Low Voltage Regime Research Articles

Defects induced persistent photoconductivity in monolayer MoS2

Understanding the relaxation mechanisms of photoexcited charge carriers in two-dimensional materials is indispensable from the fundamental point of view and for future optoelectronic applications. Through the photoconductivity and electronic transport experiments, we probe the mechanisms behind the persistent photoconductivity (PPC) in monolayer molybdenum disulfide (MoS2). The temperature (T) and power-dependent photoresponse studies reveal that the relaxation of excited charge carriers is strongly affected by the random fluctuations of local potentials. The relaxation time (τ) increases from τ ≃ 12 s at T = 16.5 K to τ ≃ 1235 s at T = 297 K, indicating PPC is a high T phenomenon in monolayer MoS2. The transport measurements demonstrate that the defect states with the density ≃4.43 × 1014 eV−1 cm–2 in a low gate voltage regime, originating from the sulfur vacancies, are responsible for these fluctuations. With a rise in temperature, the defect states undergo a transition from localization to extended states at T ≥ 100 K and thereby form the percolation network, which profoundly influences the relaxation mechanism. Our meticulous experiments and quantitative analysis provide newer insight into the origin of PPC in monolayer MoS2.

Deep learning-enabled prediction of 2D material breakdown

Characterizing electrical breakdown limits of materials is a crucial step in device development. However, methods for repeatable measurements are scarce in two-dimensional materials, where breakdown studies have been limited to destructive methods. This restricts our ability to fully account for variability in local electronic properties induced by surface contaminants and the fabrication process. To tackle this, we implement a two-step deep-learning model to predict the breakdown mechanism and breakdown voltage of monolayer MoS2 devices with varying channel lengths and resistances using current measured in the low-voltage regime as inputs. A deep neural network (DNN) first classifies between Joule and avalanche breakdown mechanisms using partial current traces from 0 to 20 V. Following this, a convolutional long short-term memory network (CLSTM) predicts breakdown voltages of these classified devices based on partial current traces. We test our model with electrical measurements collected using feedback-control of the applied voltage to prevent device destruction, and show that the DNN classifier achieves an accuracy of 79% while the CLSTM model has a 12% error when requiring only 80% of the current trace as inputs. Our results indicate that information encoded in the current behavior far from the breakdown point can be used for breakdown predictions, which will enable non-destructive and rapid material characterization for 2D material device development.

Interface and doping in carbon dots influence charge transfer and transport

Nanoscale carbon-based materials have the potential for large-scale optoelectronic and biomedical applications due to their tunable photo-physics. Here, we explore the impact of doping and the role of intervening media on both electron transfer and transport from carbon dots (CD) to an acceptor menadione (MD). In a binary mixture compared to undoped-CD (UCD), the amine-based nitrogen-doped-CD (ACD) reflects significant changes in its photo-physics like excitation-independent fluorescence, enhanced lifetime, along with a distinct drop in electron transfer rate (kET) to MD. With micro-emulsion, kET from UCD to MD is enhanced while a substantial reduction is observed for ACDs, emphasizing the critical role of doping, surface states, encapsulation, and interfaces of CTAB micelles. The impact of doping and Coulombic interaction at interfaces on the charge transport phenomena with redox-active MD is further investigated by current-sensing atomic force microscopy. For similar bias-voltage, ACD reflects a higher conductance than UCD, and the conductance is enhanced in the presence of CTAB micelles. Moreover, at a low voltage regime, the current-voltage curve shows a linear growth, while at higher voltage, Fowler-Nordheim (F–N) tunneling is observed. The presence of MD, however, leads to an abrupt rise in conductivity resulting in a transition from F–N to direct tunneling.

