High-speed small-scale InGaP/GaAs HBT technology and itsapplication to integrated circuits

We have developed the advanced performance, small-scale InGaP/GaAs heterojunction bipolar transistors (HBTs) by using WSi/Ti base electrode and buried SiO₂ in the extrinsic collector. The base-collector capacitance C_BC was further reduced to improve high-frequency performance. Improving the uniformity of the buried SiO ₂, reducing the area of the base electrode, and optimizing the width of the base-contact enabled us to reduce the parasitic capacitance in the buried SiO₂ region by 50% compared to our previous devices. The cutoff frequency f_T of 156 GHz and the maximum oscillation frequency f_max of 255 GHz were obtained at a collector current I_C of 3.5 mA for the HBT with an emitter size S_E of 0.5×4.5 μm², and f_T of 114 GHz and f_max of 230 GHz were obtained at I_C of 0.9 mA for the HBT with S_E of 0.25×1.5 μm². We have also fabricated digital and analog circuits using these HBTs. A 1/8 static frequency divider operated at a maximum toggle frequency of 39.5 GHz with a power consumption per flip-flop of 190 mW. A transimpedance amplifier provides a gain of 46.5 dB·Ω with a bandwidth of 41.6 GHz at a power consumption of 150 mW. These results indicate the great potential of our HBTs for high-speed, low-power circuit applications     

Effects of base layer thickness on reliability of CVD Si3N4 stack gate dielectrics

Effects of the base layer in Si₃N₄/SiON stack gate dielectrics, in particular, the physical thickness of the base layer, on the dielectric reliability, MOSFET performance and process controllability are investigated. It is found that the electrical characteristics such as TDDB lifetime as well as the Si₃N₄ film property in Si₃N₄/SiON stack dielectrics with the same capacitance oxide equivalent thickness strongly depend on the SiON-base layer thickness. From the TDDB measurements for both stress polarities and from the Si₃N₄ stoichiometry by the X-ray photoelectron spectroscopy analysis, the optimum SiON-base layer thickness is determined to be approximately 1 nm, in order to obtain longer TDDB lifetime and surperior n-ch MOSFET performance. The obtained results are considered to attribute to the nitrogen profile in the Si₃N₄/SiON stack dielectrics and the strained layer thickness near SiON/Si interface.     

Mechanism of Ca2+ increase in myoblasts derived from chicken embryos

The mechanism of intracellular calcium ions (Ca(2+)) increase in chicken myoblasts was studied using histological, immunohistochemical, immunoblotting and Ca(2+) imaging techniques. Mononuclear myoblasts at embryonic day 12 (E12) contained myofibrils in the peripheral cytoplasm, and the sarcoplasmic reticulum was observed in the cytoplasm. Several Ca(2+)-related receptors, namely acetylcholine (ACh) receptors, dihydropyridine receptors (DHPRs) and ryanodine receptors (RyRs), were detected in the tissue as early as E12. Western blotting analyses detected one band corresponding to RyR subtype 3 (RyR3) at E12 and two bands corresponding to RyR1 and RyR3 after E13. Ca(2+) imaging of mononuclear myoblasts in vitro revealed an intense Ca(2+)-increase response to ACh stimulation, and this effect was abolished after EGTA addition to the culture medium. Nifedipine treatment also led to a lack of Ca(2+) increase in response to ACh stimulation, while ryanodine treatment led to a weak Ca(2+)-increase response. On the other hand, multinuclear myoblasts showed a Ca(2+)-increase response to ACh stimulation in the presence of not only EGTA but also nifedipine, although ryanodine treatment led to a lack of Ca(2+) increase. These results suggest that the mechanism of Ca(2+) increase in mononuclear myoblasts involves extracellular Ca(2+) entry through DHPRs, which is amplified by Ca(2+) release from the cytoplasmic Ca(2+) store, while multinuclear myoblasts mainly depend on Ca(2+) release from the cytoplasmic Ca(2+) store.     

Numerical simulation on void bubble dynamics using moving particle semi-implicit method

In present study, the collapse of void bubble in liquid has been simulated using moving particle semi-implicit (MPS) code. The liquid is described using moving particles and the bubble–liquid interface was set to be vacuum pressure boundary without interfacial heat mass transfer. The topological shape of bubble can be traced according to the motion and location of interfacial particles. The time dependent bubble diameter, interfacial velocity and bubble collapse time were obtained under wide parametric range. The comparison with Rayleigh and Zababakhin's prediction showed a good agreement which validates the applicability and accuracy on MPS method in solving present momentum problems. The potential void induced water hammer pressure pulse was also evaluated which is instructive for further material erosion study. The bubble collapse with non-condensable gas has been further simulated and the rebound phenomenon was successfully captured which is similar with vapor-filled cavitation phenomenon. The present study exhibits some fundamental characteristics of void bubble hydrodynamics and it is also expected to be instructive for further applications of MPS method to complicated bubble dynamics problems.     

