Gate Voltage Application Research Articles

Ferroelectric Polarization Enhanced Optoelectronic Synaptic Response of a CuInP2S6 Transistor Structure.

Neuromorphic computing can simulate brain function and is a pivotal element in next-generation computing, providing a potential solution to the limitations brought by the von Neumann bottleneck. Optoelectronic synaptic devices are highly promising tools for simulating biomimetic nervous systems. In this study, we developed an optoelectronic neuromorphic device with a transistor structure constructed using ferroelectric CuInP2S6. Essential synaptic behaviors in this device are observed in response to light and electrical stimuli. The optoferroelectric coupling is revealed, and the highly tunable gate modulation of the charge carrier is realized in a single device. On this basis, the light adaptation of the biological eyes and smarter Pavlovian dogs was implemented successfully and enhanced by ferroelectric polarization. The gate voltage application promotes the migration of additional Cu+ ions in the in-plane direction, thus enhancing the synaptic performance of electrical stimulation. Meanwhile, the processing ability of convolutional kernel noise images in ferroelectric devices has been achieved. Our results offer the important observation and application of ferroelectric polarization-enhanced synaptic properties of a transistor structure and have great potential in promoting the development of two-dimensional van der Waals materials and devices.

(Invited) Extraction of Trap Level in GaN Hemt from Transient Current upon Back Gate Voltage Application

Gallium nitride (GaN) have attracted attention as a high frequency and high voltage power device material because they have wide bandgap and high mobility [1]. However, large number of defects exists in the epitaxial layer. Current collapse originated from the traps at the surface and in the buffer layer is one of the results of the defects [2]. To distinguish the bulk traps from the whole current collapse, back-gate voltage application is commonly used [3]. In this study, we measured the bulk traps by measuring the drain transient current by back gate voltage application.TLM pattern (L/W=120/100 mm) was fabricated on an epitaxial wafer; GaN(2nm)/AlGaN(26nm)/ GaN(150/350nm)/C-GaN/buffer/Si sub. Fig. 1 shows the I d-V B characteristic of the TLM, where a threshold voltage (V th) of -300/-450 V can be extracted. A large clock-wise hysteresis suggests large amount of electron trapping in the epitaxial layer. Fig. 2(a) shows the voltage application sequence for measuring the transient I d 2DEG current. A negatively high V B below the V th was applied to clear all the traps initially exists in the epitaxial layers. Then, a step voltage was applied to a certain voltage, and the transient I d was monitored until the value saturates. Fig. 2(b) shows typical transient I d on V B. To analyze the time constant (t) of the transient, the differential of the I d on logarithm time was used, as the time at the peak signal corresponds to the time constant. Fig. 3(a) shows analysis result, and the peak of this graph correspond to t. On the other hand, the amplitude is related to trap density. Fig. 3(b) shows the Alenius plot of the extracted t, and activation energy from 0.2 to 0.3 eV was detected.In conclusion, we have measured the transient I d of GaN-HEMT device with a step voltage application to the substrate back gate. From analysis, we extracted defect levels with activation energy of 0.2 to 0.3 eV. The method is useful for characterizing the trap density in the epitaxial layers.[1] T. Mizutani, et al., IEEE Trans Electron Devices, 50, 10, 2015 (2003).[2] G. Meneghesso, et al., IEEE Trans Electron Devices, 53, 12, 2932 (2006).[3] X. Chen, et al., Appl. Phys. Lett., 117, 26, 263501 (2020). Figure 1

Optical and electronic signal stabilization of plasmonic fiber optic gate electrodes: towards improved real-time dual-mode biosensing

The use of multimodal readout mechanisms next to label-free real-time monitoring of biomolecular interactions can provide valuable insight into surface-based reaction mechanisms. To this end, the combination of an electrolyte-gated field-effect transistor (EG-FET) with a fiber optic-coupled surface plasmon resonance (FO-SPR) probe serving as gate electrode has been investigated to deconvolute surface mass and charge density variations associated to surface reactions. However, applying an electrochemical potential on such gold-coated FO-SPR gate electrodes can induce gradual morphological changes of the thin gold film, leading to an irreversible blue-shift of the SPR wavelength and a substantial signal drift. We show that mild annealing leads to optical and electronic signal stabilization (20-fold lower signal drift than as-sputtered fiber optic gates) and improved overall analytical performance characteristics. The thermal treatment prevents morphological changes of the thin gold-film occurring during operation, hence providing reliable and stable data immediately upon gate voltage application. Thus, the readout output of both transducing principles, the optical FO-SPR and electronic EG-FET, stays constant throughout the whole sensing time-window and the long-term effect of thermal treatment is also improved, providing stable signals even after 1 year of storage. Annealing should therefore be considered a necessary modification for applying fiber optic gate electrodes in real-time multimodal investigations of surface reactions at the solid-liquid interface.

