Influence of surface defect charge at AlGaN-GaN-HEMT upon Schottky gate leakage current and breakdown voltage

The relation between Schottky gate leakage current and the breakdown voltage of AlGaN-GaN high-electron mobility transistors (HEMTs) is discussed based on the newly introduced simple, yet useful, surface defect charge model. This model represents the leakage current caused by the positive charge in the surface portion of AlGaN layer induced by process damage such as nitrogen vacancies. The new model has been implemented into a two-dimensional device simulator, and the relationship between the gate leakage current and the breakdown voltage was simulated. The simulation results reproduced the relationship obtained experimentally between the leakage current and the breakdown voltage. Further simulation and experiment results show that the breakdown voltage is maintained even if the defect charge exists up to the defect charge density of 2.5/spl times/10/sup 12/ cm/sup -2/, provided the field plate structure is adopted, while the breakdown voltage shows a sudden drop for the defect density over 5/spl times/10/sup 11/ cm/sup -2/ without the field plate. This result shows that the field plate structure is effective for suppressing the surface charge influence on breakdown voltage due to the relaxation of the electric field concentration in the surface portion of the AlGaN layer.     

On negative selection against ATG triplets near start codons in eukaryotic and prokaryotic genomes

The frequencies of ATG triplets in the genomes of various species were systematically analyzed, and the frequency of ATG triplets was significantly low around start codons in both prokaryotic and eukaryotic genomes. In eukaryotes, however, the frequency decrease before the start codon is much more evident than that after the start codon. In prokaryotes, on the other hand, the ATG frequency pattern around the start codon is less evident, and-more importantly-symmetric. We also computed average distances between a start codon and its nearest upstream-located ATG triplet and found a general tendency for the average distances to be longer in higher organisms.     

Measurement of the surrounding liquid motion of a single rising bubble using a Dual-Camera PIV system

A new type of Particle Image Velocimetry technique, called “Dual-Camera PIV System”, was developed in order to achieve higher-accuracy measurement at a high time resolution. It is very difficult to measure precisely a complex flow field such as a gas–liquid two phase flow using PIV, because of the existence of a strong turbulence. In the conventional dynamic PIV, a time interval of two images required for analysis depends basically on a camera frame rate. A time interval of a set of PIV images affects the measurement accuracy significantly. Therefore, it is necessary to shorten the time interval of a set of PIV images as well as to achieve high frame rates. For this specific purpose, we developed a measurement system composed of two high speed cameras. The interval of two images obtained from each camera was controlled arbitrarily. Furthermore, a recursive cross-correlation method was adopted as PIV algorithm in order to achieve high spatial resolution. The interrogation areas were determined from the number density of PIV particles. The developed system was evaluated by cross-correlation coefficient and signal–noise (S/N) ratio. As the demonstration, the surrounding liquid motion in the vicinity of a single rising bubble was measured via this measurement system.     

Direct contact melting with asymmetric load

In this study, direct contact melting with an asymmetric load was investigated both experimentally and by numerical analysis. A rectangular parallelepiped solid on a heating surface was melted under various asymmetric loads, while total load acting on the solid and brine temperature were kept constant. In the numerical analysis, the melting process keeping the force balance between the pressure in the liquid film and the loads at all times was calculated. It was found that the average heat flux into the solid was independent of the moment acting on the solid. Analytical results of the time dependencies of the amount of melting and the inclination of the solid agreed with experimental ones for each condition.     

Scale-Model Experiment and Numerical Simulation of a Steel Teeming Process

During the teeming process of molten steel from a ladle, a bathtub-type vortex may be formed. The vortex entrains undesired slag floating on the surface, lowering the quality. The formation of such vortices was studied using scale models. Since the kinematic viscosity of water is similar to that of molten steel, we used water in our experiments. The particle image velocimetry (PIV) technique was used to measure water flow patterns. Dimensional analysis showed that the Froude number was the governing Π number. A computational fluid dynamics (CFD) model successfully simulated the behavior of the vortex formed in our scale-model experiments.     

Upper Critical Field of Organic Superconductor κ-(ET)2Cu(NCS)2 Determined by Specific Heat Measurements

Both specific heat and resistance measurements of organic superconductor -(ET)₂Cu(NCS)₂ under magnetic field were performed. Upper critical fields, obtained from the specific heat data, are significantly less than those from the magnetoresistance measurements. At temperatures below 2 K and magnetic fields below 1 T, we observed additional power dissipation due to the field sweep. This effect may be attributed to the existence of irreversibility line.     

Quantum Transition to Supersolid Phase

We study a possible coexistence of superconducting state and charge density waves which, in a broad sense, might be called a supersolid phase. We investigate the infinite dimensional (d=) attractive Hubbard model by applying a sublattice dependent Gutzwiller wave function |BCS as a variational wave function describing the ground state. One may naively expect that the BCS superconducting state evolves continuously to the Bose–Einstein condensed state of bipolarons as the attractive interaction increases, as far as the system is dilute. However, we show that our variational wave function has lower energy than the simple BCS wave function for all electron densities and the interaction strengths. Our variational parameters increase (g _A,B) as we increase the interaction strength (U). The energy gap turns out to be a mixture of s and extended-s waves. In the vicinity of half-filling, we find a quantum transition from a simple superconducting phase to a supersolid phase with increase of the electron density and/or the interaction strength.