Showing papers on "Equivalent series resistance published in 2016"

Macroscopic fibres of CNTs as electrodes for multifunctional electric double layer capacitors: from quantum capacitance to device performance

Abstract: In this work we present a combined electrochemical and mechanical study of mesoporous electrodes based on CNT fibres in the context of electric double layer capacitors. We show that through control of the synthetic and assembly processes of the fibres, it is possible to obtain an active material that combines a surface area of 250 m2 g−1, high electrical conductivity (3.5 × 105 S m−1) and mechanical properties in the high-performance range including toughness (35 J g−1) comparable to that of aramid fibre (e.g. Kevlar). These properties are a consequence of the predominant orientation of the CNTs, observed by wide- and small-angle X-ray diffraction, and to the exceptionally long CNT length on the millimetre scale. Cyclic voltammetry measurements in a three-electrode configuration and using 1-butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14TFSI) ionic liquid electrolyte, show that the CNT fibres have a large quantum capacitance, evidenced by the near linear dependence of geometric capacitance (and conductivity) on potential bias. This reflects the low dimensionality of the CNT building blocks, which were purposely synthesised to have 1–5 layers and a high degree of graphitization. From the charge–discharge measurements of supercapacitor devices with symmetric CNT fibre electrodes we obtain power and energy densities as high as 58 kW kg−1 and 14 Wh kg−1, respectively. These record-high values for CNT fibre-based supercapacitors, are a consequence of the low equivalent series resistance due to the high conductivity of the fibres, the large contribution from quantum capacitance, and the wide stability window of the ionic liquid (3.5 V). Cycle life experiments demonstrate stable capacitance and energy retention over 10 000 cycles of charge–discharge at 3.5 V.

All-printed capacitors from graphene-BN-graphene nanosheet heterostructures

Abstract: This work aims to develop methodologies to print pinhole-free, vertically stacked heterostructures by sequential deposition of conductive graphene and dielectric h-BN nanosheet networks. We achieve this using a combination of inkjet printing and spray-coating to fabricate dielectric capacitors in a stacked graphene/BN/graphene arrangement. Impedance spectroscopy shows such heterostructures to act as series combinations of a capacitor and a resistor, with the expected dimensional dependence of the capacitance. The areal capacitance ranges from 0.24 to 1.1 nF/cm2 with an average series resistance of ∼120 kΩ. The sprayed BN dielectrics are pinhole-free for thicknesses above 1.65 μm. This development paves the way toward fabrication of all-printed, vertically integrated, multilayer devices.

Unified Equivalent Circuit Model and Optimal Design of $V^{2}$ Controlled Buck Converters

Abstract: $V^{2}$ control has advantages of simple implementation and fast transient response and is widely used in industry for point-of-load applications. This control scheme is elegant when output capacitors with large RC time constant are employed, such as OSCON capacitors. However, in most cases using capacitors with small RC time constant, such as ceramic capacitors, instability problem will occur. Previous modeling methods including sampled-data modeling, discrete-time analysis, time-domain analysis, and describing function are all very mathematical and difficult to apply for practical engineers as little physical insight can be extracted. Up to now, no equivalent circuit model is proposed which is able to predict the instability issue and serve as a useful design tool for $V^{2}$ control. This paper proposes a unified equivalent circuit model which is applicable to all types of capacitors by considering the effect of capacitor voltage ripple. The equivalent circuit provides the physical insight of $V^{2}$ control as a nonideal voltage source, a dual concept of previous nonideal current source for current-mode control. The equivalent circuit model is a simple yet accurate complete model and is very helpful for design purpose. Optimal design guidelines for point-of-load applications are provided. The proposed equivalent circuit model is applicable to both variable frequency modulation and constant frequency modulation. The equivalent circuit model and design guidelines are verified with Simplis simulation and experimental results.

