Switching Threshold Voltage Research Articles

Мodelling of Passive Coupling of Battery Units of Hybrid Energy Storage System

Due to the development of electric transport and the growth of “green” energy, electric energy storage systems (ESS) are increasingly being used in the world. The growth of the battery market in the last decade has been 20-30%. One of the ways to increase the efficiency of an electric power storage device is its hybridization, i.e. the use of heterogeneous battery units. The paper examines the features of passive coupling of lead-acid and lithium-ion batteries in a hybrid storage device. A model is presented for calculating the electrical characteristics of these units during operation. The possibility of choosing a hybrid drive structure that provides a comparable operating voltage range of the units (operation without voltage converters) is demonstrated. The modes of operation of a hybrid energy storage system are modeled both for simple parallel connection and for switching blocks according to a threshold algorithm. It is demonstrated that in order to equalize the discharge rate of the main and additional units, it is necessary to coordinate the capacity of the ESS, the degree of hybridization, the type of load and the electrical parameters of the batteries, which is impossible without modeling the system. When the threshold switching of the blocks takes place, additional control parameters, making it possible to change the discharge rate of the additional block and increase the economic efficiency of the hybrid ESS. Estimates of the economic efficiency of hybrid ESSs have been made for different values of the threshold switching voltage of the lithium-ion unit, as well as for three characteristic loads: an electric forklift truck, a 30-apartment apartment building and a 300-apartment residential complex. The results demonstrate the features and technical and economic potential of passive hybridization, can be used for the design of hybrid ESSs for small power systems with solar and wind power plants, in the calculation and design of generator – storage – consumer systems.

Synthesis, Characterization, and Resistive Memory Behaviors of Highly Strained Cyclometalated Platinum(II) Nanohoops.

Strained carbon nanohoops exhibit attractive photophysical properties due to their unique π-conjugated structure. However, incorporation of such nanohoops into the pincer ligand of metal complexes has rarely been explored. Herein, a new family of highly strained cyclometalated platinum(II) nanohoops has been synthesized and characterized. Strain-promoted C-H bond activation has been observed during the metal coordination process, and Hückel-Möbius topology and random-columnar packing in the solid state are found. Transient absorption spectroscopy revealed the size-dependent excited state properties of the nanohoops. Moreover, the nanohoops have been successfully employed as active materials in the fabrication of solution-processable resistive memory devices, including the use of the smallest platinum(II) nanohoop for the fabrication of a binary memory, with low switching threshold voltages of ca. 1.5 V, high ON/OFF current ratios, and good stability. These results demonstrate that strain incorporation into the structure can be an effective strategy to fundamentally fine-tune the reactivity, optoelectronic, and resistive memory properties.

Tweaking the Non-Volatile Write-Once-Read-Many-Times (WORM) Memory using Donor-Acceptor Architecture with Isatin as Core Acceptor.

Organic memory devices have attracted attention because they promise flexible electronics, low manufacturing costs, and compatibility with large-scale integration. A series of new D-A architectures were synthesized employing different donor groups and the isatin moiety as the acceptor through Suzuki-Miyaura coupling reactions. Strong intramolecular interactions were observed in the synthesized compounds, further corroborated by an optimal bandgap. The SEM investigation confirmed good molecular ordering and superior thin film surface coverage. All the compounds demonstrated notable binary Write-Once-Read-Many-Times (WORM) memory behaviour. The threshold switching voltage for these D-A systems ranged from -0.79 to -2.37 V, with the compound having isobutyl substituent showing the lowest threshold voltage and maximum ON/OFF ratio of 102, thus outperforming others. The combined effects of charge transfer and charge trapping are responsible for the resistive switching mechanism prevailing in these systems. The alterations in D-A molecules that affect molecular packing, thin film morphology, and, finally, the memory performance of the active layer are highlighted in this work.

