Kazuyoshi Fushinobu

Effect of gate voltage on hot‐electron and hot phonon interaction and transport in a submicrometer transistor

This paper studies the effects of gate voltage on heat generation and transport in a metal–semiconductor field effect transistor made of gallium arsenide (GaAs) with a gate length of 0.2 μm. Based on the interactions between electrons, optical phonons, and acoustic phonons in GaAs, a self‐consistent model consisting of hydrodynamic equations for electrons and phonons is developed. Concurrent study of the electrical and thermal behavior of the device shows that under a source‐to‐drain bias at 3 V and zero gate bias, the maximum electron temperature rise in this device is higher than 1000 K whereas the lattice temperature rise is of the order of 10 K, thereby exhibiting nonequilibrium characteristics. As the gate voltage is decreased from 0 to −2 V the maximum electron temperature increases due to generation of higher electric fields whereas the maximum lattice temperature reduces due to lower power dissipation. The nonequilibrium hot‐electron effect can reduce the drain current by 15% and must be included ...

Heat generation and transport in submicron semiconductor devices

The reduction of semiconductor device size to the submicrometer range leads to unique electrical and thermal phenomena. The presence of high electric fields (order of 10 7 V/m) energizes the electrons and throws them far fom equilibrium with the lattice. This makes heat generation a nonequilibrium process. For gallium arsenide (GaAs), energy is first transferred from the energized electrons to optical phonons due to strong polar coupling. Since optical phonons do not conduct heat, they must transfer their energy to acoustic phonons for lattice heat conduction. Based on the two-step mechanism with corresponding time scales, a new model is developed to study the process of nonequilibrium heat generation and transport in a GaAs metal semiconductor field effect transistor (MESFET) with a gate length of 0.2 μm. When 3 V is applied to the device, the electron temperature rise is predicted to be more than 1000 K. The effect of lattice heating on electrical characteristics of the device shows that the current is reduced due to decrease in electron mobility. The package thermal conductance is observed to have strong effects on the transient response of the device

Visualization of the Membrane Temperature Field of a Polymer Electrolyte Fuel Cell

Membrane temperature field of a polymer electrolyte fuel cell (PEFC) has been visualized experimentally. PEFCs need further breakthrough for deployment in the market. One of the major issues is the temperature management of the polymer membrane and the whole cell that strongly govern system performance through electrochemical reactions, ion transport, water management, and gas supply. The temperature field of the membrane, however had not been visualized due to the cell configuration. In our experiment, the thermography technique is applied to visualize an operating test cell. Despite the unique configuration, measured i-V characteristics guarantee the cell performance. The visualization results revealed several important characteristics that help us understanding the physics and suggest design knowledge. One major result is the existence of so called a hot spot. The membrane does have a temperature distribution, and a local temperature maximum may exceed the membrane design limitation. This trend, of course, is not favorable for design purposes. Also, the impact of the major operation parameters, such as current density humidification, and gas flow configuration, have been clearly exhibited. The experimental results are examined by using the results of our previously developed numerical code. The code includes the conjugate nature of the electrochemical reaction and the heat and mass transport processes. By comparing the experiment and the calculation, the mechanisms of the hot-spot generation and the parameter dependence have been explained. The results revealed the physics and suggested essential design criteria.

Freezing of a water droplet due to evaporation: heat transfer dominating the evaporation-freezing phenomena and the effect of boiling on freezing characteristics

Abstract One of the authors has proposed a novel transport/storage system for the waste cold from the gasification process of liquefied natural gas (LNG), which consists of an evaporator, a cold trap, and a pipeline. In order to estimate the performance of this system, one should know the pressure in the evaporator, in which evaporation–freezing of a PCM occurs, and in the cold trap, as well as the pressure drop of the pipeline due to the flow of low pressure vapor of the PCM. In this paper, the cooling/freezing phenomena of a water droplet due to evaporation in an evacuated chamber was experimentally examined, and the heat transfer dominating the evaporation-freezing phenomena was investigated in order to estimate the pressure in the evaporator. From the results, it was shown that the water droplet in the evacuated cell is effectively cooled by the evaporation of water itself, and is frozen within a few seconds through a remarkable supercooling state, and that the cooling rate of the water droplets were dominated by heat transfer within the droplet under the abrupt evacuation condition. The later result means that, in order to obtain an ice particle by evaporation–freezing, the surroundings of the water droplet should be evacuated at the pressure as low as the saturate pressure of water at the maximum supercooling temperature of the droplet.