An Organic Electrochemical Transistor to Monitor Salmonella Growth in Real‐Time

AbstractOrganic electrochemical transistors (OECTs) are used in research and diagnostic applications due to their facile manufacture, scalability, and biocompatibility. In these devices, the source–drain current upon gate voltage application depends on ion concentration in the electrolyte. This study investigates whether an OECT can be employed to monitor bacterial growth since it is known that the concentration of charged species increases in bacterial cultures during growth. A poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate‐based single‐well OECT, compatible with long‐term incubation of bacterial cultures, is fabricated. It is shown that the growth of Salmonella alters the transfer characteristics of the device and demonstrates how it can be applied to monitor growth in real‐time by recording the source–drain current at gate voltage +0.5 V. The signal can also be measured in filtrates of bacterial cultures, devoid of bacterial cells. This suggests that the signal originates from charged metabolic products. Bacterial biofilm formation does not alter the device response. This proof‐of‐principle study presents OECT recordings as an alternative to optical methods, allowing bacterial growth to be monitored in transparent and opaque media alike. By measuring metabolic products rather than bacterial cell multiplication, insight into the stationary phase and other nondividing states may be obtained in the future.

Electronic excitation spectra of organic semiconductor/ionic liquid interface by electrochemical attenuated total reflectance spectroscopy

The interface of organic semiconductor films is of particular importance with respect to various electrochemical devices such as transistors and solar cells. In this study, we developed a new spectroscopic system, namely electrochemical attenuated total reflectance ultraviolet (EC-ATR-UV) spectroscopy, which can access the interfacial area. Ionic liquid-gated organic field-effect transistors (IL-gated OFETs) were successfully fabricated on the ATR prism. Spectral changes of the organic semiconductor were then investigated in relation to the gate voltage application and IL species, and the magnitude of spectral changes was found to correlate positively with the drain current. Additionally, the Stark shifts of not only the organic semiconductor, but also of the IL on the organic semiconductor films were detected. This new method can be applied to other electrochemical devices such as organic thin film solar cells, in which the interfacial region is crucial to their functioning.

Substrate effect on the neuromorphic function of nanoionics-based transistors fabricated using WO3 thin film

This paper reports a systematic study on the device characteristics of neuromorphic transistors fabricated by using RF magnetron sputtered WO3 thin film as a channel layer upon atomically flat Si (100), Si (111) and glass substrates. The synaptic behaviour such as learning and forgetting, which are related to short term memory (STM) and memorizing, related to long term memory (LTM) have been demonstrated based on nanoionics phenomena. The conductance modulation using gate-voltage application conditions controls the synaptic behaviour. The switching period, time between learn and forget, has been optimized with gate voltage of within 3 V for the device fabricated on Si (100) substrate. A systematic investigation of substrate effect on the neuromorphic function of devices fabricated with different substrates reveals that the device fabricated on p-type Si(111) exhibits significantly shorter switching ratio, which makes a possibility to modify the device function by the choice of substrate.

Low Gilbert damping in epitaxial thin films of the nodal-line semimetal D03−Fe3Ga

$D{0}_{3}$-ordered ${\mathrm{Fe}}_{3}\mathrm{Ga}$ has been recently reported to show a giant anomalous Nernst effect due to a large Berry curvature along the interconnected nodal lines concentrated near the Fermi level. This peculiar band structure may be advantageous for spintronics device applications because one can expect a strong response to a gate-voltage application. This study evaluates the Gilbert damping constants, a key parameter governing the magnetization dynamics of epitaxial thin films of $D{0}_{3}\text{\ensuremath{-}}{\mathrm{Fe}}_{3}\mathrm{Ga}$ and bcc-Fe by ferromagnetic resonance measurements. We find that the Gilbert damping constant of $D{0}_{3}\text{\ensuremath{-}}{\mathrm{Fe}}_{3}\mathrm{Ga} [(6.0\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}]$ is relatively low and comparable to that of Fe $[(2.3\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}]$. The low Gilbert damping suggests that the interconnected nodal lines near the Fermi level do not hinder magnetization dynamics, making $D{0}_{3}\text{\ensuremath{-}}{\mathrm{Fe}}_{3}\mathrm{Ga}$ even more attractive as a building block of spintronics devices.

