Reverse Bias Mode Research Articles

Suppression of current-induced membrane discharge of bipolar membranes by regulating ion crossover transport

Bipolar membranes (BPMs) exhibit the unique capability to regulate the operating environment of electrochemical system through the water dissociation-combination processes. However, the industrial utilization of BPMs is limited by instability and serious energy consumption. The current-induced membrane discharge (CIMD) at high-current conditions has a negative influence on the performance of anion-exchange membranes, but the underlying ion transport mechanisms in the BPMs remain unclear. Here, the CIMD-coupled Poisson-Nernst-Planck (PNP) equations are used to explore the ion transport mechanisms in the BPMs for both reverse bias and forward bias at neutral and acid-base conditions. It is demonstrated that the CIMD effect in the reverse-bias mode can be suppressed by enhancing the diffusive transport of salt counter-ions (Na+ and Cl−) into the BPMs, and that in the forward-bias mode with acid-base electrolytes can be suppressed by matching the transport rate of water counter-ions (H3O+ and OH−). Suppressing the CIMD can promote the water dissociation in the reverse-bias mode, as well as overcome the plateau of limiting current density and reduce the interfacial blockage of salt co-ions (Cl−) in the anion-exchange layer in the forward-bias mode with acid-base electrolytes. Our work highlights the importance of regulating ion crossover transport on improving the performance of BPMs.

Unraveling the origins of the coexisting localized-interfacial mechanism in oxide-based memristors in CMOS-integrated synaptic device implementations.

The forefront of neuromorphic research strives to develop devices with specific properties, i.e., linear and symmetrical conductance changes under external stimuli. This is paramount for neural network accuracy when emulating a biological synapse. A parallel exploration of resistive memory as a replacement for conventional computing memory ensues. In search of a holistic solution, the proposed memristive device in this work is uniquely poised to address this elusive gap as a unified memory solution. Opposite biasing operations are leveraged to achieve stable abrupt and gradual switching characteristics within a single device, addressing the demands for lower latency and energy consumption for binary switching applications, and graduality for neuromorphic computing applications. We evaluated the underlying principles of both switching modes, attributing the anomalous gradual switching to the modulation of oxygen-deficient layers formed between the active electrode and oxide switching layer. The memristive cell (1R) was integrated with 40 nm transistor technology (1T) to form a 1T-1R memory cell, demonstrating a switching speed of 50 ns with a pulse amplitude of ±2.5 V in its forward-biased mode. Applying pulse trains of 20 ns to 490 ns in the reverse-biased mode exhibited synaptic weight properties, obtaining a nonlinearity (NL) factor of <0.5 for both potentiation and depression. The devices in both modes also demonstrated an endurance of >106 cycles, and their conductance states were also stable under temperature stress at 85 °C for 104 s. With the duality of the two switching modes, our device can be used for both memory and synaptic weight-storing applications.

All-transparent tandem photovoltaic-powered photodetector

Can electronics operate independently and invisibly in designated places? If yes, the user would not recognize the existence of devices and this can be applied to various applications. The device should be optically transparent and simultaneously achieve transparent electric power to satisfy this invisible freedom. Herein, we propose a dual transparent platform consisting of a broad-range translucent photodetector powered by an invisible photovoltaic unit. Transparent photovoltaic (TPV) has a NiO/TiO2 heterojunction for high optical transparency of 79.7% and can produce a maximum voltage of 0.6 V at an efficiency of 13.4%. A transparent broadband photodetector (TBD) that comprises Co3O4/ZnO has high optical transparency (70%) and provides a broad wavelength photodetection. The TBD can be tuned in the reverse bias mode along with the separate power unit (TPV), which significantly enhances the responsivity to 6.8 mA W−1. In contrast to the self-powered operation mode, the dual system (TBD and TPV power units) can actively control the bias voltage to the TBD, supplied from the TPV (0–0.6 V). TPV and TBD have an overall transparency of 56.4% and long-term stability to allow a high degree of freedom in transparent human electronics, ensuring visional transparency and an independent power supply.

Implementation of the Metamaterial Multiband Frequency Reconfigurable Antenna for IoT Wireless Standards

A reconfigurable metamaterial multiband antenna operating an octa-band resonating mode is designed for IoT wireless applications. Metamaterial cell split ring resonator (SRR) and quasi-complementary split ring resonator (CSRR) are printed on the back and front sides of the proposed antenna design to achieve the multiband characteristics. The PIN diode is implemented in a slotted ground section of the antenna structure to get different wireless standards by switching the diode ON/OFF. During reverse biased (OFF) mode of diode antenna operating at eight resonant frequencies 2.42, 4.34, 5.81, 8.45, 11.02, 13.98, 16.12, and 18.87 GHz covering the IoT wireless standards such as S, C, X, Ku, and K band to identify a variety of wireless applications such as WLAN, WAIC, ITU, Satellite, and radar communication. Simple structure, stable radiation pattern, multiband operation, frequency band reconfigurability feature, reasonable gain, and efficiency are the significant features of the proposed metamaterial multiband antenna.