An ultra-energy-efficient crosstalk-immune interconnect architecture based on multilayer graphene nanoribbons for deep-nanometer technologies

An ultra-energy-efficient interconnect structure based on multilayer graphene nanoribbon (MLGNR) interconnects for deep-nanometer technologies is proposed herein. First, a low-swing interconnect based on MLGNRs and high-performance interface circuits using carbon nanotube field-effect transistors (CNTFETs) is proposed. Then, an ultra-energy-efficient interconnect structure is obtained by actively shielding such low-swing lines. The structures under study are simulated comprehensively at the 7-nm technology node. The results indicate that the MLGNR interconnect is significantly more energy efficient than its multiwall carbon nanotube (MWCNT) counterpart in the low-voltage regime. Moreover, the proposed approach is superior to its MLGNR counterparts. The proposed structure leads to 86%, 75%, and 31% lower energy consumption over a length of 500 µm as compared with the typical, actively shielded, and low-swing MLGNR interconnects, respectively. Moreover, the impact of the ratio of the widths of the signal line to the shield line on the performance of the interconnects is evaluated. The energy consumption reduction achieved by the proposed approach is mostly preserved even when using minimum-width shield lines on wider signal lines to reduce the area overhead. Moreover, the impact of process variations on the performance of the interconnects is assessed using Monte Carlo simulations, demonstrating the robustness of the proposed approach.

Analytical description of mixed ohmic and space-charge-limited conduction in single-carrier devices

While space-charge-limited current measurements are often used to characterize charge-transport in relatively intrinsic, low-mobility semiconductors, it is currently difficult to characterize lightly or heavily doped semiconductors with this method. By combining the theories describing ohmic and space-charge-limited conduction, we derive a general analytical approach to extract the charge-carrier density, the conduction-band edge, and the drift components of the current density–voltage curves of a single-carrier device when the semiconductor is undoped, lightly doped, or heavily doped. The presented model covers the entire voltage range, i.e., both the low-voltage regime and the Mott–Gurney regime. We demonstrate that there is an upper limit to how doped a device must be before the current density–voltage curves are significantly affected, and we show that the background charge-carrier density must be considered to accurately model the drift component in the low-voltage regime, regardless of whether the device is doped or not. We expect that the final analytical expressions presented herein to be directly useful to experimentalists studying charge-transport in novel materials and devices.

An analytical study on low voltage regime of natural organic semiconductor based device: Physics of trap energy and ideality factor

Present investigation demonstrates the physics at low voltage region of natural organic semiconductor based single layer device. Theoretical explanation of diffusion driven current equations has been implemented as an efficient way to study the charge transport mechanism in that regime. GPVDM (abbreviated form of General purpose photovoltaic model), a renowned software simulation technique of organic devices has been introduced to obtain current voltage relationship and fitted with diffusion assisted equation. The plot shows high consistency with theoretical explanation which indicates reliability of the plot. Simulation based observation on trap assisted conduction at low voltage and its relation with corresponding ideality factor at that region has been explained on the basis of theoretical point of view. It has been found that ideality factor decays exponentially with increasing trap energy at low voltage. Experimental approach has finally been employed on Turmeric and Indigo dye based single layer natural semiconducting diodes to validate the outcome of the proposal. Results of the experiment lead to great similarity with the outcomes of both theoretical and simulation approach.

P‐169: Towards Fully Integrated 3D Master Equation — Kinetic Monte Carlo Predictive Device Simulations

In the past decade, the discovery of organic semiconductor materials and the development of organic electronic devices has occurred mostly by experimental trial‐and‐error. In the past years, rational design based on predictive modeling has started playing a larger role in accelerating research and development of organic semiconductor devices. To make this transition possible, we developed and validated advanced predictive simulation software, Bumblebee, based on 3D kinetic Monte Carlo (KMC) and 3D master equation (ME) methods for simulating state‐of‐the art multilayer organic light emitting diode (OLED) stacks. While the 3D‐KMC engine is optimized for high voltages, the 3D‐ME engine is especially suitable for the low voltage regime, making these two engines complementary to each other for efficient simulation of OLED device performance in different operation regimes. To demonstrate the power of predictive modelling, we show several examples of device simulation results obtained with Bumblebee for different OLED stacks. Particular focus will be given to the low current/low voltage range where the impact of traps and energetic disorder on device turn‐on/turn‐off is most important.