Electric field modulation of exchange bias at the Co/CoOx interface

We demonstrated the electric field (EF) effect on exchange bias (EB) in a perpendicularly magnetized $\mathrm{Co}/\mathrm{Co}{\mathrm{O}}_{x}$ layered structure. The antiferromagnetic $\mathrm{Co}{\mathrm{O}}_{x}$ layer was formed by naturally oxidizing the Co surface. The modulation of the EB field and the coercivity by gate voltage application through a dielectric layer was clearly observed below the blocking temperature ${T}_{\mathrm{B}}$. The modulation ratio of the EB field exhibited strong temperature dependence, and it increased as the temperature approached ${T}_{\mathrm{B}}$. One possible cause of the EB modulation detailed herein is the modulation of the electronic state at the $\mathrm{Co}/\mathrm{Co}{\mathrm{O}}_{x}$ interface, which is fundamentally different from the case in which multiferroic antiferromagnets are used.

Investigation of gating effect in Si spin MOSFET

A gate voltage application in a Si-based spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) modulates spin accumulation voltages, where both electrical conductivity and drift velocity are modified while keeping constant electric current. An unprecedented reduction in the spin accumulation voltages in a Si spin MOSFET under negative gate voltage applications is observed in a high electric bias current regime. To support our claim, the electric bias current dependence of the spin accumulation voltage under the gate voltage applications is investigated in detail and compared with a spin drift diffusion model including the conductance mismatch effect. The drastic decrease in the mobility and spin lifetime in the Si channel is ascribable to the optical phonon emission at the high electric bias current, which consequently reduced the spin accumulation voltage.

Wide-Range Resistance Modulation on a SmNiO3 Chemical Transistor

A SmNiO3 (SNO) chemical field effect transistor (FET) using ion liquid as gate insulator shows large and non-linier resistance change depending on Vg application condition. The resistance modulation corresponds to redox reaction at the interface between ionic liquid and SNO channel (Ni3+→Ni2++hole), however, the quantitative understanding for SNO chemical transistor operation is still lacking. We systematically studied the correlation between Ni valence state and modulated resistance ratio. The channel resistance of the SNO chemical FET changed nonlinearly over a wide range for different temperatures, Vg magnitudes, and Vg application durations. The correlation between the modulated resistance and the Ni valence state was quantitatively revealed using X-ray photoelectron spectroscopy (XPS). A model describing the modulated resistance value (Rmod) with the Vg application conditions: operation temperature (T), a magnitude of gate voltage (Vg) and Vg application duration (t), was proposed in Eq. (1) by considering the kinetics of the reduction reaction on the SNO channel. Rmod=exp[A∙exp[B∙Vg]∙t]----Eq.(1) The parameter A is the Ni2+ creation speed without Vg application, which is experimentally obtained. The parameter B corresponds to the acceleration parameter and is temperature-dependent. Equation (1) allows us to calculate the time development of the resistance value under various Vg application conditions. Figure (a) shows the modulated resistance evaluated as a function of the Vg magnitude and duration time. With increasing Vg and t, the resistance nonlinearly increases. One can see that our proposed model enables the resistance to be predicted for given Vg application conditions and selective resistance modulation over a wide range of resistances has been demonstrated. Figure (b) shows a comparison between the experimental time evolution of the modulated resistance (blue circles) and values calculated from eq. 1 (green triangles) under various Vg application conditions (lower panel). The calculation of the modulated resistance shows good agreement with the experimental values at Vg of 2.6 V (left) and 3.2 V (middle). However, at Vg = 3.6 V (right), the experimentally obtained resistance value (∼104) was 2 orders smaller than the calculated value (∼106). This difference seems to be caused by the simplified model, where the Ni2+ creation speed is assumed to be constant during Vg application. At the actual SNO surface, the speed of Ni2+ creation (Ni3+ consumption) may decrease with the passage of time because of the exhaustion of Ni3+ on the SNO surface. The discrepancy between the experimental and calculated values should increase with Vg. Additionally, at a Vg of more than 3.0 V, the leak current (the current between the gate and drain electrodes) became large, suppressing the operation of the FET [1]. The model was able to quantitatively describe the resistance modulation behavior in the SmNiO3 chemical FET, and wide range resistance control was demonstrated. The control of a chemical FET with flexible and nonlinear responses, which are absent in a conventional FET, is expected to contribute to the generation of new properties, such as biomimetic switches and/or sensors behaving similarly to the human brain. In the presentation, the improvement of resistance modulation efficiency by the channel size reduction on the SMNiO3 chemical transistors will be shown. [1] D. Kawamoto et al., ACS Appl. Electron. Mater., 1 (2019) 82-87. Figure 1