High Photosensitivity Nanocrystalline p-Cu2S/n-FTO Heterojunction Photodetectors Prepared by Dip Coating Method

Objective: The aim of present work is to fabricate nanocrystalline p-Cu2S/n-FTO heterojunction photo detectors with high photosensitivity. Methods: Copper sulphide (Cu2S) is semiconductor compound belonging to I-VI group. Cost effective chemical method was used to synthesize thioglycerol capped Cu2S nanoparticles with improved optoelectronic and physical properties. These nanoparticles were investigated by ultraviolet-visible (UV-Vis) absorption spectroscopy, photoluminescence (PL) and transmission electron microscopy (TEM). Cu2S films were deposited on fluorine doped tin oxide (FTO) glass slides using cost effective dip coating method at different dip times and heat treated in air atmosphere. Wide band gap fluorine doped tin oxide, n-type semiconductor was used as transparent window material together with narrow band gap Cu2S, p-type semiconductor used as absorber layer in case of Cu2S/FTO heterojunction photo detectors under reverse biased mode and illumination. Results: Cu2S films were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Analysis by X-rays (EDAX), Mapping and Atomic Force Microscopy (AFM). Selected Area Electron Diffraction (SAED) images showed ring patterns indicating that Cu2S nanoparticles are polycrystalline in nature and spot patterns indicating that Cu2S nanoparticles are monocrystalline in nature. Conclusion: Previously, preparation of optoelectronic devices such as photo detectors using Cu2S films has not been reported. In this study, photo detectors having enhanced photosensitivity were prepared using Cu2S films. I-V characteristics of p-Cu2S/n-FTO heterojunctions under dark and illumination conditions were examined. It was observed that film conductivity decreases under dark condition and increases under illumination condition with increase in dip time in case of Cu2S/FTO heterojunction photo detectors.

Thermionic emission assisted charge conduction mechanisms across LaMnO3 / La0.7Ca0.3MnO3 interface of manganite thin film structure

In the present communication, temperature dependence of resistive switching (RS) behavior across chemically grown LaMnO3/La0.7Ca0.3MnO3 (LMO/LCMO) interface have been discussed by employing various theoretical approaches and mechanisms. Temperature dependent RS behavior of LMO/LCMO interface highlights impactful switching between high resistance state (HRS) and low resistance state (LRS) under reverse bias mode through an active role of depletion region. Trapping–detrapping processes assisted charge conduction has been identified as a source of RS nature of LMO/LCMO interface. Thermionic emission model and space charge limited conduction (SCLC) mechanism have been employed to understand the charge conduction and charge transport across LMO/LCMO interface. Present structure of Ag/LMO/LCMO/Ag has also been investigated for possible retention and endurance behaviors under different external parameters which verify the stability, reliability, reproducibility, non–volatile nature and applicability of LMO/LCMO interface.

Nanofluidic diodes based on asymmetric bio-inspired surface coatings in straight glass nanochannels.

In this study, we present nanofluidic diodes fabricated from straight glass nanochannels and functionalized using bio-inspired polydopamine (PDA) and poly-L-lysine (PLL) coatings. The resulting PDA coatings are shown to be asymmetric due to a combination of transport considerations which can be leveraged to provide a measure of control over the effective channel geometry. By subsequently introducing a layer of amine-bearing PLL chains covalently bound to the PDA, we enhance heterogeneities in the charge and ion distributions within the channel and enable significant current rectification between forward-bias and reverse-bias modes; our PDA-PLL-coated channels yielded a rectification ratio greater than 1000 in a 100 nm channel filled with 0.01× phosphate-buffered saline solution (PBS). We further demonstrated that at higher ionic strength conditions, reducing the solution pH increased the number of protonated amines within the PLL layer, amplifying the charge disparities along the channel and leading to greater rectification. As nanofluidic diodes with bipolar surface charge distributions tend to provide superior performance compared to those with a single wall charge polarity, we imposed a more bipolar charge distribution in our devices by partially coating our PDA-PLL-coated channels with negatively charged polyacrylic acid (PAA). These enhanced bipolar channels exhibited greater current rectification than the PDA-PLL-coated channels, reaching rectification ratios in excess of 100 even in more physiologically-relevant 1× PBS solutions. Our fabrication approach and the results herein provide a promising platform from which the scientific community can build upon in the relentless endeavor for improved sensitivity in biosensors and other analytical devices.