GeSe ovonic threshold switch: the impact of functional layer thickness and device size

Three-dimensional phase change memory (3D PCM), possessing fast-speed, high-density and nonvolatility, has been successfully commercialized as storage class memory. A complete PCM device is composed of a memory cell and an associated ovonic threshold switch (OTS) device, which effectively resolves the leakage current issue in the crossbar array. The OTS materials are chalcogenide glasses consisting of chalcogens such as Te, Se and S as central elements, represented by GeTe6, GeSe and GeS. Among them, GeSe-based OTS materials are widely utilized in commercial 3D PCM, their scalability, however, has not been thoroughly investigated. Here, we explore the miniaturization of GeSe OTS selector, including functional layer thickness scalability and device size scalability. The threshold switching voltage of the GeSe OTS device almost lineally decreases with the thinning of the thickness, whereas it hardly changes with the device size. This indicates that the threshold switching behavior is triggered by the electric field, and the threshold switching field of the GeSe OTS selector is approximately 105 V/μm, regardless of the change in film thickness or device size. Systematically analyzing the threshold switching field of Ge–S and Ge–Te OTSs, we find that the threshold switching field of the OTS device is larger than 75 V/μm, significantly higher than PCM devices (8.1–56 V/μm), such as traditional Ge2Sb2Te5, Ag–In–Sb–Te, etc. Moreover, the required electric field is highly correlated with the optical bandgap. Our findings not only serve to optimize GeSe-based OTS device, but also may pave the approach for exploring OTS materials in chalcogenide alloys.

Silver nanoparticles induced enhanced stability, dielectric anisotropy, and electro-optical parameters of a nematic liquid crystalline material 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzene

Nanocomposites of 4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzene nematic liquid crystals and silver nanoparticles have been prepared. UV absorption spectroscopy, Fourier transform infrared and Raman spectroscopies were used to characterize the morphology of silver nanoparticles (Ag-NPs). Thermodynamic, optical, and dielectric studies have been done to explore the change in the physical parameters of the composites with respect to the pure sample. Due to dispersion of silver nanoparticles, transition temperature, associated enthalpy and order parameters of the nanocomposites have increased. Dielectric study shows enhanced dielectric anisotropy for Ag-NPs dispersed nanocomposites. The relaxation frequency of an observed mode has shifted towards high frequency region for the nanocomposites. Optical band gap for pure and nanocomposites samples has been estimated from UV absorption peaks and it is found that optical properties of the nanocomposites are almost invariant. Threshold (switching) voltage, rotational viscosity and splay elastic constant for pure and dispersed samples have also been obtained from electro-optical studies.

Mole Fraction and Device Reliability Analysis of Vertical-Tunneling-Attributed Dual-Material Double-Gate Heterojunction-TFET with Si0.7Ge0.3 Source Region at Device and Circuit Level

This paper puts forward a new vertical-tunneling-attributed dual-material double-gate heterojunction-TFET (VTDMDG-HTFET). The device structure of VTDMDG-HTFET includes Si[Formula: see text]Ge[Formula: see text] material for the designing of source region. The mechanism of vertical tunneling in VTDMDG-HTFET provides superior electric field at tunneling interface and leads to improved transfer characteristics. VTDMDG-HTFET also includes metal gate work-function engineering approach to facilitate high ON-current and small OFF-current. Application of high-[Formula: see text] dielectric material HfO2 in the form of gate oxide enhances the capacitive-coupling at channel-source interface. Moreover, the work includes the comparison of proposed VTDMDG-HTFET with the conventional vertical-tunneling based dual-material double-gate-TFET (VTDMDG-TFET). It has been observed from the simulation results that SiGe source-based VTDMDG-HTFET offers better DC characteristics in terms of superior ON-current, smaller OFF-current, steeper subthreshold-slope, lower threshold voltage, higher transconductance along with smaller drain-induced barrier-lowering (DIBL) effects as compared to its counterpart silicon-source-based VTDMDG-TFET. In this work, reliability of VTDMDG-HTFET has also been examined and compared with VTDMDG-TFET in terms of temperature sensitivity and effect of different interface trap charges. The device simulation and investigations have been carried out using technology computer aided design (TCAD) tool. Moreover, device circuit mixed-mode simulation analysis is carried out for conventional and proposed device based resistive load inverters. A comparison is performed between both the inverters in terms of prorogation delay and switching threshold voltages. Further, the mixed mode simulation analysis is also performed for the proposed device based inverter under the influence of different interface trap charges (ITCs). Study shows that reliability performance of the proposed device is better as compared to its counterpart conventional device. Hence, vertical tunneling and SiGe source-based VTDMDG-HTFET can be a better choice for various applications such as analog/RF, analog and digital electronic circuits, energy harvesting, gas-sensing and memory devices.