Gate-tunable non-volatile photomemory effect in MoS2 transistors

Non-volatile memory devices have been limited to flash architectures that are complex devices. Here, we present a unique photomemory effect in transistors. The photomemory is based on a photodoping effect—a controlled way of manipulating the density of free charges in monolayer using a combination of laser exposure and gate voltage application. The photodoping promotes changes on the conductance of leading to photomemory states with high memory on/off ratio. Such memory states are non-volatile with an expectation of retaining up to 50% of the information for tens of years. Furthermore, we show that the photodoping is gate-tunable, enabling control of the recorded memory states. Finally, we propose a model to explain the photodoping, and we provide experimental evidence supporting such a phenomenon. In summary, our work includes the phototransistors in the non-volatile memory devices and expands the possibilities of memory application beyond conventional memory architectures.

Correlation between Ni Valence and Resistance Modulation on a SmNiO3 Chemical Transistor

The resistance modulation under various gate voltage (Vg) application conditions was systematically studied for a chemical field effect transistor (FET) composed of a SmNiO3 (SNO) film channel and ...

Electric field effect on magnetism in a MgO/Pd/Co system with a solid-state capacitor structure

The electric field effect on the magnetism in a MgO/Pd/Co system, in which a magnetic moment is induced in the Pd layer owing to the ferromagnetic proximity effect, has been investigated using various experimental methods. An electric field was applied to the surface of the Pd layer through a solid-state HfO2/MgO dielectric bilayer by applying a gate voltage with a back-gating configuration. Changes in the magnetic properties of the system as a result of gate voltage application were detected using magnetization and polar-Kerr effect measurements as well as X-ray absorption and X-ray magnetic circular dichroism (XMCD) spectroscopies. A systematic change in the magnetic moment of the system by the application of a gate voltage is observed. The magnetic hysteresis loops obtained by the polar-Kerr effect measurement and the element-specific XMCD signal at the Co L3-edge clearly show a reproducible change in the coercivity that is dependent on the gate voltage.

Influence of the magnetization reversal mechanism on the electric field modulation of coercivity in Pt/Co structures

We studied the effect of electric field (EF) on the coercivity of two Pt/Co structures with different Co layer thicknesses. Opposite signs of the coercivity change by the gate voltage application are observed in these samples, whereas the sign of the magnetic anisotropy change is the same. We performed direct observations of the domain structures during magnetization reversal under the gate voltage and found that two samples showed significant differences in the reversal process and EF effect. This result indicates that the sign reversal of the electric field effect on coercivity observed in the present structures originates from the difference in the macroscopic magnetization reversal process.

High carrier mobility in quasi-suspended few-layer graphene on printed graphene oxide layers

Heterostructures of graphene (G) or multilayer graphene (MLG) transferred on the graphene oxide (GO) printed layer are considered in the present study. Hillocks with a height of about 60–100 nm are found at the background GO relief of 10–15 nm. Graphene in these heterostructures completely follows the GO relief. The quasi-suspended layers on the hillocks are observed for MLG, and the distance between MLG and GO is estimated up to 20–40 nm. An increase in the MLG thickness is suggested to increase the distance between MLG and GO. Carrier mobility in G/GO heterostructures is found to equal 300–500 cm2/V s. The formation of quasi-suspended MLG/GO structures leads to an increase in the carrier mobility up to 4500 cm2/V s with an increase in the MLG thickness (3–8 nm). The change in the carrier mobility in MLG as a function of voltage sweep direction is also observed. The effect is supposedly connected with the ability of the quasi-suspended layer to corrugate under the gate voltage application. The capsulation of heterostructures using GO films leads to the carrier mobility degradation to 300–500 cm2/V s in one–four weeks. The quasi-suspended structures are promising for flexible and/or printed electronics at the use as graphene channels for sensors, detectors and other applications.

Molecule desorption induced by gate-voltage application in MOS structure

For the first time, we demonstrate desorption from a MOS surface by applying gate voltages (VG). We observed CH4, CO, and CO2 desorption from a MOS (Fe nanofilm/a-SiO2/Si) surface in vacuum only when applying negative VG, suggesting the occurrence of electronic excitation by hot-hole injection. This demonstration is the first step in the application of MOSs to electrically controlled catalysts.