Impact of crystalline defects in 4H-SiC epitaxial layers on the electrical characteristics and blocking capability of SiC power devices

In this study, we report the impact of structural 4H-SiC epitaxial defects on the electrical characteristics and blocking capabilities of SiC power devices. The detection and classification of the various crystal defects existing in 4H-SiC epitaxial layers and substrates was carried out with a commercial inspection tool using an optical microscope with a photoluminescence channel (PL). After the fabrication of dedicated test structures, devices that contain a single crystal defect were selected and electrically tested in reverse bias mode. Photon emission microscopy was performed to enable the localization of the leakage current spots within the devices. Thus, a direct correlation of the various crystal defects with the reduced blocking capability mechanism was made. This evaluation helps to set directions and build a strategy towards the reduction of critical defects in order to improve the performance of SiC devices for high power applications.

Magnetoresistive nature assisted field effect configuration for LaMnO3 / La0·7Ca0·3MnO3 interface

In this communication, LaMnO3/La0·7Ca0·3MnO3/LaAlO3 (LMO/LCMO/LAO) structure was prepared by low cost sol–gel based chemical solution deposition (CSD) method. Field effect configuration (FEC) based LMO/LCMO interface resistive nature has been investigated for the LMO/LCMO/LAO structure. Temperature dependent interface resistivity modulation factor has been understood using forward and reverse bias modes. Temperature dependent LMO/LCMO interface resistivity has been discussed in detail under different applied magnetic fields, different applied interface electric fields and different magnetic field directions to identify simultaneous possible existence of magnetoresistance (MR), electroresistance (ER) and anisotropic magnetoresistance (AMR) effects across the same LMO/LCMO interface. Zener double exchange (ZDE) polynomial law has been fitted theoretically to the obtained interface resistive nature to understand the possible spin fluctuations across the LMO/LCMO interface.

Impact of Epitaxial Defects on Device Behavior and their Correlation to the Reverse Characteristics of SiC Devices

In this work we report on the impact of various crystalline defects present in 4H-SiC epitaxial layers on the electrical blocking characteristics of SiC power devices. Dedicated test structures were fabricated and electrically characterized in reverse bias mode. SiC substrate and epitaxial crystal defects, as well defects due to front-end processing were detected and classified using commercial inspection tools. Devices with a single defect-type were studied which leads to a direct correlation of the leakage current spot position within the device and the obtained blocking characteristics. This gives a better understanding of each crystal defect impact on device ́s performance which leads to an improvement in the reliability and cost reduction of SiC power devices.

Temperature-dependent charge barrier height of amorphous germanium contact detector

The exploration of germanium (Ge) detectors with amorphous Ge (a-Ge) contacts has drawn attention to the searches for rare-event physics such as dark matter and neutrinoless double-beta decay. The charge barrier height (CBH) of the a-Ge contacts deposited on the detector surface is crucial to suppress the leakage current of the detector in order to achieve a low-energy detection threshold and high-energy resolution. The temperature-dependent CBH of a-Ge contacts for three Ge detectors is analyzed to study the bulk leakage current (BLC) characteristics. The detectors were fabricated at the University of South Dakota using homegrown crystals. The CBH is determined from the BLC when the detectors are operated in the reverse bias mode with a guard-ring structure, which separates the BLC from the surface leakage current (SLC). The results show that CBH is temperature dependent. The direct relation of the CBH variation to temperature is related to the barrier inhomogeneities created on the interface of a-Ge and crystalline Ge. The inhomogeneities that occur at the interface were analyzed using the Gaussian distribution model for three detectors.

Bias-controlled photocurrent generation process in GaN-based ultraviolet p–i–n photodetectors fabricated with a thick Al2O3 passivation layer

GaN-based p–i–n ultraviolet photodetectors (UV-PDs) with a 20 nm thick Al2O3 passivation layer are fabricated and the photocurrent generation mechanisms of the device at different biases are studied by performing different electro-optical measurements. Our UV-PDs reveal good rectifying current–voltage behavior but their generated photocurrent to UV light varies sub-linearly with the impinging power. This sub-linear trend is ascribed to the effect of strong carrier trapping. Furthermore, in reverse bias mode, PDs show higher photocurrent than forward bias, suggesting the higher charge carrier separation process caused by the exacerbated electric field. The simulated results are also agreed well with the experimental observations.