A Modular High-Voltage Pulse-Generator With Sequential Charging for Water Treatment Applications

Abstract: High-voltage pulse-generators can be used effectively for bacterial decontamination in water treatment applications. Applying a pulsed electric field to the infected water sample guarantees killing of harmful germs and bacteria. In this paper, a modular high-voltage pulse-generator with sequential charging is proposed for water treatment via underwater pulsed streamer corona discharge. The proposed generator consists of series-connected modules similar to an arm of a modular multilevel converter. The modules’ capacitors are charged sequentially from a relatively low-voltage dc supply, then they are connected in series and discharged into the load. Two configurations are proposed in this paper, one for low repetitive pulse rate applications, and the other for high repetitive pulse rates. In the first topology, the equivalent resistance of the infected water sample is used as a charging resistance for the generator’s capacitors during the charging process. While in the second topology, the water resistance is bypassed during the charging process, and an external charging resistance with proper value is used instead. In this paper, detailed designs for the proposed pulse-generators are presented and validated by simulation results using MATLAB. A scaled down experimental setup has been built to show the viability of the proposed concept.

Electrical characterizations of Au/ZnO/n-GaAs Schottky diodes under distinct illumination intensities

Abstract: The Au/ZnO/n-GaAs Schottky barrier diode was fabricated and examined regarding to its current–voltage characteristics under distinct illumination intensities at room temperature. The reverse biased current increases with increasing illumination level while forward biased current is almost unchanged with illumination which states that the fabricated diodes exhibit photosensitive character or photodiode behavior. Hence, the shunt resistance is decreased with illumination while the series resistance is almost remained constant. The increment in the ideality factor after illumination can be ascribed to the assumption of inhomogeneities at M/S interface. Considering the ideality factor and the voltage dependent effective barrier height, the energy distribution profiles of surface states (Nss) were formed by the forward bias current–voltage data and increased with increasing illumination level. The Nss values acquired by considering series resistance are lower than those acquired by ignoring series resistance. Consequently, surface states can serve as recombination centers and have great importance especially in reverse bias current–voltage characteristics.

Spectral Capacitance of Series and Parallel Combinations of Supercapacitors

Abstract: The porous nature of the electrode material in supercapacitors and the apparent conductivity of their electrolyte cause their impedance to show a complex frequency-dependent behavior, which in turn makes it incorrect to treat them as ideal capacitors even at a few millihertz frequency. This is particularly crucial if the intended application requires a configuration using stacked supercapacitor banks, where errors in defining the metrics of the individual components accumulate. While manufacturers provide supercapacitor ratings at DC only, we show using a detailed impedance spectroscopy study of all possible series and parallel combinations of two different commercial 1 Farad carbon-carbon supercapacitors, that these nominal DC capacitances are not suitable to evaluate the equivalent capacitance. Instead, using a model consisting of a series resistance and a constant phase element, we employ a real effective capacitance (in proper Farad units) suitable for direct application. This effective capacitance can be used to define a frequency-dependent quality factor of a supercapacitor, and enables the easy calculation of series and parallel associations of identical or different devices.

Transparent metal oxide films based sensors for solar tracking applications

Abstract: Titanium dioxide:zinc oxide (TiO2:ZnO) composite thin films were prepared onto glass and p-type silicon substrates by the sol–gel spin coating technique. The composite metal oxide films indicate a high transmittance more than 90% in visible region. Optical constants such as optical band gap and refractive index of the films were determined using transmittance and reflectance spectra. TiO2:ZnO/p-Si diodes exhibited a high photocurrent responsivity under various solar illuminations. Electronic parameters like ideality factor, series resistance and barrier height were determined from I–V characteristic curves, Cheung model and Norde equations. The capacitance–voltage and conductance–voltage of the diodes were measured in the range from 10 kHz to 1 MHz. The capacitance of the diodes exhibited a decrease in capacitance and increase in conductance with increasing frequency. The obtained results suggest that TiO2:ZnO/p-Si diodes can be used as a sensor in optic communications and optoelectronic applications.

