSFET) by affecting the interfacial properties between the top silicon and buried oxide layers. In order to replenish these interfacial properties, microwave annealing (MWA) was performed on the device under various conditions. Because MWA utilizes the direct energy transfer (DET) method, the RTA-induced thermal damage on the back interface was effectively removed despite a short processing time. The improvement was comparable to that of conventional thermal annealing at a higher temperature for a long period of time. Therefore, MWA is expected to be a very effective post-RTA heat-treatment technology for fabricating ultrathin-body SOI MOSFETs because of its low thermal budget.

Effects of Al Addition on Resistive-Switching Characteristics of Solution Processed Zn-Sn-O ReRAMs.

Resistive random access memory (ReRAM) using amorphous Al-doped zinc tin oxide (a-AZTO) as resistive switching layer was fabricated by solution based deposition method. The a-AZTO films with different compositions (Al:Zn:Sn 0:1:1, 0.1:1:1 and 0.2:1:1) were prepared by solution based deposition method using low process temperature. The developed a-AZTO resistive switching nonvolatile memory devices with a Ti/Al-Zn-Sn-O (AZTO)/Pt structure exhibited typical bipolar switching behavior, remaining in a stable resistance state with exhibiting switching responses over 100 cycles, and a retention time of 104 s. The resistive switching characteristics of a-AZTO ReRAMs were improved by increasing Al concentration. We demonstrate the electrical characteristics of a-AZTO TFTs are comparable to IGZO TFTs. The replacement of IGZO film with AZTO film can effectively reduce the consumption of indium (In), a toxic and rare metal, prominently reducing the production cost of transparent amorphous oxide semiconductor (TAOS) based electrical devices and improving environmental safety. Therefore, it is expected that the a-AZTO ReRAMs integrated with a-AZTO TFTs in the backplane of emerging transparent flat panel displays (FPDs) is potentially beneficial for the realization of system-on-panel (SoP) applications.

Effects of Microwave Annealing on High-k Dielectric HfO2 Thin Films.

In this study, the effect of microwave annealing (MWA), which has been attracting considerable attention as a low-thermal-budget method, on the high-k materials required for next-generation ultrahigh-integration semiconductor devices is investigated. Metal-oxide-semiconductor (MOS) capacitor-structure devices are fabricated using HfO2 films, which are typical high-k materials; post deposition annealing (PDA) is then performed by MWA and compared to conventional thermal annealing (CTA). The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the fabricated HfO2 MOS capacitors are measured to evaluate the leakage current, time-zero dielectric breakdown (TZDB), time-dependent dielectric breakdown (TDDB), and frequency dispersion. Compared to the as-deposited state, the PDA-treated HfO2 dielectric film is found to have reduced trap density, improved dielectric breakdown, frequency dispersion, and lifetime. In particular, the high-k HfO2 thin films treated with MWA exhibit excellent surface properties, including a low root-mean square roughness (Rq) of 0.141 nm, thickness of 49.3 nm, and low inter face trap density (Dit) of 7.82 × 1012 cm-2eV-1 and electrical properties such as a high breakdown electric field (EBD) of 10 MV/cm, and high breakdown to charge (QBD) of 1,000 C/cm². As a result, the MWA-treated high-k HfO2 thin films are smoother and denser than the CTA-treated thin films, and show better electrical properties and reliability, suggesting that the defects at Si the interface as well as in the high-k films are effectively removed.

Silicon-on-Insulator Double-Gate Ion-Sensitive Field-Effect Transistors Using Flexible Paper Substrate-Based Extended Gate for Cost-Effective Sensor Applications.

We developed ion-sensitive field effect transistors (ISFETs) with disposable paper extended gates (EGs). The sensing and the measuring part of conventional ISFETs are not separated and integrated on the same device. Therefore, if the sensing part is contaminated by reaction in a chemical solution, there is a problem that an expensive measuring part manufactured through a complicated process becomes also unusable. To overcome this problem and provide a cost-effective sensor platform, we constructed a high-sensitivity ISFET sensor with a measuring transistor part and a separate EG sensing part. In particular, in this experiment, a double-gate transistor capable of amplifying the sensitivity using the capacitive coupling effect between a top-gate and a bottom-gate, which differs from a general single gate transistor on an SOI substrate, was fabricated. As a result, the pH sensitivity of 1199.92 ± 32.4 mV/pH could be achieved using paper EG and dual-gate mode sensing operation, which greatly exceeds the theoretical Nernst limit (59.15 mV/pH at 25 °C) in single gate mode sensing operation. We also measured non-ideal effects such as the hysteresis and drift behavior of paper EGs, and demonstrated that they have excellent stability and reliability for long-term measurements. In addition, hysteresis and sensitivity were measured after 3, 7, 14 and 30 days to verify the aging effect of the continuous use of paper EGs. As a result, paper EGs showed stable operating characteristics for 30 days. Therefore, we expect that double-gate ISFETs with flexible paper EGs will have a significant impact on label-free, low-environmental impact, cost-effective, disposable, and flexible FET-based biosensor applications.

Improvement in Resistance Switching of SiC-Based Nonvolatile Memory by Solution-Deposited HfOx Thin Film.

In this study, we implemented a resistance change memory (ReRAM) device using a SiC layer with excellent physical properties. We fabricated devices composed of Ti/SiC/Pt and Ti/HfOx/SiC/Pt structures and investigated their memory characteristics. The Ti/SiC/Pt ReRAM devices exhibited stable bipolar resistive switching characteristics but had a relatively small memory window, whereas the Ti/HfOx/SiC/Pt ReRAM devices had a large memory window and low operating voltage. In addition, the Ti/HfOx/SiC/Pt ReRAM devices exhibited stable endurance characteristics over 500 cycles and excellent retention characteristics at room temperature and high temperatures for 1×104 s. Further, the Ti/HfOx/SiC/Pt ReRAM devices exhibited multi-level conduction states by modulating the reset stop voltage, and each resistance level had excellent endurance characteristics. The average transmittance of the HfOx/SiC bilayer in visible light was 87%. Such a high value indicates that the HfOx/SiC bilayer fabricated by the stacking method is expected to be a suitable material for highly reliable nonvolatile memory and transparent electronic devices, even in harsh environments.