Atsushi Nakajima

Combustion of fuel vapor-drop-air systems: Part I—Open burner flames

Abstract The burning characteristics of fuel vapor-drop-air systems (propane-kerosene drop-air systems) have been examined using an inverted-cone-flame-burner apparatus. A small amount of kerosene drops added t a propane-air mixture markedly accelerates the burning velocity for an overall fuel-air ratio and extends the region of stable burning leanwards. A small amount of propane added to a kerosene drop-air mixture, however, never exhibits such combustion-promoting effects. There exists an optimum value in the quality of kerosene drops to be added. The weaker the intensity of turbulence in the flame zone, or the finer the drops, the greater combustion-promoting effects are observed. Kerosene mists consisting of submicron droplets, however, exhibit no such combustion-promoting effects.

Combustion of fuel vapor-drop-air systems: Part II—Spherical flames in a vessel

Abstract The burning characteristics of fuel vapor-drop-air systems (propane-kerosene drop-air systems) have been studied using a cylindrical combustion vessel, 58 mm deep and 148 mm in diameter, with a centrally located spark gap. As in the case of burner flames reported in the previous paper, a small amount of kerosene drops added to a propane-air mixture intensities the burning process, raising the maximum pressure for an overall fuel-air ratio and shrotening the time to reach the maximum pressure from ignition. In addition, both the burning and propagating velocities of a flame for an overall fuel-air ratio are markedly accelerated by a very small amount of kerosene drops added. The combustion-promoting effects of drops are more prominent for leaner mixtures and get less prominent as the mixture becomes richer beyond the stoichimetric. An optimum value exists in the quantity of drops added to a propane-air mixture.

Physical Mechanism of Buffer-Related Current Transients and Current Slump in AlGaN/GaN High Electron Mobility Transistors

Two-dimensional transient analyses of AlGaN/GaN high electron mobility transistors (HEMTs) are performed in which a deep donor and a deep acceptor are considered in a buffer layer. Quasi-pulsed current–voltage (I–V) curves are derived from the transient characteristics. When the drain voltage is raised abruptly, electrons are injected into the buffer layer and captured by deep donors, and when it is lowered abruptly, the drain currents remain at low values for some periods and begin to increase slowly as the deep donors begin to emit electrons, showing drain-lag behavior. The gate lag could also occur due to deep levels in the buffer layer, and it is correlated with relatively high source access resistance in AlGaN/GaN HEMTs. It is shown that the current slump is more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because the trapping effects become more significant. The drain lag could be a major cause of current slump in the case of higher off-state drain voltage. It is suggested that to minimize current slump in AlGaN/GaN HEMTs, an acceptor density in the buffer layer should be made low, although there may be a trade-off relationship between reducing current slump and obtaining sharp current cutoff.

Analysis of field-plate effects on buffer-related lag phenomena and current collapse in GaN MESFETs and AlGaN/GaN HEMTs

A two-dimensional transient analysis of field-plate GaN MESFETs and AlGaN/GaN HEMTs is performed in which a deep donor and a deep acceptor are considered in a semi-insulating buffer layer, and quasi-pulsed current–voltage curves are derived from them. How the existence of a field plate affects buffer-related drain lag, gate lag and current collapse is studied. It is shown that in both MESFET and HEMT, the drain lag is reduced by introducing a field plate because electron injection into the buffer layer is weakened by it, and the buffer-trapping effects are reduced. It is also shown that the field plate could reduce buffer-related current collapse and gate lag in the FETs. The dependence of lag phenomena and current collapse on the field-plate length and on the SiN passivation layer thickness is also studied. The work suggests that in the field-plate structures, there is an optimum thickness of the SiN layer to minimize the buffer-related current collapse and drain lag in GaN MESFETs and AlGaN/GaN HEMTs.