Thermal effects on resistive switching in manganite–silicon thin film device

In this article, we report the results of the fabrication and studies of Y0.95Ca0.05MnO3/Si device (referred hereafter as YCMO/Si) by pulsed laser deposition (PLD) and its temperature-dependent resistive switching (RS) behaviours measured across the YCMO/Si interface. These temperature (100–300 K)-dependent hysteretic current–voltage (I–V) characteristics have been understood on the basis of various possible charge conduction mechanisms involving the thermal effects on the charge carriers during four different cycles of the RS behaviours. Variations in the values of barrier height and the ratio of free to trapped charge carrier densities with temperature have been discussed for reverse bias mode of this YCMO/Si device. Temperature-dependent temperature coefficient of resistance (TCR) under different applied forward voltages shows an interesting variations in TCR with applied forward voltage, which proves this device as a potential candidate for practical applications.

Recessed AlGaN/GaN Heterojunction-Based Hydrogen Sensor Operated by Reverse Bias Mode

A Pt-functionalized hydrogen sensor was fabricated on a recessed AlGaN/GaN heterojunction platform where the thickness of the recessed AlGaN barrier layer under the Pt catalyst was 10 nm. Sensing characteristics were investigated under two different bias modes: forward and reverse bias operations. Unlike conventional diode type sensors, the recessed heterojunction sensor exhibited superior sensing characteristics under the reverse bias operation mode in comparison to the forward bias operation mode. The thin AlGaN barrier layer enhanced the field emission process under reverse bias operation, thus enabling a high response, which had another benefit of significantly lowering standby power consumption compared to the forward bias operation mode. A response of ~870 % and a response time of ~11 sec were achieved at 200°C with a standby power consumption of 0.03 W/cm2.

On the Trap Locations in Bulk FinFETs After Hot Carrier Degradation (HCD)

In this brief, typical locations of the interface and oxide traps generated by the hot carrier degradation (HCD) in FinFETs are studied with experiments and “atomistic” TCAD simulations under the worst case stress conditions. The typical round-Fin locations of different types of traps are analyzed by comparing the experimentally extracted results with the calibrated TCAD simulations. Then, the traps’ different impacts on HCD variations in both forward and reverse bias modes are employed to disclose the typical lateral locations of different traps. The results suggest that for both n- and p-type of FinFETs under the worst case stress conditions, the interface traps and oxide traps (type 1) are mainly distributed in the midchannel region closer to the source side on the Fin side, whereas oxide traps (type 2) are mainly distributed in the midchannel region closer to the drain side on the Fin top. Therefore, the two types of oxide traps originate in the different generation locations but may share the same atomistic structures in the same device. The results are helpful to the physical understandings of HCD in the FinFET technology.

A comprehensive mathematical model of water splitting in bipolar membranes: Impact of the spatial distribution of fixed charges and catalyst at bipolar junction

Bipolar ion-exchange membranes (BPM) have a long history and many uses. Still, the number of applications continues to grow every year, as does the interest in a better understanding of the mechanisms of ion transport and water splitting. We reconsider the existing theoretical background for the BPMs and propose a one-dimensional stationary model. For the first time, the Nernst-Planck-Poisson equations are coupled with the equations, which locally describe the water splitting kinetics. The catalytic effects exerted by the functional groups and the catalyst (if present), the protonation-deprotonation reactions of these groups as well as the effect of strong electric field (the second Wien effect) are taken into account when describing the reverse-bias mode. The effective rate constant of water splitting occurring in the absence of electric field is determined from the acid dissociation constants (Ka). The concentration profiles, the thickness of the reaction zone and space charge region (SCR) are simulated and analyzed. The results of numerical computation are compared with the known analytical estimations. Thus, the basic assumptions of the theory of BPMs are examined, and the conditions under which these assumptions are valid or not are found. Current-voltage curves and partial currents are calculated and compared with experimental data. The occurrence of two limiting currents at pKa close to 7 is predicted due to current-induced membrane discharge. According to simulations, any increase in the SCR thickness leads to a decrease of the water spitting rate at a fixed voltage due to a decrease in the second Wien effect. A very thin (about 4 nm) catalytic layer within the bipolar junction is sufficient to obtain 100 mA cm−2 current density at 1 V even when the anion-exchange and cation-exchange membrane layers do not contain catalytically active functional groups.

