High Current Levels Research Articles

Off-state breakdown and leakage current transport analysis of AlGaN/GaN high electron mobility transistors

Off-state breakdown characteristics of AlGaN/GaN high-electron-mobility transistors have been studied based on drain current injection method. It is found that at low drain current injection level, the observed premature breakdown is caused by excess gate-to-drain leakage current. Nevertheless, at high drain injection current level, buffer-leakage-dominated breakdown proceeds gate-leakage-dominated breakdown as the gate bias increases from pinch-off voltage to deep-depletion voltage. In both breakdown regions, the breakdown voltages show negative temperature coefficients. The buffer-leakage-induced breakdown should be defect-related, which is confirmed by temperature-dependent buffer leakage measurements.

Qualidade de sementes de milho em resposta à adubação nitrogenada e à inoculação com bactérias diazotróficas

In the search for elevating high current levels of productivity and reduction in production costs of corn seeds in Brazil, new technologies are being incorporated into the production system. The aim of this study was to evaluate the physiological quality of seeds of maize in response to nitrogen fertilization and inoculation with Azospirillum brasiliense and Herbaspirillum seropedicae. Two experiments were installed, and in the first the design adopted was a randomized block, and in the second experiment, the design used was a completely randomized, both with nine treatments (control without N and without inoculation, control without N and with A. brasilense, control without N and with H. seropedicae, 30 kg N ha-1 in sowing, 30 kg N ha-1 in sowing + A. brasilense, 30 kg N ha-1 in sowing + H. seropedicae, 30 kg N ha-1 in sowing + 90 kg N ha-1 in topdressing, 30 kg N ha-1 in sowing + A. brasilense + 90 kg N ha-1 in topdressing; 30 kg N ha-1 in sowing + H. seropedicae + 90 kg N ha-1 in topdressing) and four replications. The variables evaluated were: percentage and first count of germination, dry mass and length of seedling, accelerated aging, cold test and electrical conductivity. The dry mass production are affected by inoculation with diazotrophic bacteria in association with nitrogen fertilization. Inoculation of corn seeds with A. brasilense, H. seropedicae and the use of nitrogen fertilizer promotes increase in productivity of corn seed to the extent of 4.73; 5.17 to 8.90%, respectively. The use of A. brasilense improves the physiological quality of seeds of maize by cold test without soil.

Three-Dimensional Magnetic and Mechanical Finite-Element Analysis of the HTS Insert Coil

Future accelerator magnets will need to reach field in the 20-T range. Reaching such a magnetic field is a challenge only attainable using high-temperature superconductor (HTS) material. The high current densities and stress levels needed to satisfy the design criterion of such magnets make YBaCuO superconductor the most appropriate candidate. The HFM EUCARD program is aimed to design and manufacture a dipole insert made of HTS material generating 6 T inside a Nb3Sn dipole of 13 T at 4.2 K. In the HTS insert, engineering current densities higher than 250 A/mm2 under 19 T are required to reach the performances. This paper summarizes the results of magnetic and mechanical finite element analysis of the HTS dipole insert magnet. The nonlinearity of the ferromagnetic material is also taken into account. The mechanical analysis of the magnet is carried out under Lorentz forces.

Oil whip-induced wear in journal bearings

This paper investigates the effect of oil whirl and oil whip in fluid film radial bearings due to possible metallic contact. The degree of metallic contact and thereby wear and tear between rotating shafts and bearing bushes is assessed by measuring electric currents through the oil film. The current as well as the voltage varied in accordance with the contact ratio between the shaft and bush in the fluid film radial bearing. The gauge signal thus indicates the degree of metallic contact based on the thickness of the oil film in the load zone. Some experimental results are provided to illustrate that at low normalised loads involving oil whirl and oil whip, no electric current is detected, while high levels of electric current are registered at high load levels when no oil whirl or oil whip occurred. It is therefore concluded that at low loads, oil whirl and oil whip have little influence on wear and tear in a journal bearing.

Bivariate quadratic method in quantifying the differential capacitance and energy capacity of supercapacitors under high current operation

Capacitance and resistance are the fundamental electrical parameters used to evaluate the electrical characteristics of a supercapacitor, namely the dynamic voltage response, energy capacity, state of charge and health condition. In the British Standards EN62391 and EN62576, the constant capacitance method can be further improved with a differential capacitance that more accurately describes the dynamic voltage response of supercapacitors. This paper presents a novel bivariate quadratic based method to model the dynamic voltage response of supercapacitors under high current charge–discharge cycling, and to enable the derivation of the differential capacitance and energy capacity directly from terminal measurements, i.e. voltage and current, rather than from multiple pulsed-current or excitation signal tests across different bias levels. The estimation results the author achieves are in close agreement with experimental measurements, within a relative error of 0.2%, at various high current levels (25–200 A), more accurate than the constant capacitance method (4–7%). The archival value of this paper is the introduction of an improved quantification method for the electrical characteristics of supercapacitors, and the disclosure of the distinct properties of supercapacitors: the nonlinear capacitance–voltage characteristic, capacitance variation between charging and discharging, and distribution of energy capacity across the operating voltage window.