Study of self-compliance behaviors and internal filament characteristics in intrinsic SiOx-based resistive switching memory

Abstract: Self-compliance characteristics and reliability optimization are investigated in intrinsic unipolar silicon oxide (SiOx)-based resistive switching (RS) memory using TiW/SiOx/TiW device structures. The program window (difference between SET voltage and RESET voltage) is dependent on external series resistance, demonstrating that the SET process is due to a voltage-triggered mechanism. The program window has been optimized for program/erase disturbance immunity and reliability for circuit-level applications. The SET and RESET transitions have also been characterized using a dynamic conductivity method, which distinguishes the self-compliance behavior due to an internal series resistance effect (filament) in SiOx-based RS memory. By using a conceptual “filament/resistive gap (GAP)” model of the conductive filament and a proton exchange model with appropriate assumptions, the internal filament resistance and GAP resistance can be estimated for high- and low-resistance states (HRS and LRS), and are found to be independent of external series resistance. Our experimental results not only provide insights into potential reliability issues but also help to clarify the switching mechanisms and device operating characteristics of SiOx-based RS memory.

Fractional order equivalent series resistance modelling of electrolytic capacitor and fractional order failure prediction with application to predictive maintenance

Abstract: Being one of the most used passive components in power electronics, electrolytic capacitors have the shortest life span due to their wear-out failure which is mainly caused by vaporisation and deterioration of capacitor electrolyte. Knowing these two phenomena increase equivalent series resistance (ESR) of the capacitor, tracking ESR value over the system operating time can be a good indicator for state of health of an electrolytic capacitor. To set the maintenance schedule, various ESR monitoring algorithms have been investigated in literature. These classical real-time algorithms lead the maintenance program to be either risky if the prediction is longer than the actual life-time or more expensive if it is shorter than the actual life span. This study presents a generalised equivalent model using fractional order (FO) element for electrolytic capacitor to estimate the ESR and impedance of faultless running capacitor. Furthermore, a novel failure predictive model using Mittag-Leffler function is proposed to track the ESR increment and estimate the failure time. Hence, the predictive maintenance of the system with capacitors nearing their failure time can be set more precisely. These two FO models are compared against classical ESR and life-time prediction models to illustrate the enhanced performances of the proposed models.

Capacitance roll-off and frequency-dispersion capacitance–conductance phenomena in field plate and guard ring edge-terminated Ni/SiO2/4H-nSiC Schottky barrier diodes

Abstract: In this work, field plate and guard ring edge-terminated Ni/4H-nSiC Schottky barrier diodes (SBD) were fabricated using standard photolithography process. Strange peaks in capacitance–conductance curves, capacitance roll-off, and a high value of ideality factor (η = 1.3) in fabricated SBD were seen as a signature of interface trap states (Nss) at the residual oxide (2.2 nm)/4H-nSiC interface and series resistance (Rs). Schottky capacitance spectroscopic, High–low capacitance–voltage (C–V) and forward-bias current–voltage (I–V) techniques, in the frequency range from 100 Hz to 1 MHz, determines Nss of the order of 1012 cm−2 eV−1 and were found exponentially distributed in the bandgap of SiC. Using Hill–Coleman's method, the density Nss was calculated to be 1.15 × 1015 cm−2 eV−1 at 100 Hz and 7.81 × 1012 cm−2 eV−1 at 1 MHz, which explains the larger value of capacitance at low frequencies. Relaxation times and capture cross sections of Nss were also estimated. Calculated values of Nss were used in a Silvaco simulation that emphasize that bulk level defects present in the SiC also contributes in the experimentally observed strange peaks in C–V characteristics of fabricated SBD. At higher current levels, calculated values of Rs (V, f), confirm an increase of leakage current through residual oxide and describes the capacitance roll-off phenomena in the fabricated SBD.

Lifetime Monitoring of Electrolytic Capacitor to Maximize Earnings From Grid-Feeding PV System

Abstract: Electrolytic capacitors are popularly used in single-phase grid-feeding solar photovoltaic (PV) inverters to suppress the second harmonic and switching frequency voltage ripples. With aging equivalent series resistance (ESR) of capacitor increases and its capacitance value decreases, which lead to increase in dc-link voltage ripple. Oscillations in PV operating point around its maximum power point (MPP), results in reduction of average output power and revenue generated. To address this, frequent replacement of capacitors are required, which may lead to increased cost. Therefore, capacitors must be replaced at an optimal period to ensure maximum earnings. To realize this, a technique for monitoring of power extraction efficiency (PEE) is proposed in this paper. Further, criteria for replacement of capacitor based on the measured values of PEE is suggested. Mathematical model relating the capacitance and ESR values to the PEE is derived. Effect of variation in temperature and solar radiation on PEE is discussed. Detailed simulation studies are carried out using MATLAB-Simulink. A scaled down laboratory prototype of inverter is developed. The proposed technique is implemented in the existing digital processor/controller used for MPP tracking, thereby avoiding additional circuits/sensors. PEE estimated by simulation and experimentation are found to be within 1% of each other.

