Masayoshi Naito

High-speed low-loss p-n diode having a channel structure

A p-n diode having a channel structure (static shielding diode, SSD) is proposed to increase the reverse blocking voltage of a low-loss high-speed p-n diode. It is shown by numerical analysis and experiment that a low-loss, high-speed SSD with high blocking capability can be realized by surrounding the p-layer and a portion of the n-layer with a highly doped p+-layer. In this method, the blocking voltage can be increased by a factor of 2 to 3.5 without sacrificing the low forward voltage drop and fast reverse recovery. The SSD with a 0.81-V forward voltage drop at 80 A/cm2, a 180-V blocking voltage at 150°C, and a 87-ns reverse recovery time can be fabricated.

Modelling temporal kinetic oscillations for electrochemical oxidation of formic acid on Pt

Abstract A model is presented which suitably reproduces the observed kinetic potential oscillations for the electrochemical oxidation of formic acid on Pt. Coupled ordinary differential equations are formulated concerning three variables: the electrode potential and the amounts of adsorbed water and carbon monoxide. Two points prove to be essential in this model to an obtain oscillation behavior similar to the observed behavior: the interaction between the two adsorbed species increases with the electrode potential and the saturation surface coverage of the adsorbed carbon monoxide is less than 1. These two points are consistent with experimental observations.

High current characteristics of asymmetrical p-i-n diodes having low forward voltage drops

A closed form solution for the forward characteristics of a very asymmetrical step-junction p-i-n diode at high current levels is derived and discussed. It is found that the forward voltage decreases significantly when the amount of the impurities per unit area of one of the highly doped regions is reduced. The theory is confirmed by experiment, and the diodes obtained show a low forward loss together with high reverse blocking voltage and fast reverse recovery.

General relationship between complex impedance and linear stability in electrochemical systems

The impedance of the electrochemical system is derived in an explicit analytical form in relation to the stability of the system under various driving conditions. It is shown that the complex impedance is represented as the ratio of characteristic polynomials of the Jacobian matrices of linearized system under potentiostatic control and under galvanostatic control. Thus it is definitely shown that the zeros of the impedance are the eigenvalues of the Jacobian of the system under potentiostatic control, and that the poles are the eigenvalues of the Jacobian under galvanostatic control. The obtained impedance formulas are used to derive or prove several electrochemical characteristics. A direct analytical relationship between the hidden negative impedance and the galvanostatic Hopf bifurcation is also derived.

One-dimensional analysis of turnoff phenomena for a gate turnoff thyristor

Turnoff phenomena for a one-dimensional gate turnoff thyristor (GTO) are investigated using exact numerical solutions of a full set of semiconductor device equations. It is shown that the time responses of the hole and the electron densities around the center junction J2 are responsible for the dynamic behavior of the GTO. The storage time almost corresponds to the time period required for J2 to come out of saturation. The fall time is the period from the coming out of saturation of J2 to the point when the cathode emitter junction recovers. Time variations in the rates of replenishment and removal of holes in the p-base during the dynamic turnoff process are discussed, and an understanding of the switching mechanism, which is not obtainable in the generally used static transistor analogy, is obtained. Though a one-dimensional model is employed in this paper, it still provides a great deal of insight into the devices operation.

Dynamics of potential oscillations in the electrochemical oxidation of formic acid on Pt

A mechanistic model for the galvanostatic potential oscillation in the electrochemical oxidation of formic acid on Pt (H. Okamoto, N. Tanaka and M. Naito, Chem. Phys. Lett., 1996, 248, 289) is analyzed here from the viewpoints of nonlinear dynamics and bifurcation. The model assumes the potential-dependent adsorption, desorption, and reaction of water and carbon monoxide. It is shown that the adiabatic elimination of the potential is warranted in the dynamics and so the essential variables for the dynamics are only chemical ones. The potential oscillation occurs in a certain range of the applied current due to a Hopf instability. The potential dependence of the adsorption and desorption of water to and from the electrode plays an essential role in the Hopf bifurcation. The current dependence of the stability of the system is also a result of the potential dependence of the amount of adsorbed water.

Estimating the active demension of the dynamics in a time series based on an information criterion

Abstract We describe a method for estimating the active dimension of the dynamics generating a time series from the observed time series. The active dimension is the minimum number of variables needed to describe the essential part of the dynamics behind the time series, and is equal to the minimal embedding dimension at which the delay-coordinate reconstruction becomes a local embedding. The achievement of embedding was examined based on the results of time series prediction with the optimal metric. To bring further objectivity to the estimation, we introduced an information criterion, the minimum description length criterion. We demonstrated the effectiveness of the method by applying it to chaotic time series obtained from numerical calculations and from an electrochemical experiment. The advantages of the proposed method are the transparency of its parameter settings and its applicability to relatively short time series.

Estimating the number of reaction intermediates for formaldehyde oxidation by analysis of the chaotic behavior

Abstract The number of main chemical reaction intermediates in electrochemical oxidation of formaldehyde on a platinum electrode is determined to be three by analyzing the chaotic patterns of potential oscillations with a constant current. In a chaotic situation, the number of independent variables, which is the number of reaction intermediates appearing near the rate-determining step, is between the first or second integer greater than the correlation dimension and the minimal necessary embedding dimension. For the reaction studied, the upper and lower limits for the number of independent variables are both found to be three from the Grassberger Procaccia plot. This agrees with the result derived from chemical measurements and is thought to be a good base for mathematical modeling of the reaction system.

Numerical analysis of turn-off characteristics for a gate turn-off thyristor with a shorted anode emitter

Turn-off current waveform for a gate turn-off thyristor (GTO) with a shorted anode emitter has been calculated numerically by solving the semiconductor basic equations in an equivalent one-dimensional model device. This model is derived from the analysis of current and carrier distributions obtained by a two-dimensional calculation of the on-state of GTO. A calculated turn-off current waveform agrees well with the experimental waveform. The computational time of one case is about 2 min. It is shown that this one-dimensional analysis method is useful for the calculation of the turn-off time. Using this one-dimensional model during the turn-off process and the two-dimensional model in the on-state, the relation between turn-off time and the forward voltage drop can be obtained in relation to the shorted emitter structure. It is shown that the shorted emitter structure is useful to improve this tradeoff relation.

Potential Oscillations Related to Proton Concentration in Formaldehyde Oxidation

We have determined the effect of proton concentration on the appearance of chaos and windows of periodic oscillations in the galvanostatic oxidation of formaldehyde on platinum at 43°C. When proton concentration increases from 0.1 to 5 mol dm -3 , the highest currents at which periodic oscillations and chaos appear decrease, and the oscillation amplitudes increase, while the oscillation periods change in a complex way. When perchloric acid is used instead of sulfuric acid, the highest currents at which oscillations appear increase by about 50%. These results (except those concerning periods) have been explained by using voltammograms and considering the reaction mechanism. We have found new types of alternating periodic and chaotic sequences, which connect the known sequences. We have used these findings to plot bifurcation diagrams.