Transient Phase Change in the Cathode Side of a PEM Fuel Cell

We numerically investigated the temporal variation and spatial distribution of interfacial mass-transfer phase-change rate in the cathode side gas diffusion layer of a proton exchange membrane (PEM) fuel cell during startup. For this purpose, a three-dimensional transient two-phase nonisothermal model was used. A nonequilibrium evaporation-condensation interfacial mass-transfer rate is incorporated in the model, which enables us to take supersaturation and subsaturation into consideration. This helps us to investigate the most significant contribution of the phase-change rate to the transient response and thermal behavior of the cell. The effects of the operating temperature and channel inlet humidity on the phase-change rate are also investigated. It is observed that when condensation is dominant, the transient time decreases. It is also observed that the maximum temperature decreases with time due to vapor-phase diffusion and phase change.

Electro-Thermal Behavior of a Sub-Micrometer Bulk CMOS Device: Modeling of Heat Generation and Prediction of Temperatures

The electro-thermal behavior of a bulk CMOS device is analyzed using the hydrodynamic model equation. The analysis is first applied to an introductory example of a single field-effect-transistor (FET) to indicate the importance of incorporating a non-equilibrium state between charge carriers and phonons in the analytical model. Then, the system of hydrodynamic model equations is described in detail, which takes into account carrier generation/recombination process and non-equilibrium between charge carriers and phonons. A supposed bulk CMOS device has nano-meter dimensions and thus is vulnerable to malfunction due to crosstalk between the constituent FETs. The simulation of electro-thermal transients in the entire CMOS domain is performed focusing on the behavior in a short period after switching of the gate voltage from low to high. The result shows a significant level of crosstalk that may lead to impairment of the switching function of the CMOS. Also shown is the development of a hot spot at the source region of the activated FET in addition to the one at the gate/drain corner. The deliberate omission of sub-continuum mechanisms, particularly the carrier generation/recombination, from the analytical model produced erroneous distributions of charge carrier density and temperature, thus proving the significance of this mechanism in defining heat generation and heat flow in the device.

Optical Measurement Technique of Water Contents in Polymer Membrane for PEFCs

The feasibility of an optical technique is examined for the measurement of the membrane water content in polymer electrolyte fuel cells (PEFCs). Transmission of the infrared light of 1.92 μm wavelength is used to measure the water content in the polymer electrolyte membrane. A calibration procedure is examined, and the technique is applied for the transient measurement of a Nafion membrane that gives the value of water diffusion coefficient, consistent with previous reports. The technique is then applied to an operating PEFC to show its applicability for in situ measurement.

Numerical Calculation of Sub-Micron Hot Spot in Si Devices

Numerical calculation of silicon MOSFET is performed. Conjugate nature of the thermal and electrical behavior in the device is considered, and the lattice temperatures is solved as well as the electron concentration and the electron temperature. The calculated results shows the importance of considering both the electron and lattice temperatures for device modeling; the electron temperature has a significant impact on the calculated electron concentration and the lattice temperature. Submicron local hot spot is observed in the device, and its characteristics are discussed.© 2003 ASME

Surface Electronic/Atomic Structure and Activation Energy on Pt(111), Pt3Cu(111), and PtCu(111) for PEFC Cathode

Theoretical analysis of oxygen reduction reaction (ORR) on cathode Pt(111), Pt3Cu(111), and PtCu(111) surfaces in a polymer electrolyte fuel cell (PEFC) is performed to investigate the ORR mechanisms on Pt(111) and the effect of alloying. From density functional theory (DFT) calculations, we found that the difference in adsorption energy of O2 molecules on the surface has a strong relation with the number of d-electrons and the position of the d-band center of the surface Pt atoms. From the results of the activation energy calculations using the unity bond index–quadratic exponential potential (UBI-QEP) method, we show that the oxygen atom coverage on Pt(111) surface has a strong influence on the ORR activity. In addition, it was found that Pt3Cu(111) and PtCu(111) surfaces have lower coverage compared with that of Pt(111) surface, which results in the enhancement of ORR activity.

Heat transfer regime map for electronic devices cooling

A simple analytical model that predicts the temperature rise of a small heater on an unheated substrate is presented. The model approximates uniform temperature over both the heater and the substrate, although they have spatial distributions in actual case. A rigorous numerical calculation has been performed to verify the model results. A regime map is constructed based on the analytical model. Each regime in the map is named in order to show the time dependency and the governing heat transfer mode of the heater temperature rise.