The Correct Equation for the Current Through Voltage-Dependent Capacitors

Two different equations for the current through voltage-dependent capacitances are used in the literature. One equation is obtained from the time derivative of charge that is considered as capacitance–voltage product: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\it dQ/dt=d[C(V)V]/dt=C(V)[dV/dt]+V[dC(V)/dt].}$ </tex-math></inline-formula> In the second equation, the term <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\it V[dC(V)/dt]}$ </tex-math></inline-formula> does not exist: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\it dQ/dt=} {\it C(V)[dV/dt]}$ </tex-math></inline-formula> . This paper clears the ongoing confusion caused by the difference between these two equations. We use the voltage-dependent parasitic capacitance of a commercial Schottky diode in reverse bias mode to test experimentally both equations. The result is that it is incorrect to add the term <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\it V[dC(V)/dt]}$ </tex-math></inline-formula> in the first equation with the measured capacitance. We also perform a theoretical analysis, which shows that the differential capacitance, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\it C(V)=dQ/dV}$ </tex-math></inline-formula> , in the correct current equation corresponds to the physical parameters of the diode capacitance.

Structural, Optical and Electrical Analysis of Pentacene Thin Film

The significant electrical characteristics of pentacene thin film was studied by fabricating silver-pentacene-aluminium sand witch device. This metal-semiconductor-metal (MSM) device can act as the diode . When investigating, it is found that the diode can act as an equivalent plate capacitor as reverse bias mode exclusive of current flow. In forward bias mode, studied diode as an equivalent circuit with a parallel circuit of resistance –capacitance. This paper describes analysis of capacitance by measuring I-V characteristics and impedance measurement. The free samples of pentacene are also analysed using field emission scanning electron microscope (FESEM) and X-ray diffraction micrographs (XRD) . The X-ray diffraction micrograph shows the configuration of pentacene in the (-1 0 2) plane as the strongest orientation .The particle size are analysed using FESEM. The optical characteristics are analysed using UV spectrometer. The spectra of absorption, reflection and refraction are studied well.

Thermionic emission driven resistive switching behaviour in Ca and Sr doped YMnO3 thin film devices

In this communication, we report the resistive switching (RS) behavior of pulsed laser deposition (PLD) grown with 5% Ca and Sr doped YMnO3 [referred with general formula as Y0.95A0.05MnO3 (YAMO), where A = Ca or Sr; YCMO if A is Ca and YSMO if A = Sr] on (100) single crystalline Nb:SrTiO3 (SNTO) substrates. The thickness of both the YCMO and YSMO thin film devices were ∼300 nm. The hysteretic current–voltage (I–V) characteristics were measured at 100 and 300 K across the YAMO/SNTO interfaces using current perpendicular to plane (CPP) transport measurement geometry. Both these devices exhibit distinct RS behaviours and these are highly influenced by the temperature, ionic size of dopants (i.e. Ca and Sr) and structural disorder as well as lattice mismatch between the films and substrates. While the YCMO/SNTO device exhibits an effective RS behavior in the reverse bias mode, the YSMO/SNTO device in the forward bias mode. The current compliance effects were observed for both the thin film devices and in both the bias modes and their modifications with different parameters. These results are discussed in the light of oxygen vacancies, their distribution and mobility. Thermionic emission model has been employed to understand the charge conduction mechanism across the YAMO/SNTO junctions. The variations in the barrier height has been obtained by fitting the experimental data of I–V with different temperatures, resistance states, dopants and bias modes and discussed in detail. Retention behaviours in the context of switching cycles and time duration have been studied at 100 and 300 K for the verification of device stability, reliability and reproducibility.

Transport properties and electroresistance of manganite based heterostructure

In the present communication, ZnO/La0.7Sr0.3MnO3/Al2O3 (ZnO/LSMO/Al2O3) heterostructure was grown using chemical solution deposition (CSD) technique. X–ray diffraction (XRD) measurement confirms the (100) crystallographic oriented growth of LSMO manganite and polycrystalline hexagonal growth of ZnO layer. Temperature dependent resistivity behavior under different applied magnetic fields, performed for two different geometries [current in plane (CIP) and current perpendicular to plane (CPP)], suggests the modifications in metal to insulator transition temperature TP and alterations in resistivity with applied magnetic field and measurement geometry. This also implies the better charge conduction across the ZnO/LSMO interface (CPP) as compared to LSMO film layer (CIP). Various charge conduction mechanisms have been employed to understand the charge transport for observed low temperature resistivity minimum, metallic behavior, insulating state and magnetoresistance (MR) of LSMO film and ZnO/LSMO interface. Electroresistance (ER) and field effect configuration (FEC) have been investigated across the ZnO/LSMO interface by recording the LSMO channel resistance, at room temperature, that suggest the tuning of signature and value of ER by considering forward and reverse bias modes across the interface. FEC studies show the large ER ∼ +750% and −85% across the LSMO channel with its tunability under different interface biases.