Diesel emission control system using combined process of nonthermal plasma and exhaust gas components' recirculation

A NO_x aftertreatment system, using nonthermal plasma (NTP) reduction and exhaust gas components' recirculation, is investigated. A pilot-scale system is applied to a stationary diesel engine. In this system, NO_x is first removed by adsorption, and subsequently, the adsorbent is regenerated by thermal desorption. NO_x desorbed is reduced by using nitrogen NTP. Moreover, NO_x, CO₂, and water vapor recirculated into the engine intake reduce NO_x. In this study, approximately 57% of the NO_x of the exhaust (NO_x: 240-325 ppm, flow rate = 300 NL/min) can be continuously treated for 58 h. A system energy efficiency of 120 g (NO₂)/kWh is obtained.     

Slow crystallization process from amorphous PbTiO3

The effect of isothermal annealing of amorphous PbTiO₃ at a temperature below the crystallization temperature, T_crys, has been investigated. Time dependent dielectric constant in the annealing process has been observed. From the experimental results, both the existence of the glass transition temperature, T_g, and the steadiness of dielectric constant at room temperature have been established.     

High performance light water reactor

The objective of the high performance light water reactor (HPLWR) project is to assess the merit and economic feasibility of a high efficiency LWR operating at thermodynamically supercritical regime. An efficiency of approximately 44% is expected. To accomplish this objective, a highly qualified team of European research institutes and industrial partners together with the University of Tokyo is assessing the major issues pertaining to a new reactor concept, under the co-sponsorship of the European Commission. The assessment has emphasized the recent advancement achieved in this area by Japan. Additionally, it accounts for advanced European reactor design requirements, recent improvements, practical design aspects, availability of plant components and the availability of high temperature materials. The final objective of this project is to reach a conclusion on the potential of the HPLWR to help sustain the nuclear option, by supplying competitively priced electricity, as well as to continue the nuclear competence in LWR technology. The following is a brief summary of the main project achievements:

• A state-of-the-art review of supercritical water-cooled reactors has been performed for the HPLWR project.

• Extensive studies have been performed in the last 10 years by the University of Tokyo. Therefore, a ‘reference design’, developed by the University of Tokyo, was selected in order to assess the available technological tools (i.e. computer codes, analyses, advanced materials, water chemistry, etc.). Design data and results of the analysis were supplied by the University of Tokyo. A benchmark problem, based on the ‘reference design’ was defined for neutronics calculations and several partners of the HPLWR project carried out independent analyses. The results of these analyses, which in addition help to ‘calibrate’ the codes, have guided the assessment of the core and the design of an improved HPLWR fuel assembly.

• Preliminary selection was made for the HPLWR scale, boundary conditions, core and fuel assembly design, reactor pressure vessel, containment, turbine and balance of plant.

• A review of potentially applicable materials for the HPLWR was completed and a preliminary selection of potential in-vessel and ex-vessel candidate materials was made.

• A thorough review of heat transfer at supercritical pressures was completed together with a thermal-hydraulics analysis of potential HPLWR sub-channels. This analytical tool supports the core and fuel assembly design.

• The RELAP5 and the CATHARE 2 codes are being upgraded to supercritical pressures. Thus they can be used to support the HPLWR core design and to perform plant safety analyses.

• Assessment of the HPLWR design constraints, based on current LWR technology was documented. This document stresses the various criteria that must be satisfied in the design (e.g. material, temperature, power, safety criteria, etc.) based on experience gained in the design of PWR.

• Preliminary economic assessment concluded that the HPLWR has the potential to be economically competitive. However, an accurate assessment can only be done after the HPLWR design has been fixed. A more accurate economic assessment may be performed after the conclusion of this project.

Solid-state diffusion bonding of silicon nitride using titanium foils

This article presents an effective way to control the interfacial reaction during solid-state diffusion bonding of silicon nitride (Si₃N₄) using titanium foils. The interfacial structure and its growth kinetics were analyzed in detail with scanning electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). The actual phase sequence of the joint interfaces bonded at temperatures between 1473 and 1673 K is concluded to be Si₃N₄/Ti₅Si₃(N)/α-Ti(N)+Ti₅Si₃(N), which is different from the phase sequence observed at room temperature after bonding. The joints are very weak due to the formation of a brittle Ti₅Si₃(N) layer at the interface. To suppress the growth of the Ti₅Si₃ layer, a nitrogen-solution treatment of titanium foils prior to each bonding experiment is implemented. Although a perfect prevention of the Ti₅Si₃(N) layer formation is not achieved with this treatment, it is shown that the growth of the layer is effectively suppressed enough to improve the joint strength to a level 3 times higher than the case in which pure titanium is employed.     

Random-coiled conformation of polypeptide chains

Summary Random-coiled conformation of poly(L-leucine), which has -branched side-chain, was theoretically analyzed by a conformational energy calculation based on intra-residue interactions. Calculated characteristic ratio 7.62 was obtained by using the transformation matrix statistically averaged over the entire side-chain conformational space of L-Leu residue. This value is smaller than those of poly(L-phenylalanine) (11.24) and poly(L-tyrosine) (12.33) which have -branched side-chain.