Study on Performance Improvement of AC-DC Converter

Study on Performance Improvement of AC-DC Converter

Stable and unstable dynamics of Overhauser fields in a double quantum dot

Nonlinear dynamics of nuclear spin ensembles driven by a two-electron system in a double quantum dot in the Pauli spin blockade (SB) regime is studied experimentally in conjunction with numerical simulation. Dynamic nuclear spin polarization (DNP) is systematically studied by evaluating the current level and its fluctuations. We interpret large current noise in the SB regime as stable feedback noise, where identical Overhauser fields of the two dots are preferred. In contrast, stepwise increases of current in the shallow Coulomb blockade region can be understood as unstable dynamics with significant imbalance of the Overhauser fields, which cancels the external magnetic field in one of the two dots. There, an extremely small transverse Overhauser field can easily lift the SB transport, giving the highest current level, when longitudinal components cancel the applied field.

Analysis of dominant carrier recombination mechanisms depending on injection current in InGaN green light emitting diodes

Two kinds of green InGaN light emitting diodes (LEDs) have been investigated in order to understand the different slopes in logarithmic light output power-current (L-I) curves. Through the analysis of the carrier rate equation and by considering the carrier density-dependent the injection efficiency into quantum wells, the slopes of the logarithmic L-I curves can be more rigorously understood. The low current level, two as the tunneling current is initially dominant. The high current level beyond the peak of the external quantum efficiency (EQE) diminishes below one as the carrier overflow becomes dominant. In addition, the normalized carrier injection efficiency can be obtained by analyzing the slopes of the logarithmic L-I curves. The carrier injection efficiency decreases after the EQE peak of the InGaN LEDs, determined from the analysis of the slopes of the logarithmic L-I curves.

Implementation of superconducting fault current limiter for flexible operation in the power substation

The concentration of large-scale power loads located in the metropolitan areas have resulted in high fault current levels during a fault thereby requiring the substation to operate in the double busbar configuration mode. However, the double busbar configuration mode results in deterioration of power system reliability and unbalanced power flow in the adjacent transmission lines which may result in issues such as overloading of lines. This paper proposes the implementation of the superconducting fault current limiter (SFCL) to be installed between the two substation busbars for a more efficient and flexible operation of the substation enabling both single and double busbar configurations depending on the system conditions for guaranteeing power system reliability as well as fault current limitations. Case studies are being performed for the effectiveness of the SFCL installation and results are compared for the cases where the substation is operating in single and double busbar mode and with and without the installation of the SFCL for fault current mitigation.

Coherent Raman Scattering Microscopy

Reviewed by Alberto Diaspro, SPIE Fellow, Istituto Italiano di Tecnologia, Genova, Italy Research and development of modern optical microscopes are guided by several aspects, from resolution obsession to imaging of living specimens, from a low-level of perturbation to long-term experiments on biological systems. One of the most outstanding goals is the possibility of performing at the highest current levels, including 4-D (x,y,z,t) nanoscopy/superresolution, label-free imaging. Coherent Raman scattering (CRS) microscopy represents a key label-free imaging method for studying living cells and tissues at a high time rate. Ji-Xin Cheng and Xiaoliang Sunney Xie, two of the most representative scientists in this area, assembled the perfect book for disseminating the method. Their experience as scientists and authors and their robustness and consideration in the field allowed them to edit a great book that, starting from the foundation of CRS microscopy, covers the field in a comprehensive and exhaustive way.