Comprehensive Study on Electrical and Hydrogen Gas Sensing Characteristics of Pt/ZnO Nanocrystalline Thin Film-Based Schottky Diodes Grown on n-Si Substrate Using RF Sputtering

Abstract: This paper presents a comprehensive study on the electrical characteristics of Pt/ZnO thin film Schottky contacts fabricated on n-Si substrates by RF sputtering, and its application as a Hydrogen sensor. The basic structural, surface morphological, and optical properties of the ZnO thin film were also been explored. Pt/ZnO thin film junction was characterized using current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature, exhibiting rectifying behavior with barrier height, ideality factor and series resistance of 0.71 eV (I–V) /0.996 eV (C–V), 2.5 and ∼95 Ω respectively. The lack of congruence between the values of Schottky barrier heights calculated from I–V and C–V measurements is interpreted. Cheung's method and modified Norde's functions were employed along with the conventional thermionic emission model, to incorporate the impact of series resistance in the calculation of diode parameters. We unveiled, the Hydrogen sensing characteristics displayed by the Pt/ZnO thin film-based sensor to different concentrations (200–1000 ppm) of Hydrogen at 350 °C. The sensor has exhibited good recoverable transient characteristics under a series of Hydrogen exposure cycles with a maximum sensitivity of 57% at 1000 ppm of Hydrogen.

Architectural Modifications for Flexible Supercapacitor Performance Optimization

Abstract: We have developed material and architectural alternatives for flexible supercapacitors and investigated their effect on practical performance. The substrate alternatives include paperboard as well as various polyethylene terephthalate (PET) films and laminates, with aqueous NaCl electrolyte used in all devices. In all the supercapacitors, activated carbon is used as the active layer and graphite ink as the current collector, with various aluminium or copper structures applied to enhance the current collectors’ conductivity. The capacitance of the supercapacitors was between 0.05 F and 0.58 F and their equivalent series resistance (ESR) was from <1 Ω to 14 Ω, depending mainly on the current collector structure. Furthermore, leakage current and selfdischarge rates were defined and compared for the various architectures. The barrier properties of the supercapacitor encapsulation have a clear correlation with leakage current, as was clearly shown by the lower leakage in devices with an aluminium barrier layer. A cycle life test showed that after 40000 charge-discharge cycles the capacitance decreases by less than 10%.

Supercapacitors Improve the Performance of Linear Power-Management Circuits: Unique new design options when capacitance jump from micro-farads to farads with a low equivalent series resistance

Abstract: For over a century, the electronics design community has used electrolytic or polypropylene capacitors with values from 10 pF to 100,000 nF, and they come with voltage ratings from less than 10 V to several thousand volts. They are traditionally used for filters, dc blocking, and very short-term energy storage in electronic circuits. Within the last two decades, a new form of capacitors, electric doublelayer capacitors (EDLCs), entered the electronic marketplace, filling the gap between rechargeable batteries and electrolytic capacitors. Today, these devices are generally known as supercapacitors (SCs), ultracapacitors (UCs), electrochemical capacitors (ECs), and EDLCs.

Short channel effects in graphene-based field effect transistors targeting radio-frequency applications