Numerical Analysis of Breakdown Voltage Enhancement in AlGaN/GaN HEMTs With a High-

2-D analysis of breakdown characteristics in AlGaN/GaN high electron mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer. The dependence of the OFF-state breakdown voltage on the relative permittivity of the passivation layer εr and the thickness of the passivation layer d are studied. It is shown that as εr increases, the OFF-state breakdown voltage increases. This is because the electric field at the drain edge of the gate is weakened as εr increases. This occurs because in the insulator the applied voltage tends to drop uniformly in general, and hence when the insulator is attached to the semiconductor, the voltage drop along the semiconductor becomes smoother at the drain edge of the gate if the εr of the insulator is higher. It is also shown that the OFF-state breakdown voltage increases as d increases because the electric field at the drain edge of the gate is weakened as d increases. It is concluded that AlGaN/GaN HEMTs with a high- k and thick passivation layer should have high breakdown voltages.

k

Two-dimensional transient analysis of gate-field-plate AlGaN/GaN high electron mobility transistors with a backside electrode is performed by considering a deep donor and a deep acceptor in a buffer layer. Effects of introducing a field plate and a backside electrode on buffer-related lag and current collapse are studied. It is shown that gate field plate introduction is effective in reducing lag and current collapse when the acceptor density in the buffer layer is high. On the other hand, backside electrode introduction is shown to be effective in reducing drain lag and current collapse, particularly when the acceptor density in the buffer layer is relatively low, because the fixed potential at the backside electrode reduces electron injection into the buffer layer and the resulting trapping effects.

Passivation Layer

We present here analysis of oxidation reaction in liquid by a plasma-jet irradiation under various gas flow patterns such as laminar and turbulence flows. To estimate the total amount of oxidation reaction induced by reactive oxygen species (ROS) in liquid, we employ a KI-starch solution system, where the absorbance of the KI-starch solution near 600 nm behaves linear to the total amount of oxidation reaction in liquid. The laminar flow with higher gas velocity induces an increase in the ROS distribution area on the liquid surface, which results in a large amount of oxidation reaction in liquid. However, a much faster gas flow conversely results in a reduction in the total amount of oxidation reaction in liquid under the following two conditions: first condition is that the turbulence flow is triggered in a gas flow channel at a high Reynolds number of gas flow, which leads to a marked change of the spatial distribution of the ROS concentration in gas phase. Second condition is that the dimpled liquid surface is formed by strong gas flow, which prevents the ROS from being transported in radial direction along the liquid surface.

Analysis of backside-electrode and gate-field-plate effects on buffer-related current collapse in AlGaN/GaN high electron mobility transistors

In this paper, we carry out a 2-D transient analysis of field-plate GaAs metal-semiconductor field-effect transistors (FETs) by taking surface states into account. Quasi-pulsed current-voltage curves are derived from the transient characteristics. We show that drain lag and current slump (power slump) due to surface states are reduced by introducing a field plate because the fixed potential at the field plate mitigates the trapping effects of the surface states. The dependence of lag and current slump on the field-plate length and the passivation layer thickness is also studied. We show that it is possible to reduce the current slump and maintain the high-frequency performance of GaAs FETs at optimum values of the field-plate length and the layer thickness.

Effects of gas flow on oxidation reaction in liquid induced by He/O2 plasma-jet irradiation

Two-dimensional transient analysis of field-plate AlGaN/GaN HEMTs and GaN MESFETs is performed, considering a deep donor and a deep acceptor in the semiinsulating GaN buffer layer. Quasi-pulsed I-V curves are derived from the transient characteristics. It is studied how the existence of a field plate affects buffer-related drain lag, gate lag and current collapse. It is shown that in both FETs, the drain lag is reduced by introducing a field plate, because electron injection into the buffer layer is weakened by it, and trapping effects are reduced. It is also shown that the buffer-related current collapse and gate lag are reduced in the field-plate structures. The dependence on SiN passivation layer thickness under the field plate is also studied, suggesting that there is an optimum thickness of the SiN layer to minimize buffer-related current collapse and drain lag in GaN HEMTs and MESFETs.

Two-Dimensional Analysis of Field-Plate Effects on Surface-State-Related Current Transients and Power Slump in GaAs FETs

We present the development of a low-frequency nonthermal plasma-jet system, where the surrounding-gas condition of the plasma jet is precisely controlled in open air. By restricting the mixing of the ambient air into the plasma jet, the plasma jet can be selectively changed from a N2 main discharge to an O2 main discharge even in open air. In the plasma-jet system with the controlled surrounding gas, the production of reactive oxygen and nitrogen species is successfully controlled in deionized water: the concentration ratio of NO2 − to H2O2 is tuned from 0 to 0.18, and a high NO2 − concentration ratio is obtained at a N2 gas ratio of 0.80 relative to the total N2/O2 gas mixture in the main discharge gas. We also find that the NO2 − concentration is much higher in the plasma-activated medium than in the plasma-activated deionized water, which is mainly explained by the contribution of amino acids to NO2 − generation in the medium.