Statistical modelling of organic thin film transistor behaviour

Abstract Three analyses of the expressions describing the electrical characteristics of organic thin film transistors (OTFT's) are presented. The first is the field-independent approach to mobility originally used for inorganic semiconductor materials, often referred to as the Square Law (SQL). The second is appropriate for both the Multiple Trapping and Release (MTR) and the Variable Range Hopping (VRH) descriptions of mobility, where dependence on a transverse field is consistent with the Universal Mobility Law (UML). The third is appropriate for the Extended Gaussian Disorder (EGD) description where an exponential dependence of mobility on the transverse field occurs. In each case master equations have been derived, including Schottky contact effects, where the polarity of the voltage drop across the source and drain contacts is correctly taken into account for the first time. The effect of the bulk semiconductor material beyond the accumulation layer is also accounted for, and defines the sub-threshold performance in a low-voltage regime. A new statistical modelling procedure has been developed to extract the key parameters of these expressions simultaneously from experimental data. For the analysis of TRANSFER data, no more than five parameters are used in the SQL, UML and EGD treatments. All three models are considered so that the effect the choice of model has on the extracted parameters can be revealed; analysis of data from different metallophthalocyanines is used to illustrate the different effects. When the contact resistances correctly take into account possible Schottky-like behaviour, all three descriptions provide equally excellent fits to the data from TRANSFER experiments. In a following report, a family of copper phthalocyanine-related semiconductors will be examined in detail using these new analysis procedures to explore the effect of non-peripheral substituent bulk, and aza-nitrogen replacement by CH, on mobility.

A low-current atmospheric pressure discharge generating atomic magnesium fluxes

In the present work, we studied the stable operation of an atmospheric pressure discharge with magnesium electrodes in air and argon flows. We employed a plasma generator consisting of a central cathode and a coaxial anode. At a direct discharge current up to 200 mA, spontaneous transitions to a low-voltage (30–40 V) burning regime were observed when the current was slightly raised by several milliamperes. This phenomenon is reflected in the optical spectrum by the appearance of corresponding lines of singly charged and exited magnesium atoms. It has been found that for a discharge fed by a pulsed power supply, at a unipolar pulse repetition rate of 100 kHz, with the working gas of argon, the aerosol emission becomes stabilized. The aerosol consists of magnesium particles suspended in the argon flow that eventually mixes up with the surrounding air. Based on our estimates, we have made an assumption that the physical nature of the discharge maintenance lies in the ion–electron emission due to the interaction of argon ions with the magnesium cathode surface.

Spin-polaronic effects in electric shuttling in a single molecule transistor with magnetic leads

Current-voltage characteristics of a spintromechanical device, in which spin-polarized electrons tunnel between magnetic leads with anti-parallel magnetization through a single level movable quantum dot, are calculated. New exchange- and electromechanical coupling-induced (spin-polaronic) effects that determine strongly nonlinear current-voltage characteristics were found. In the low-voltage regime of electron transport the voltage-dependent and exchange field-induced displacement of quantum dot towards the source electrode leads to nonmonotonic behavior of differential conductance that demonstrates the lifting of spin-polaronic effects by electric field. At high voltages the onset of electron shuttling results in the drop of current and negative differential conductance, caused by mechanically-induced increase of tunnel resistivities and exchange field-induced suppression of spin-flips in magnetic field. The dependence of these predicted spin effects on the oscillations frequency of the dot and the strength of electron-electron correlations is discussed.

Modeling and Demonstration of Oxygen Vacancy-Based RRAM as Probabilistic Device for Sequence Learning

The joint device-algorithm development of a resistive RAM (RRAM)-based sequence learning system is presented. The low-voltage gradual RESET of oxygen-vacancy-based RRAM was characterized and modeled by kinetic Monte Carlo simulations based on the hour-glass model. In the low-voltage regime, the RESET becomes stochastic and depends on the SET-history of the device. The stochastic RESET is detrimental to deep neural network training, which requires precise weight updates. However, this intrinsic RRAM effect can be employed as a local learning rule for brain-inspired computing. Therefore, a novel and non-deep learning sequence learning approach that employs RRAM as active computational elements is proposed. Two applications using this algorithm are explored in both simulations, employing a parameterized learning rule and a hardware demonstration: sequence denoising and gait authentication. This article shows promise for the applicability of RRAM in non-deep learning applications targeting low-power online learning at the edge.