Structure and electronic properties of thin Ge2Sb2Te5 films produced by DC ion-plasma spattering

The optical properties of Ge2Sb2Te5 thin films were studied as a function of thickness. An increase in optical band gap with decreasing film thickness has been observed. The current– voltage characteristics measured in Ge2Sb2Te5 thin films in the current mode are studied. A decrease in switching time and threshold voltage with decreasing film thickness is established.

NbO2 selector device with Ge2Sb2Te5 thermal barrier for low off current (300 nA) and low power operation

This study investigated the impact of a Ge2Sb2Te5 (GST) thermal barrier on the performance of NbO2-based selector devices. Our findings showed that the GST barrier could significantly decrease the off-state leakage current from 3 μA to 300 nA without increasing the threshold switching voltage owing to its insulation properties and high thermal resistance. We also found that the GST barrier can effectively contain the Joule heat within the NbO2 switching region, as confirmed through a cryogenic analysis of the thermal resistance of GST. The results showed that the GST/NbO2 device had a thermal resistance 3.48 times higher than that of a single-layer NbO2 device. Our results provide design guidelines for utilizing a barrier layer to reduce the leakage current in low-power threshold switching devices.

Metal doped polyaniline as neuromorphic circuit elements for in-materia computing

ABSTRACT Polyaniline-based atomic switches are material building blocks whose nanoscale structure and resultant neuromorphic character provide a new physical substrate for the development next-generation, nanoarchitectonic-enabled computing systems. Metal ion-doped devices consisting of a Ag/metal ion doped polyaniline/Pt sandwich structure were fabricated using an in situ wet process. The devices exhibited repeatable resistive switching between high (ON) and low (OFF) conductance states in both Ag+ and Cu2+ ion-doped devices. The threshold voltage for switching was>0.8 V and average ON/OFF conductance ratios (30 cycles for 3 samples) were 13 and 16 for Ag+ and Cu2+ devices, respectively. The ON state duration was determined by the decay to an OFF state after pulsed voltages of differing amplitude and frequency. The switching behaviour is analagous to short-term (STM) and long-term (LTM) memories of biological synapses. Memristive behaviour and evidence of quantized conductance were also observed and interpreted in terms of metal filament formation bridging the metal doped polymer layer. The successful realization of these properties within physical material systems indicate polyaniline frameworks as suitable neuromorphic substrates for in materia computing.

Superior electro-optical switching properties in polymer dispersed liquid crystals prepared with functionalized carbon nanotube nanocomposites of LC for switchable window applications

In this work, we are reporting the results on electro-optical switching properties of polymer dispersed liquid crystal (PDLC) devices for switchable window related applications. The PDLC cells are prepared by mixing octadecylamine functionalized single-walled carbon nanotubes (ODA-SWCNT) nanocomposites of liquid crystals (LC) and acrylate-based monomer by polymerization induced phase separation method. The important switchable device parameters such as threshold voltage, switching voltage, response time, contrast ratio and effective refractive index are investigated. The PDLC cells prepared with 0.05 wt% ODA-SWCNT nanocomposites of the LC, 7CB show almost zero threshold field and 36% less driving field compared to PDLC prepared with pure 7CB. A distinct optical response (frequency doubling) phenomenon is observed for PDLC prepared with 0.03 and 0.05 wt% ODA-SWCNT nanocomposites of 7CB compared to PDLC cells prepared with pure 7CB and lower concentrations (<0.03 wt%) of ODA-SWCNT. The relaxation of optical transmittance is observed at low frequency even when the applied electric field (Eapp) is ON which is due to the strong induced internal electric field (due to free ions) which nullifies the Eapp. The physical mechanism of this unique optical behavior is discussed in detail. The PDLC prepared with 0.01 wt% ODA-SWCNT show 84% faster response time compared to PDLC prepared with pure 7CB. The working of prototype PDLC devices for use in switchable window related applications is reported. The results indicate that, the PDLC prepared with 0.05 wt% ODA-SWCNT nanocomposites of 7CB show high ON state clarity and high OFF state opacity compared to PDLC prepared with pure LC and lower concentrations of ODA-SWCNT.