Abstract: Channel length scaling in graphene field effect transistors (GFETs) is key in the pursuit of higher performance in radio frequency electronics for both rigid and flexible substrates. Although two-dimensional (2D) materials provide a superior immunity to short channel effects (SCEs) than bulk materials, they could dominate in scaled GFETs. In this work, we have developed a model that calculates electron and hole transport along the graphene channel in a drift-diffusion basis, while considering the 2D electrostatics. Our model obtains the self-consistent solution of the 2D Poisson's equation coupled to the current continuity equation, the latter embedding an appropriate model for drift velocity saturation. We have studied the role played by the electrostatics and the velocity saturation in GFETs with short channel lengths Severe scaling results in a high degradation of GFET output conductance. The extrinsic cutoff frequency follows a scaling trend, where the index fulfills The case corresponds to long-channel GFETs with low source/drain series resistance, that is, devices where the channel resistance is controlling the drain current. For high series resistance, decreases down to and it degrades to values of because of the SCEs, especially at high drain bias. The model predicts high maximum oscillation frequencies above 1 THz for channel lengths below 100 nm, but, in order to obtain these frequencies, it is very important to minimize the gate series resistance. The model shows very good agreement with experimental current voltage curves obtained from short channel GFETs and also reproduces negative differential resistance, which is due to a reduction of diffusion current.

GaInN-based tunnel junctions with graded layers

Abstract: We demonstrated low-resistivity GaInN-based tunnel junctions using graded GaInN layers. A systematic investigation of the samples grown by metalorganic vapor phase epitaxy revealed that a tunnel junction consisting of a 4 nm both-sides graded GaInN layer (Mg: 1 × 1020 cm−3) and a 2 nm GaN layer (Si: 7 × 1020 cm−3) showed the lowest specific series resistance of 2.3 × 10−4 Ω cm2 at 3 kA/cm2 in our experiment. The InN mole fraction in the 4 nm both-sides graded GaInN layer was changed from 0 through 0.4 to 0. The obtained resistance is comparable to those of standard p-contacts with Ni/Au and MBE-grown tunnel junctions.

Planar Lithographed Superconducting LC Resonators for Frequency-Domain Multiplexed Readout Systems

Abstract: Cosmic microwave background (CMB) polarization experiments are increasing the number of transition edge sensor (TES) bolometers to increase sensitivity. In order to maintain low thermal loading of the sub-Kelvin stage, the frequency-domain multiplexing (FDM) factor has to increase accordingly. FDM is achieved by placing TES bolometers in series with inductor–capacitor (LC) resonators, which select the readout frequency. The multiplexing factor can be raised with a large total readout bandwidth and small frequency spacing between channels. The inductance is kept constant to maintain a uniform readout bandwidth across detectors, while the maximum acceptable value is determined by bolometer stability. Current technology relies on commercially available ceramic chip capacitors. These have high scatter in their capacitance thereby requiring large frequency spacing. Furthermore, they have high equivalent series resistance (ESR) at higher frequencies and are time consuming and tedious to hand assemble via soldering. A solution lies in lithographed, planar spiral inductors (currently in use by some experiments) combined with interdigitated capacitors on a silicon (Si) substrate. To maintain reasonable device dimensions, we have reduced trace and gap widths of the LCs to 4 $$\upmu $$ m. We increased the inductance from 16 to 60 $$\upmu $$ H to achieve a higher packing density, a requirement for FDM systems with large multiplexing factors. Additionally, the Si substrate yields low ESR values across the entire frequency range and lithography makes mass production of LC pairs possible. We reduced mutual inductance between inductors by placing them in a checkerboard pattern with the capacitors, thereby increasing physical distances between adjacent inductors. We also reduce magnetic coupling of inductors with external sources by evaporating a superconducting ground plane onto the backside of the substrate. We report on the development of lithographed LCs in the 1–5 MHz range for use with FDM systems. These resonators will be used by CMB polarization experiments such as Polarbear-2, Simons Array, and SPT-3G. Existing FDM systems have multiplexing factors up to 16 $$\times $$ . We report the extension to 40 $$\times $$ , i.e., Polarbear-2, and 68 $$\times $$ , i.e., SPT-3G. We present the design criteria of Polarbear-2’s LC circuits, the fabrication techniques, and the testing. Concerns such as yield, accuracy in frequency, loss, and mutual inductance between spatially neighboring channels will be discussed.

RF power absorption by plasma of a low-pressure inductive discharge

Abstract: This paper aims to analyze the mechanism of power absorption and to reveal, both experimentally and numerically, the basic factors determining the ability of plasma to absorb RF power. This is done by determining the plasma equivalent resistance value under different conditions in a low-pressure RF inductive discharge such as different antenna shape, working gas pressure, electron density, operating frequency and geometrical dimensions of the plasma source. Experimental and numerical results show that the plasma equivalent resistance changes non-monotonously with an increase in electron density, increases with an increase in neutral gas pressure, and that the maximum plasma equivalent resistance shifts toward higher electron densities when the operating frequency is increased.