A Fully Static True-Single-Phase-Clocked Dual-Edge-Triggered Flip-Flop for Near-Threshold Voltage Operation in IoT Applications

A Dual-Edge-Triggered (DET) flip-flop (FF) that can reliably operate at low voltage is proposed in this paper. Unlike the conventional Single-Edge-Triggered (SET) flip-flops, DET-FFs can improve energy efficiency by latching input data at both clock edges. When combined with aggressive voltage scaling, significant efficiency improvement is expected. However, prior DET-FF designs were susceptible to Process, Voltage and Temperature (PVT) variations, limiting their operation at low voltage regimes. A fully static true-single-phase-clocked DET-FF is proposed to achieve reliable operation at voltages as low as a near-threshold regime. Instead of the two-phase or pulsed clocking scheme in conventional DET-FFs, a True-Single-Phase-Clocking (TSPC) scheme is adopted to overcome clock overlap issues and enable low-power operation. Fully static implementation also enables robust operation in a low voltage regime. The proposed DET-FF is designed in 28nm CMOS technology, and a comprehensive analysis including post-layout Monte Carlo simulation for wide PVT ranges is performed to validate the design approaches. Extensive analysis and comparison with prior-art DET-FFs confirmed that the proposed DET-FF can operate at the lowest voltage of 0.28 V for a temperature range of −40 °C to 120 °C while maintaining nearly-best energy efficiency and power-delay-product.

Lattice Boltzmann electrokinetics simulation of nanocapacitors.

We propose a method to model metallic surfaces in Lattice Boltzmann Electrokinetics (LBE) simulations, a lattice-based algorithm rooted in kinetic theory which captures the coupled solvent and ion dynamics in electrolyte solutions. This is achieved by a simple rule to impose electrostatic boundary conditions in a consistent way with the location of the hydrodynamic interface for stick boundary conditions. The proposed method also provides the local charge induced on the electrode by the instantaneous distribution of ions under voltage. We validate it in the low voltage regime by comparison with analytical results in two model nanocapacitors: parallel plates and coaxial electrodes. We examine the steady-state ionic concentrations and electric potential profiles (and corresponding capacitance), the time-dependent response of the charge on the electrodes, and the steady-state electro-osmotic profiles in the presence of an additional, tangential electric field. The LBE method further provides the time-dependence of these quantities, as illustrated on the electro-osmotic response. While we do not consider this case in the present work, which focuses on the validation of the method, the latter readily applies to large voltages between the electrodes, as well as to time-dependent voltages. This work opens the way to the LBE simulation of more complex systems involving electrodes and metallic surfaces, such as sensing devices based on nanofluidic channels and nanotubes, or porous electrodes.

Investigation of the temperature-dependent electrical properties of Au/PEDOT:WO3/p-Si hybrid device