Auditory-Nerve Responses to Varied Inter-Phase Gap and Phase Duration of the Electric Pulse Stimulus as Predictors for Neuronal Degeneration

After severe hair cell loss, secondary degeneration of spiral ganglion cells (SGCs) is observed-a gradual process that spans years in humans but only takes weeks in guinea pigs. Being the target for cochlear implants (CIs), the physiological state of the SGCs is important for the effectiveness of a CI. For assessment of the nerve's state, focus has generally been on its response threshold. Our goal was to add a more detailed characterization of SGC functionality. To this end, the electrically evoked compound action potential (eCAP) was recorded in normal-hearing guinea pigs and guinea pigs that were deafened 2 or 6 weeks prior to the experiments. We evaluated changes in eCAP characteristics when the phase duration (PD) and inter-phase gap (IPG) of a biphasic current pulse were varied. We correlated the magnitude of these changes to quantified histological measures of neurodegeneration (SGC packing density and SGC size). The maximum eCAP amplitude, derived from the input-output function, decreased after deafening, and increased with both PD and IPG. The eCAP threshold did not change after deafening, and decreased with increasing PD and IPG. The dynamic range was wider for the 6-weeks-deaf animals than for the other two groups. Excitability increased with IPG (steeper slope of the input-output function and lower stimulation level at the half-maximum eCAP amplitude), but to a lesser extent for the deafened animals than for normal-hearing controls. The latency was shorter for the 6-weeks-deaf animals than for the other two groups. For several of these eCAP characteristics, the effect size of IPG correlated well with histological measures of degeneration, whereas effect size of PD did not. These correlations depend on the use of high current levels, which could limit clinical application. Nevertheless, their potential of these correlations towards assessment of the condition of the auditory nerve may be of great benefit to clinical diagnostics and prognosis in cochlear implant recipients.

Mode stability in photonic-crystal surface-emitting lasers with large κ1DL

We study mode stability in photonic-crystal surface-emitting lasers (PCSELs) with large coupling-coefficient-length product κ1DL(>6). We observe that mode competition occurs at high current levels above threshold. Our combined experimental and theoretical study provides the first evidence of the mode competition originating from the high-order band-edge modes. The decreased threshold margin between these competing high-order modes and the main lasing mode with increasing cavity length as well as the spatial hole burning effect may deteriorate the single-mode stability. Our finding is essential for designing single-mode high-power PCSELs for which the strategy to suppress the high-order modes must be considered.

Highly transparent low capacitance plasma enhanced atomic layer deposition Al2O3-HfO2 tunnel junction engineering

The development of metallic single electron transistor (SET) depends on the downscaling and the electrical properties of its tunnel junctions. These tunnel junctions should insure high tunnel current levels, low thermionic current, and low capacitance. The authors use atomic layer deposition to fabricate Al2O3 and HfO2 thin layers. Tunnel barrier engineering allows the achievement of low capacitance Al2O3 and HfO2 tunnel junctions using optimized annealing and plasma exposure conditions. Different stacks were designed and fabricated to increase the transparency of the tunnel junction while minimizing thermionic current. This tunnel junction is meant to be integrated in SET to enhance its electrical properties (e.g., operating temperature, ION/IOFF ratio).

DFIG 풍력발전기가 연계된 배전선로의 고조파 공진 특성에 관한 연구

There were telecommunication noise and malfunctions of the electronic devices occurred over a wide area due to the high harmonic voltage and/or current levels of the Back-to-back converter in the DFIG wind power system even though the magnitude of all harmonics is within the international standards. The triangular carrier signals of the PWM used in the power converter system is related to the telecommunication noise because they are in the range of audible frequencies and amplified by a variety of the standing waves that were excited by harmonic voltage sources in the weak grid system such as a long distance distribution transmission lines. This paper describes the characteristics of the harmonics in the wind turbine-generator, numerical analysis and simulation of the harmonics resonance phenomena in the distribution lines as well as measuring induced voltage of the telecommunication lines in parallel with power lines in order to verify the root cause of the telecommunication noise. These noise problems can occur in a wind turbine power system with a non-linear converter at any time, as well as photovoltaic power system. So, the preliminary review of suitable filter devices and switching frequencies of the PWM have to be required by considering the stability of the controller at the design stage but as part of the measures the effect of the telecommunication cable shields was analyzed by comparing the measured data between multi-conductor with/without shields so as to attenuate the sources of the harmonics voltage induced into the telecommunication lines and to apply the most cost-effective measures in the field.

Physically based analytical model of the blocking I–V curve of Trench IGBTs

In this paper a physical model for the evaluation of the I–V curve in avalanche condition of the vertical PNP of an IGBT is presented. A two-zone linear approximation for the electric field in the depletion region is adopted and the slope of the two regions is defined from the total current and the ratio between the electrons and holes fluxes. The ratio between the electrons and holes which flow into the depletion region is evaluated by means of an accurate charge control model which provides the β of the PNP up to high injection current levels.