Tailoring of physical properties of glassy selenium (g-Se) by using multi-walled carbon nanotubes (MWCNTs)

The present research reports the studies of the different physical properties of a novel kind of glass nanocomposite sample that belongs to the nano-structuring of glassy selenium (g-Se) by choosing multiwall carbon nanotubes (MWCNTs) as a dopant. The structural analysis of the sample is done by employing different techniques [e.g., Scanning electron microscopy (SEM), X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC), and Raman spectroscopy]. The detailed analysis reveals that the MWCNTs are incorporated successfully in the glass matrix of Se. The occurrence of thermally activated glass transition/crystallization in the present glass nanocomposite sample has been investigated by thermal analysis of calorimetric measurements. The thermo-mechanical and electrical properties of this sample have been studied by micro-indentation test, and current–voltage measurements respectively. We have also compared different physical properties of current glass composites with that of g-Se for understanding the consequences of MWCNTs doping in the parent g-Se. The detailed analysis shows that the electrical conductivity, hardness, threshold voltage of resistive switching, and modulus of elasticity of the parent sample (i.e., g-Se) are increased drastically after the doping of MWCNTs, whereas there is a drastic reduction in the thermal stability and volume of the micro-voids.

Elemental Redistribution During the Crystallization of Ge–Cu–Te Thin Films for Phase-Change Memory

Herein, a GeCu2Te2 alloy is proposed as a phase-change material for application in nonvolatile phase-change random access memory (PRAM). The crystallization kinetics and microchemical changes during phase transformation are investigated, and their correlation with the electrical behaviors of the GeCu2Te2 thin films are examined. The key findings are as follows: (ⅰ) the GeCu2Te2 alloy shows a higher crystallization temperature (∼185 °C) than the classic Ge2Sb2Te5 (GST) thin films, thus demonstrating superior thermal stability; (ⅱ) the crystallization kinetics demonstrate a decreasing in the Avrami exponent n from 4, which is related to the growth-dominated crystallization process evidenced by the micromorphology; (ⅲ) a massive redistribution of the chemical elements along the depth of the thin films during crystallization is considered to be driven by selective surface oxidation at amorphous state, and stress buildup during crystallization. In addition, the crystallization-induced stress is determined as ∼168 MPa by utilizing the wafer curvature and X-ray diffraction methods for the GeCu2Te2 thin films. Finally, the lower threshold switching voltage ∼1.72 V for amorphous GeCu2Te2 thin films is beneficial for reducing the SET operating power consumption. The authors believe that these results are valuable for the optimal phase change material design.

Ultrahigh tunability of resistive switching in strongly correlated functional oxide

Vanadium dioxide (VO2) is a strongly correlated material that undergoes reversible resistive switching. However, the underlying mechanism of such resistive switching is still under debate. In this work we report an ultrahigh decrease in the VO2 switching threshold-voltage by more than 35 V and a giant 680% change by modifying the material stoichiometry through tungsten (W) doping. This remarkable effect of chemical doping on the resistive switching characteristics of VO2 illuminates on the fundamental mechanism of the insulator-to-metal transition, which reveals that such abrupt resistive switching is attributed to the electro-thermal actuation process. Furthermore, we report ultralow voltage spontaneous electrical oscillation in W-doped VO2 for the first time. Advances achieved here through the design of a new class of transition metal oxide based strongly correlated materials are of paramount importance for emerging electronics.