Improvement of conversion efficiency for solar cell with metal–oxide‐semiconductor diode

The energy loss of solar cell due to the recombination of holes and electrons which are excited by the incident solar beam is approximately 20% of all the loss-mechanisms. To recover the energy loss, some promising methods have been researched [1,2] .We presented the novel structure of the solar cell that has the metal-oxide-semiconductor (MOS) diode at the side wall of the electricity generation layer [3] . The purpose of this research is to simulate influence of the field-effect on the recombination of carriers and show the increase of the conversion efficiency of the solar cell. The configuration of the new solar cell is as follows. The gate electrode and oxide surround the pin or pn - n stacked layer. If the positive voltage is applied to the gate electrode, the static potential is formed in the stacked layer. The holes and electrons which are excited by the incident solar beam are separated by the drift potential. The holes and electrons move the inside area far from the oxide/stacked layer interface and in the vicinity of the interface, respectively, and it resulted in the decrease of the recombination probability. The simulation is pursuit by the ATLAS, which is the two-dimensional device simulation frame work, by SILVACO. Ltd. Fig . 1 shows the calculated conversion efficiency for the c-Si or p-Si as the function of the gate voltage. It is found that the conversion efficiency increases by the gate voltage application. The increase ratios of the conversion efficiency under the gate voltage application are 1.62 and 1.56 times for c-Si and p-Si in comparison with non-gate application equal to the conventional structure, respectively. And also, it is found that the effect of the gate voltage application on the conversion efficiency becomes remarkable as the life time increases. The reason of this phenomenon is thought to be due to the prevention of the recombination for the excited carriers by the space

In-situ electron holography of surface potential response to gate voltage application in a sub-30-nm gate-length metal-oxide-semiconductor field-effect transistor

The response of the electrostatic potential distribution within a metal-oxide-semiconductor field-effect transistor (MOSFET) to an external electric field was revealed using electron holography cross-sectional in-situ observation while applying the gate voltage to a transistor scaled down to a 25-nm gate length. Charging effects due to electron irradiation were taken into account by using complementary numerical device simulation. Direct observation of the channel potential and its response to the gate voltage can be used to determine the gate electrode effective work-function for scaled MOSFETs.

Electrical Property of DNA Field-Effect Transistor: Charge Retention Property

We discovered the charge retention property of the field-effect transistor (FET) in a Si gate/SiO2/DNA channel structure. The DNA FET with the Si source and drain showed hole conduction, and the drain current was controlled by the gate voltage application. In addition, the experimental results that currents similar to the space change limited currents (SCLCs) and hysteresis were observed in the drain current–drain voltage (Id–Vd) characteristics indicate that the negative charges captured at the trap sites in the DNA enhance the hole currents. Also, the drain currents increased as the repetition number of the measurement increased. However, by inserting the refresh process of gate voltage application of -50 V between each measurement, the current increase was restrained. This phenomenon indicates that the trap and detrap process of electrons occurs in the DNA channel depending on the gate voltage application. The charge retention mechanism was also discussed.

Two-dimensional electron gas transport properties in AlGaN/GaN single- and double-heterostructure field effect transistors

Two-dimensional electron gas transport properties have been investigated in both AlGaN/GaN single- and AlGaN/GaN/AlGaN double-heterostructure field effect transistors, by means of the Hall effect measurement under the gate-voltage application. In AlGaN/GaN single-heterostructures, the dependencies of the electron mobility on the electron density have systematically been examined from 30 to 400 K. Below room temperature, the mobility has been shown to assume a maximum value at a critical electron density where electrons begin to overflow into the AlGaN barrier layer. Above room temperature, even at high electron densities that exceed the channel electron capacity, the degradation in the mobility has been found to be small. This is a feature favorable for high-power and high-temperature device operation. In AlGaN/GaN/AlGaN double-heterostructures, a striking effect has been observed that the mobility is drastically enhanced compared with that in the AlGaN/GaN single-heterostructure at low temperatures. The dependencies of the mobility on the electron density measured at 4.2 K in both heterostructures have been analyzed from the viewpoint of the electron distribution in the channel. The observed mobility enhancement in the double-heterostructure has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Transport properties specific to AlGaN/GaN single- and double-heterostructures have thus successfully been revealed through the mobility-density relations that have been determined by the Hall effect measurement under the gate-voltage application.