Design and Evaluation of a High-Frequency LTCC Inductor Substrate for a Three-Dimensional Integrated DC/DC Converter

High operating frequency and integration technique are two main approaches to achieve high power density for the switching mode power supply. The emerging gallium nitride (GaN)-based power device enables a multimegahertz high-efficiency point-of-load (POL) converter with high current capability. The low-temperature cofire ceramic (LTCC)-based integration technique successfully extends the three-dimensional (3-D) integrated POL module from the low current level to the high current level (>10 A). This paper presents the low-profile LTCC inductor substrate design and evaluation for a multimegahertz 3-D integrated POL converter with large output current. The detailed study about the impact of frequency on the LTCC inductor shows that the high frequency not only shrinks the volume of the inductor, but also simplifies the inductor structure. The comparison between the LTCC inductor and the discrete inductor demonstrates that the LTCC inductor dramatically boosts the converter light-load efficiency due to its nonlinear inductance. Because of the low-profile design, the power density of the single-phase POL module with LTCC inductor achieves 1.1 kW/in3 at 5 MHz. The performance of the LTCC inductor can be further improved by the inverse coupling, which results in more than 40% core thickness and core loss reduction. Therefore, the power density of a two-phase integrated POL module is pushed to 1.5 kW/in3, which is around ten times of the power density of state-of-the-art industry products with the same current level.

Unexpected Sources of Basal Plane Dislocations in 4H-SiC Epitaxy

The suppression of degradation inducing basal plane dislocations (BPDs) in the critical drift layer of SiC power devices has occurred mainly by preventing BPDs in the substrate from propagating into the drift layer. As optimized epitaxial growth has produced drift layers free of BPDs over a large fraction of the wafer, other sources of BPDs have become important. Two alternate sources are discussed. The first is epitaxial inclusions, which mainly consist of grossly misoriented 4H-SiC. The local stress field around the inclusion introduces a cluster of BPDs. In low-BPD epitaxy, the outermost BPDs can glide centimeters forming half-loop arrays that have BPD segments along their whole length. The second source of BPDs not normally considered is BPDs that are converted to threading edge dislocation before reaching the drift layer. Experiments suggest that for high current levels and/or over time, device degradation is possible.

Electromechanical Forces in Transformers Caused by Inrush Currents: An Analytical, Numerical and Experimental Approach

Within the extensive range of reasons that cause internal faults in transformers, the axial and radial electromechanical forces produced by the inrush currents show themselves as noteworthy. In fact, such events may result in high levels of transient current, which may last for quite significant periods of time. Due to this effect, there exist a number of phenomena likely to manifest themselves. One such effect consists of the occurrence of electromechanical forces and their corresponding influences on the mechanical structure and lifetime of the equipment. In this sense, this article has as its objective the analysis of the electromechanical forces in transformers, through an analytical, numerical, and experimental approach. In order to examine and identify the qualitative and quantitative nature of the phenomenon, the studies are further corroborated through the use of the modeling technique using the averages of the finite element method in its 3D version. Finally, comparative analyses are made between the analytical and computational results with those derived from experimental tests, which showed the potential of the presented studies for the purpose of subsidizing projects and providing the design of the supporting mechanical structures of the transformer windings.

HTS Dipole Insert Developments

Future accelerator magnets will need to reach a magnetic field in the 20 T range. Reaching such a magnetic field is a challenge only reachable using high temperature superconductor (HTS) material. The high current densities and stress levels needed to satisfy the design criterion of such magnets make YBaCuO superconductor the most appropriate candidate especially when produced using the IBAD route. The HFM EUCARD program is aimed at designing and manufacturing a dipole insert made of HTS material generating 6 T inside a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn dipole of 13 T at 4.2 K. In the HTS insert, engineering current densities higher than 250 MA/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> under 19 T are required to reach the performances. The stress level is consequently very high. The insert protection is also a critical issue as HTS shows low quench propagation velocity. The coupling with the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn dipole makes the problem even more difficult. The magnetic and mechanical designs of the HTS insert will be presented as well as the technological developments underway to realize this compact dipole insert.

Electroluminescence Analysis and Simulation of the Effects of Injection and Temperature on Carrier Distribution in InGaN-Based Light-Emitting Diodes with Color-Coded Quantum Wells

This paper reports on an extensive analysis of the electroluminescence characteristics of InGaN-based LEDs with color-coded structure, i.e., with a triple quantum well structure in which each quantum well has a different indium content. The analysis is based on combined electroluminescence measurements and two-dimensional simulations, carried out at different current and temperature levels. Results indicate that (i) the efficiency of each of the quantum wells strongly depends on device operating conditions (current and temperature); (ii) at low current and temperature levels, only the quantum well closer to the p-side has a significant emission; (iii) emission from the other quantum wells is favored at high current levels. The role of carrier injection, hole mobility, carrier density and non-radiative recombination in determining the relative intensity of the quantum wells is discussed in the text.