Series Resistance Reduction in Stacked Nanowire FETs for 7-nm CMOS Technology

Abstract: Vertically stacked nanowire field effect transistors currently dominate the race to become mainstream devices for 7-nm CMOS technology node. However, these devices are likely to suffer from the issue of nanowire stack position dependent drain current. In this paper, we show that the nanowire located at the bottom of the stack is farthest away from the source/drain silicide contacts and suffers from higher series resistance as compared to the nanowires that are higher up in the stack. It is found that upscaling the diameter of lower nanowires with respect to the upper nanowires improved uniformity of the current in each nanowire, but with the drawback of threshold voltage reduction. We propose to increase source/drain trench silicide depth as a more promising solution to this problem over the nanowire diameter scaling, without compromising on power or performance of these devices.

Analysis of interface states of FeO-Al2O3 spinel composite film/p-Si diode by conductance technique

Abstract: The interface states and series resistance properties of the Al/FeO-Al2O3/p-Si diode were investigated by the capacitance (C) and conductance (G) measurements. The measured capacitance and conductance values were corrected to eliminate the effect of series resistance to obtain the real capacitance and conductance values of the diode. The C and G characteristics indicate the presence of interface states at the interface of the diode. The interface states density, N ss, was determined using Hill–Coleman method, and it was found that the density of interface states is decreased with the frequency. The obtained results suggest that the series resistance and interface states affect significantly the electronic parameters of the Al/FeO-Al2O3/p-Si diode.

Low resistivity ZnO-GO electron transport layer based CH3NH3PbI3 solar cells

Abstract: Perovskite based solar cells have demonstrated impressive performances. Controlled environment synthesis and expensive hole transport material impede their potential commercialization. We report ambient air synthesis of hole transport layer free devices using ZnO-GO as electron selective contacts. Solar cells fabricated with hole transport layer free architecture under ambient air conditions with ZnO as electron selective contact achieved an efficiency of 3.02%. We have demonstrated that by incorporating GO in ZnO matrix, low resistivity electron selective contacts, critical to improve the performance, can be achieved. We could achieve max efficiency of 4.52% with our completed devices for ZnO: GO composite. Impedance spectroscopy confirmed the decrease in series resistance and an increase in recombination resistance with inclusion of GO in ZnO matrix. Effect of temperature on completed devices was investigated by recording impedance spectra at 40 and 60 oC, providing indirect evidence of the performance of solar cells at elevated temperatures.

Short channel effects in graphene-based field effect transistors targeting radio-frequency applications

Abstract: Channel length scaling in graphene field effect transistors (GFETs) is key in the pursuit of higher performance in radio frequency electronics for both rigid and flexible substrates. Although two-dimensional (2D) materials provide a superior immunity to Short Channel Effects (SCEs) than bulk materials, they could dominate in scaled GFETs. In this work, we have developed a model that calculates electron and hole transport along the graphene channel in a drift-diffusion basis, while considering the 2D electrostatics. Our model obtains the self-consistent solution of the 2D Poisson's equation coupled to the current continuity equation, the latter embedding an appropriate model for drift velocity saturation. We have studied the role played by the electrostatics and the velocity saturation in GFETs with short channel lengths L. Severe scaling results in a high degradation of GFET output conductance. The extrinsic cutoff frequency follows a 1/L^n scaling trend, where the index n fulfills n < 2. The case n = 2 corresponds to long-channel GFETs with low source/drain series resistance, that is, devices where the channel resistance is controlling the drain current. For high series resistance, n decreases down to n= 1, and it degrades to values of n < 1 because of the SCEs, especially at high drain bias. The model predicts high maximum oscillation frequencies above 1 THz for channel lengths below 100 nm, but, in order to obtain these frequencies, it is very important to minimize the gate series resistance. The model shows very good agreement with experimental current voltage curves obtained from short channel GFETs and also reproduces negative differential resistance, which is due to a reduction of diffusion current.