Comment on A. Agarwal and S. Haney, “Some Critical Materials and Processing Issues in SiC Power Devices” [J. Electron. Mater. 37, 646 (2008)]

Agarwal and Haney [J. Electron. Mater. 37, 646 (2008)] have recently suggested that bulk defects may limit the inversion-layer mobility in SiC metal oxide semiconductor field-effect transistors. However, we believe that the physics of charge trapping and Coulomb scattering by bulk traps quantitatively contradicts this model.     

Overpotential decay behavior—II. Generalized treatment for reaction pathways involving discharge,recombination and electrochemical desorption of adsorbed intermediates

Theoretical equations are developed to represent the overpotential decay behavior of a polarized electrode following current interruption in the course of a faradaic reaction. Three mechanistic schemes are considered: (a) Fast discharge—slow recombination and/or slow electrochemical desorption; (b) Slow discharge—fast recombination of fast electrochemical desorption; (c) Coupled discharge—electrochemical desorption or recombination. Analytical solutions are presented for the above cases under conditions of negligible backward reaction rate and interaction parameter g. Some experimental and theoretical difficulties specific to further generalization of the theory of overpotential buildup and decay processes are outlined, and the manner in which the coverage variation with overpotential could influence the measured stoichiometric number is briefly discussed.     

Electrostatic microactuators for precise positioning of neural microelectrodes

Microelectrode arrays used for monitoring single and multineuronal action potentials often fail to record from the same population of neurons over a period of time likely due to micromotion of neurons away from the microelectrode, gliosis around the recording site and also brain movement due to behavior. We report here novel electrostatic microactuated microelectrodes that will enable precise repositioning of the microelectrodes within the brain tissue. Electrostatic comb-drive microactuators and associated microelectrodes are fabricated using the SUMMiT V (Sandia's Ultraplanar Multilevel MEMS Technology) process, a five-layer polysilicon micromachining technology of the Sandia National labs, NM. The microfabricated microactuators enable precise bidirectional positioning of the microelectrodes in the brain with accuracy in the order of 1 microm. The microactuators allow for a linear translation of the microelectrodes of up to 5 mm in either direction making it suitable for positioning microelectrodes in deep structures of a rodent brain. The overall translation was reduced to approximately 2 mm after insulation of the microelectrodes with epoxy for monitoring multiunit activity. The microactuators are capable of driving the microelectrodes in the brain tissue with forces in the order of several micro-Newtons. Single unit recordings were obtained from the somatosensory cortex of adult rats in acute experiments demonstrating the feasibility of this technology. Further optimization of the insulation, packaging and interconnect issues will be necessary before this technology can be validated in long-term experiments.     

Electron-Scattering Mechanisms in Heavily Doped Silicon Carbide MOSFET Inversion Layers

Hall-effect measurements of n-channel MOS devices were used to determine the main scattering mechanisms limiting mobility in SiC MOSFETs. MOS-gated Hall characterization, which was performed as a function of gate bias and body bias, indicates that surface-roughness scattering and Coulomb scattering are the main scattering mechanisms limiting electron mobility in SiC MOSFETs at room temperature. A charge-sheet model, including incomplete ionization and Fermi-Dirac statistics, is used to calculate the surface electric fields in order to develop an expression for surface-roughness scattering. In the samples used for this paper, at electron sheet densities less than 1.8times10¹² cm^-2, Coulomb scattering dominates, while surface roughness is dominant at higher sheet densities.     

Overpotential decay behavior—I. Complex electrode reactions involving adsorption

Evaluation of the open-circuit overpotential decay behaviour of a previously polarized electrode provides a useful way of examining the kinetics of the electrode reaction and the electrochemical adsorption behaviour of reaction intermediates involved in the electrode process. Limiting cases have previously been considered. Analytical and numerical solutions of equations for the open-circuit decay of potential of a polarized electrode are now given for a complete range of coverages of the electrode by electro-active intermediates arising in a complex sequence and obeying a Frumkin-type isotherm. The behavior for various values of the interaction/heterogeneity factor g in the adsorption isotherm is derived. The course of the potential-log (time) relations for various g values are compared with the experimental behavior observed in chlorine evolution on C and Ir. The overpotential decay and the Tafel slopes are closely related to the adsorption pseudo-capacitance behaviour of the electrode interface bearing electroactive chemisorbed intermediates.     