The electrical properties of Au/PEDOT:WO3/p-Si hybrid devices were studied in terms of current–voltage (I–V) and capacitance–voltage (C–V) measurements. Poly (3,4-ethylene dioxythiophene/tungsten trioxide (PEDOT:WO3) composite was prepared by an in situ chemical oxidative polymerization of monomer in 1-butyl-3-methylimidazoliumtetrafluoroborate (BMIMBF4). Optical and structural properties of the PEDOT:WO3 thin film was characterized by using FTIR, UV–Vis and AFM techniques. The bandgap energy of PEDOT:WO3 thin film was determined as 2.07 eV from UV–Vis spectrum. It was seen that the I–V plots of the Au/PEDOT:WO3/p-Si hybrid devices were non-linear and C−2–V plots were linear in the reverse bias defining rectification behavior. The values of barrier height obtained from the I–V and C−2–V plots of the fabricated devices were found to be 0.729 ± 0.012 eV and 0.817 ± 0.011 eV at room temperature in the dark environment, respectively. Devices have a high rectification behavior with a rectification ratio of 3.645 × 105 at ± 1 V. The temperature-dependent I–V characteristics of one of the devices were also analyzed on the basis of the thermionic emission theory at low forward bias voltage regime. It was observed that the values of ideality factor decrease while the values of barrier height increase with increasing temperature. This kind of temperature dependence was attributed to the presence of the barrier inhomogeneity at the hybrid film/inorganic semiconductor interface. Then, by analysing of the forward bias I–V characteristics at double logarithmic scale, it was seen that the carrier transport in the Au/PEDOT:WO3/p-Si hybrid device demonstrates the space-charge-limited current (SCLC) conduction mechanism controlled by a trap distribution above the valence band edge dominates in the range 0.1–0.3 V voltages. Furthermore, by analyzing the reverse bias I–V–T characteristics, it was shown that Schottky emission was the dominating current conduction mechanism in the temperature range of 240–320 K.

MXene artificial muscles based on ionically cross-linked Ti3C2T x electrode for kinetic soft robotics.

Existing ionic artificial muscles still require a technology breakthrough for much faster response speed, higher bending strain, and longer durability. Here, we report an MXene artificial muscle based on ionically cross-linked Ti3C2T x with poly(3,4 ethylenedioxythiophene)-poly(styrenesulfonate), showing ultrafast rise time of within 1 s in DC responses, extremely large bending strain up to 1.37% in very low input voltage regime (0.1 to 1 V), long-term cyclic stability of 97% up to 18,000 cycles, markedly reduced phase delay, and very broad frequency bandwidth up to 20 Hz with good structural reliability without delamination under continuous electrical stimuli. These artificial muscles were successfully applied to make an origami-inspired narcissus flower robot as a wearable brooch and dancing butterflies and leaves on a tree as a kinetic art piece. These successful demonstrations elucidate the wide potential of MXene-based soft actuators for the next-generation soft robotic devices including wearable electronics and kinetic art pieces.

Frequency-dependent force between ac-voltage-biased plates in electrolyte solutions.

We analyze the frequency dependence of the force between ac-voltage-biased plates in electrolyte solutions. To this end we solve analytically the Poisson-Nernst-Planck transport model in the dilute concentration and low voltage regime for a 1:1 symmetric electrolyte with blocking electrodes under a dc+ac applied voltage. The total force, which is the resultant of the electric and osmotic forces, shows a complex dependence on plate separation, frequency, ion concentration, and compact layer properties, different from that predicted from electrostatic current models or equivalent circuit models, due to the relevance of the osmotic force contribution in almost the whole range of frequencies. For the total dc force, we show that it decays at fixed ion concentration, linearly with plate separation for separations larger than a few times the Debye screening length. This linear dependence is due to the assumption about the conservation of the number of ions in the system. Moreover, the 1ω and 2ω ac harmonics of the total force show a broad peak at intermediate frequencies; it is centered at about the inverse of the charging time of the double layer capacitance, and covers the frequency range between the inverse of the diffusion time and the inverse of the electrolyte dielectric relaxation time. Finally, the 1ω ac harmonic component attains its high frequency asymptotic value at frequencies much higher than the inverse of the electrolyte dielectric relaxation time due to the very slow relaxation of the osmotic 1ωharmonic component at high frequencies. The derived analytical expressions for the total force remain valid up to voltages of the order of the thermal voltage, as has been assessed by means of numerical calculations. The numerical calculations are also used to explore the onset of higher force harmonics for larger applied voltages. Understanding the frequency dependence of the force acting on voltage-biased plates in electrolyte solutions can be of relevance for electrical actuation strategies in microelectromechanical systems and for the interpretation of some emerging electric scanning probe force microscopy techniques operating in electrolyte solutions.

Anomalous Conductance near Percolative Metal-Insulator Transition in Monolayer MoS2 at Low Voltage Regime.