Meta-stable initial condition for improving the switching probability in azobenzene derivatives on surface

The trans isomer of azobenzene (AB) and its derivatives is the most abundant isomer at thermodynamic equilibrium and is known to switch between its trans and cis isomers when triggered by light, electrons/holes and electrical field in adlayers on surfaces. However, the equilibrium initial condition limits the operation of molecular switches on surface due to large activation barrier for switching (trans to cis), hence the switching probability is very low. We show in this article, condensation of meta-stable (non-equilibrium) cis state of azobenzene derivatives on graphite. When electrons/holes induced switching is performed on these molecular switches from the meta-stable initial condition, the switching probability is enhanced by several folds. The molecules are switching between two associated cis states, instead of trans–cis switching. The switching between the cis states has relatively low energy barrier as revealed by density functional theory (DFT) calculations is at the origin of the enhanced switching probability. We also observe that reversible trans–cis and cis–cis’ switching on graphite is mediated through a stable charged intermediate state, leading to a low threshold voltage for switching compared to neutral molecules.

Impact of Surface Roughness and Material Properties of Inert Electrodes on the Threshold Voltages and Their Distributions of ReRAM Memory Cells

The paper explores the impact of the surface roughness (SR) and other material properties of metal electrodes on the statistical distributions of the switching threshold voltages, Vform, Vset, and Vreset, of a resistive memory (ReRAM) cell. The surface roughness of Pt, Ru, Co, and Cu in Cu/TaOx/Pt, Cu/TaOx/Ru, and Cu/TaOx/Co devices is extensively characterized and related to the switching characteristics of the devices. We find that SF has both impacts on the mean and on the standard deviation of the Vform, Vset, and partly also on the Vreset distributions. The surface roughness of free metal surfaces of Pt, Ru, and Co when annealed at 600 °C and 900 °C is found to increase significantly, but this increase is substantially suppressed when passivated by the thin switching layer of the solid electrolyte, such as TaOx or SiO2. The increase of SR of those metals with increasing temperature correlates well with their melting temperature. Overall, we find that the differences in surface roughness along with material properties such as the thermal conductivity and work function explain well the differences between the distributions of the threshold voltages of the ReRAM devices with Pt, Ru, and Co serving as the inert electrode.

Shift of switching threshold in low-dimensional semiconductor-based complementary inverters via inkjet printing

MoS2 crystals grown by chemical vapor deposition are suited for realization of practical 2D semiconductor-based electronics. In order to construct complementary circuits with n-type MoS2, another p-type semiconductor, whose performance can be adjusted corresponding to that of MoS2 in the limited chip area, has to be sought. Herein, we present a method for tuning switching threshold voltages of complementary inverters simply via inkjet printing without changing their channel dimensions. Random networks of inkjet printed single-walled carbon nanotubes are formed as p-channels beside MoS2, and their density and thickness are controlled by varying the number of printed layers. As a result, p-type transistor characteristics as well as inverter characteristics are facilely tuned only by varying the number of printed layers.

Top gate engineering of field-effect transistors based on wafer-scale two-dimensional semiconductors