Studies on the nutritional quality of some cucurbit kernel proteins

The nutritional quality of proteins of four cultivated cucurbit species was measured by nitrogen balance. The biological value of muskmelon, watermelon and pumpkin proteins were 59, 64, and 67 respectively. A value of 70 for bittergourd kernels was calculated from chemical score. Lysine was the first limiting essential amino acid in all the proteins. The leucine:isoleucine and leucine:lysine ratios did not show imbalance.     

THD reduction with reactive power compensation for fuzzy logic DVR based solar PV grid connected system

Dynamic voltage restorer (DVR) is used to protect sensitive loads from voltage disturbances of the distribution generation (DG) system. In this paper, a new control approach for the 200 kW solar photovoltaic grid connected system with perturb and observe maximum power point tracking (MPPT) technique is implemented. Power quality improvement with comparison is conducted during fault with proportional integral (PI) and artificial intelligence-based fuzzy logic controlled DVR. MPPT tracks the actual variable DC link voltage while deriving the maximum power from a photovoltaic array and maintains DC link voltage constant by changing modulation index of the converter. Simulation results during fault show that the fuzzy logic based DVR scheme demonstrates simultaneous exchange of active and reactive power with less total harmonic distortion (THD) present in voltage source converter (VSC) current and grid current with fast tracking of optimum operating point at unity power factor. Standards (IEEE-519/1547), stipulates that the current with THD greater than 5% cannot be injected into the grid by any distributed generation source. Simulation results and validations of MPPT technique and operation of fuzzy logic controlled DVR demonstrate the effectiveness of the proposed control schemes.     

Power quality investigation of a solar PV transformer-less grid-connected system fed DVR

This paper presents a single stage transformer-less grid-connected solar photovoltaic (PV) system with an active and reactive power control. In the absence of active input power, the grid-tied voltage source converter (VSC) is operated in a reactive power generation mode, which powers the control circuitry, and maintains a regulated DC voltage to the VSC. A data-based maximum power point tracking (MPPT) control scheme which performs power quality control at a maximum power by reducing the total harmonic distortion (THD) in grid injected current as per IEEE-519/1547 standards is implemented. A proportional-integral (PI) controller based dynamic voltage restorer (DVR) control scheme is implemented which controls the grid side converter during single-phase to ground fault. The analysis includes the grid current THD along with the corresponding variation of the active and reactive power during the fault condition. The MPPT tracks the actual variable DC link voltage while deriving the maximum power from the solar PV array, and maintains the DC link voltage constant by changing the modulation index of the VSC. Simulation results using Matlab/Simulink are presented to demonstrate the feasibility and validations of the proposed novel MPPT and DVR control systems under different environmental conditions.     

Influence of barrier thickness on the high-power performance ofAlGaN/GaN HEMTs

The dependence of current slump in AlGaN/GaN HEMTs on the thickness of the AlGaN barrier was observed. Power measurements on a 2×125×0.3 μm AlGaN/GaN HEMT made on Silicon Carbide (SiC) substrates with an AlGaN thickness of 10 nm gave a saturated output power of 1.23 W/mm at 8 GHz whereas a device with the same dimensions fabricated on samples with an AlGaN barrier of 20 nm gave a saturated output power of 2.65 W/mm at the same frequency. RF load line measurements clearly show the reduction of RF full channel current as compared to dc full channel current and the increase in the RF knee voltage compared to the dc knee voltage, with the effect being more pronounced in thin barrier samples. Passivation improved the large signal performance of these devices. A 1×150×0.3 μm transistor made on AlGaN(20 nm)/GaN structure gave a saturated output power of 10.7 W/mm (40% power added efficiency) at 10 GHz after passivation. This represents the state of the art microwave power density for AlGaN/GaN HEMTs. Heating of the transistors during high-power operation of these devices becomes the important factor in limiting their performance after passivation