Conductivity of the insulating phase increases generally at an elevated drain-source voltage due to the field-enhanced hopping or heating effect. Meanwhile, a transport mechanism governed by percolation in a low compensated semiconductor gives rise to the reduced conductivity at a low-field regime. Here, in addition to this behavior, we report the anomalous conductivity behavior to transform from a percolative metallic to an insulating phase at the low voltage regime in monolayer molybdenum disulfide (MoS2). Percolation transport at low source-drain voltage is governed by inhomogeneously distributed potential in strongly interacting monolayer MoS2 with a substrate, distinct from the quantum phase transition in multilayer MoS2. At a high source-drain voltage regime, the insulating phase is transformed further to a metallic phase, exhibiting multiphases of metallic-insulating-metallic transitions in monolayer MoS2. These behaviors highlight MoS2 as a model system to study various classical and quantum transports as well as metal-insulator transition in two-dimensional systems.

Progress Report on "From Printed Electrolyte-Gated Metal-Oxide Devices to Circuits".

Printed electrolyte-gated oxide electronics is an emerging electronic technology in the low voltage regime (≤1 V). Whereas in the past mainly dielectrics have been used for gating the transistors, many recent approaches employ the advantages of solution processable, solid polymer electrolytes, or ion gels that provide high gate capacitances produced by a Helmholtz double layer, allowing for low-voltage operation. Herein, with special focus on work performed at KIT recent advances in building electronic circuits based on indium oxide, n-type electrolyte-gated field-effect transistors (EGFETs) are reviewed. When integrated into ring oscillator circuits a digital performance ranging from 250 Hz at 1 V up to 1 kHz is achieved. Sequential circuits such as memory cells are also demonstrated. More complex circuits are feasible but remain challenging also because of the high variability of the printed devices. However, the device inherent variability can be even exploited in security circuits such as physically unclonable functions (PUFs), which output a reliable and unique, device specific, digital response signal. As an overall advantage of the technology all the presented circuits can operate at very low supply voltages (0.6 V), which is crucial for low-power printed electronics applications.

A TaOx-Based Electronic Synapse With High Precision for Neuromorphic Computing

Neuromorphic computing is a promising candidate for breaking the von Neumann bottleneck and developing high-efficient computing systems. Here we present a W/TaOx/Pt high-precision electronic synapse with excellent analog properties for neuromorphic computing. The device exhibits the potential of 10-bit weight precision, which is state of the art in conductance levels. Furthermore, the device shows linear weight update behavior in a specific conductance range, linear I-V curves in low voltage regime, long time retention, and precise modulation of weight. These characteristics are very helpful for improving the accuracy of neuromorphic networks. Finally, a $400\times 60\times 10$ three-layer perceptron was constructed with W/TaOx/Pt synapses for MNIST classification and ~92% accuracy was achieved.

Empirical and Theoretical Modeling of Low-Frequency Noise Behavior of Ultrathin Silicon-on-Insulator MOSFETs Aiming at Low-Voltage and Low-Energy Regime.

This paper theoretically revisits the low-frequency noise behavior of the inversion-channel silicon-on-insulator metal-oxide-semiconductor field-effect transistor (SOI MOSFET) and the buried-channel SOI MOSFET because the quality of both Si/SiO2 interfaces (top and bottom) should modulate the low-frequency fluctuation characteristics of both devices. It also addresses the low-frequency noise behavior of sub-100-nm channel SOI MOSFETs. We deepen the discussion of the low-frequency noise behavior in the subthreshold bias range in order to elucidate the device’s potential for future low-voltage and low-power applications. As expected, analyses suggest that the weak inversion channel near the top surface of the SOI MOSFET is strongly influenced by interface traps near the top surface of the SOI layer because the traps are not well shielded by low-density surface inversion carriers in the subthreshold bias range. Unexpectedly, we find that the buried channel is primarily influenced by interface traps near the top surface of the SOI layer, not by traps near the bottom surface of the SOI layer. This is not due to the simplified capacitance coupling effect. These interesting characteristics of current fluctuation spectral intensity are explained well by the theoretical models proposed here.