l The threshold voltage of the MoS 2 field-effect transistor was controlled by the work function of top gate metal (Au, Cu, Ag, and Al). l The oxidation of the Al gate produced surplus oxygen vacancy (positive charge) and the dipole effect in the HfO 2 dielectric layer. l Transmission electron microscope was employed to explore the thin Al 2 O 3 interlayer in the Al/HfO 2 interface. l The proposed MoS 2 inverter has an optimized inverter switching threshold voltage and DC voltage gain, which possesses significant potential for integrated multistage logic circuits. The investigation of two-dimensional (2D) materials has advanced into practical device applications, such as cascaded logic stages. However, incompatible electrical properties and inappropriate logic levels remain enormous challenges. In this work, a doping-free strategy is investigated by top gated (TG) MoS 2 field-effect transistors (FETs) using various metal gates (Au, Cu, Ag, and Al). These metals with different work functions provide a convenient tuning knob for controlling threshold voltage ( V th ) for MoS 2 FETs. For instance, the Al electrode can create an extra electron doping (n-doping) behavior in the MoS 2 TG-FETs due to a dipole effect at the gate-dielectric interface. In this work, by achieving matched electrical properties for the load transistor and the driver transistor in an inverter circuit, we successfully demonstrate wafer-scale MoS 2 inverter arrays with an optimized inverter switching threshold voltage ( V M ) of 1.5 V and a DC voltage gain of 27 at a supply voltage ( V DD ) of 3 V. This work offers a novel scheme for the fabrication of fully integrated multistage logic circuits based on wafer-scale MoS 2 film.

Self-rectifying threshold resistive switching based non-volatile memory of CBD/CBD grown vertical n-ZnO nanowire/p-Si heterojunction diodes

Vertically oriented ZnO nanowires are grown on p-Si substrate by employing two-step sequential chemical bath deposition technique. The ZnO nanowire exhibits n-type doping due to the presence of oxygen vacancies. The electrical characterizations of n-ZnO NWs/p-Si heterojunction diodes exhibit a self-rectifying, threshold resistive switching behavior. Such switching behavior is explained by oxygen vacancy assisted conducting filament formation mechanism. The relevant charge transport is governed by TC-SCLC and multistep recombination-tunneling processes through the interface traps. Threshold-voltage for resistive switching is observed to be increasing with increasing bias sweep rate. The device shows superior memory endurance for forward and reverse voltage sweep of 50 cycles in fast sweep mode. The ratio of HRS to LRS resistances shows one order of difference. The retention time of such resistive switching memory is recorded to be 4000 seconds, suggesting its non-volatile functionality. Thus, the n-ZnO NWs/p-Si heterojunction can be employed for fabricating promising non-volatile memory devices with excellent endurance and retentions. Copyright © 2018 VBRI Press.

Performance of Parallel Connected SiC MOSFETs under Short Circuits Conditions

This paper investigates the impact of parameter variation between parallel connected SiC MOSFETs on short circuit (SC) performance. SC tests are performed on parallel connected devices with different switching rates, junction temperatures and threshold voltages (VTH). The results show that VTH variation is the most critical factor affecting reduced robustness of parallel devices under SC. The SC current conducted per device is shown to increase under parallel connection compared to single device measurements. VTH shift from bias–temperature–instability (BTI) is known to occur in SiC MOSFETs, hence this paper combines BTI and SC tests. The results show that a positive VGS stress on the gate before the SC measurement reduces the peak SC current by a magnitude that is proportional to VGS stress time. Repeating the measurements at elevated temperatures reduces the time dependency of the VTH shift, thereby indicating thermal acceleration of negative charge trapping. VTH recovery is also observed using SC measurements. Similar measurements are performed on Si IGBTs with no observable impact of VGS stress on SC measurements. In conclusion, a test methodology for investigating the impact of BTI on SC characteristics is presented along with key results showing the electrothermal dynamics of parallel devices under SC conditions.

Cyclometalated Platinum(II) Complexes with Donor-Acceptor-Containing Bidentate Ligands and Their Application Studies as Organic Resistive Memories.

A series of heteroleptic cyclometalated platinum(II) complexes, [Pt(C^N)(O^O)], (1-10) with various donors and acceptors has been synthesized and characterized by 1 H NMR spectroscopy, elemental analyses, infrared spectroscopy and mass spectrometry. The X-ray structure of 2 has also been determined. The electrochemical and photophysical properties of the platinum(II) complexes were studied. These experimental results have been supported by computational studies. Furthermore, two of the complexes have been employed as the active material in the fabrication of resistive memory devices, exhibiting stable binary memory performance with low operating voltage, high ON/OFF ratio and long retention time.