Recent progress in the SiC material and ultrahigh-voltage devices is reviewed. Regarding the material issues, fast epitaxial growth of high-purity epitaxial layers and reduction of basal plane ...dislocations have made significant strides. Growth technology of 100μm-thick epitaxial layers intentionally doped to 1 × 1014cm−3 with a basal plane dislocation density of 0.1cm−2 has been established. The carrier lifetimes can remarkably be enhanced by several techniques and trials of lifetime control have been successful. Using very thick (> 100µm) and high-purity epitaxial layers, 15–27kV SiC pin diodes and various switching devices such as insulated gate bipolar transistors, thyristors, bipolar junction transistors, and metal-oxide-semiconductor field effect transistors have been demonstrated. Although the performance of these ultrahigh-voltage devices is promising, further improvement of the performance and reliability is mandatory for system applications. Technological challenges in both the material and device fabrication are discussed.
Severe regional haze pollution events occurred in eastern and central China in January 2013, which had adverse effects on the environment and public health. Extremely high levels of particulate ...matter with aerodynamic diameter of 2.5 μm or less (PM2.5) with dominant components of sulfate and nitrate are responsible for the haze pollution. Although heterogeneous chemistry is thought to play an important role in the production of sulfate and nitrate during haze episodes, few studies have comprehensively evaluated the effect of heterogeneous chemistry on haze formation in China by using the 3-D models due to of a lack of treatments for heterogeneous reactions in most climate and chemical transport models. In this work, the WRF-CMAQ model with newly added heterogeneous reactions is applied to East Asia to evaluate the impacts of heterogeneous chemistry and the meteorological anomaly during January 2013 on regional haze formation. As the parameterization of heterogeneous reactions on different types of particles is not well established yet, we arbitrarily selected the uptake coefficients from reactions on dust particles and then conducted several sensitivity runs to find the value that can best match observations. The revised CMAQ with heterogeneous chemistry not only captures the magnitude and temporal variation of sulfate and nitrate, but also reproduces the enhancement of relative contribution of sulfate and nitrate to PM2.5 mass from clean days to polluted haze days. These results indicate the significant role of heterogeneous chemistry in regional haze formation and improve the understanding of the haze formation mechanisms during the January 2013 episode.
Extreme haze episodes repeatedly shrouded Beijing during the winter of 2012-2013, causing major environmental and health problems. To better understand these extreme events, we performed a ...model-assisted analysis of the hourly observation data of PM2.5 and its major chemical compositions. The synthetic analysis shows that (1) the severe winter haze was driven by stable synoptic meteorological conditions over northeastern China, and not by an abrupt increase in anthropogenic emissions. (2) Secondary species, including organics, sulfate, nitrate, and ammonium, were the major constituents of PM2.5 during this period. (3) Due to the dimming effect of high loading of aerosol particles, gaseous oxidant concentrations decreased significantly, suggesting a reduced production of secondary aerosols through gas-phase reactions. Surprisingly, the observational data reveals an enhanced production rate of secondary aerosols, suggesting an important contribution from other formation pathways, most likely heterogeneous reactions. These reactions appeared to be more efficient in producing secondary inorganics aerosols than organic aerosols resulting in a strongly elevated fraction of inorganics during heavily polluted periods. (4) Moreover, we found that high aerosol concentration was a regional phenomenon. The accumulation process of aerosol particles occurred successively from cities southeast of Beijing. The apparent sharp increase in PM2.5 concentration of up to several hundred mu g m-3 per hour recorded in Beijing represented rapid recovery from an interruption to the continuous pollution accumulation over the region, rather than purely local chemical production. This suggests that regional transport of pollutants played an important role during these severe pollution events.
Here, we show that silicon carbide (SiC) complementary logic gates composed of p- and n-channel junction field-effect transistors (JFETs) fabricated by ion implantation operate at 623 K with a supply ...voltage as low as 1.4 V. The logic threshold voltage shift of the complementary JFET (CJFET) inverter is only 0.2 V from 300 to 623 K. Furthermore, temperature dependencies of the static and dynamic characteristics of the CJFET inverter are well explained by a simple analytical model of SiC JFETs.
Lateral p-i-n diodes with small i-region width (less than <inline-formula> <tex-math notation="LaTeX">5 ~\mu \text{m} </tex-math></inline-formula>) were fabricated by direct ion implantation into a ...high-purity semi-insulating silicon carbide (SiC) substrate. The breakdown voltage of the diodes increased with increasing the i-region width and a lateral effective breakdown electric field of 1.7-1.9 MV/cm was obtained. The experimental breakdown voltage well agreed with the technology computer-aided design (TCAD) simulation, indicating that deep levels contributing the semi-insulating property do not seriously deteriorate the breakdown voltage in case of the small i-region width.
Recent progress and current understanding of carrier lifetimes and avalanche phenomena in silicon carbide (SiC) are reviewed. The acceptor level of carbon vacancy (VC), called the Z1/2 center, has ...been identified to be the primary carrier lifetime killer in SiC. The VC defects can be eliminated by the introduction of excess carbon atoms followed by carbon diffusion in the bulk region. The true bulk lifetime after VC elimination was estimated to be approximately 110 µs. The doping dependence of carrier lifetimes in n- and p-type SiC is also presented. The impact ionization coefficients of electrons and holes were extracted in the temperature range of 298 to 423 K. The intrinsic critical electric field strength of SiC 0 0 0 1 was determined to be 2.0, 2.5, and 3.3 MV cm−1 for doping densities of 1 × 1015, 1 × 1016, and 1 × 1017 cm−3, respectively, at room temperature; it slightly increased at elevated temperature. The obtained set of impact ionization coefficients has enabled us to accurately predict the breakdown voltage of SiC devices, including its temperature dependence. Due to the unusually low impact ionization coefficient of electrons, the breakdown voltage of a SiC p+n junction is about 6%-9% higher than that of an n+p junction with a given doping density.
The temperature dependence of impact ionization coefficients of electrons and holes along 4H-SiC 〈0001〉 was determined in a wide temperature range from 156 K to 561 K. Photomultiplication ...measurements were conducted using several types of 4H-SiC photodiodes (PDs) with different structures under high vacuum conditions. The hole ionization coefficient showed a negative temperature dependence, which can be attributed to enhanced phonon scattering at elevated temperatures. Temperature dependence of the electron ionization coefficient was small, and this result may be ascribed to the simultaneous effects of enhanced phonon scattering and shrinkage of minigaps in the conduction band. Accuracy of the obtained results was verified by comparing the calculated breakdown voltage with the experimental breakdown voltage of various diodes.
Antiviral neuraminidase inhibitors, such as oseltamivir, zanamivir, and peramivir, are widely used for treatment of influenza virus infection. We reported previously that oseltamivir inhibits the ...viral growth cycle, ameliorates symptoms, and reduces viral antigen quantities. Suppressed viral antigen production, however, induces a reduction of acquired antiviral humoral immunity, and increases the incidence of re-infection rate in the following year. To achieve effective treatment of influenza virus infection, it is necessary to overcome these adverse effects of antiviral neuraminidase inhibitors. Feeding of yogurt fermented with Lactobacillus delbrueckii ssp. bulgaricus (L. bulgaricus) OLL1073R-1 is reported to have immune-stimulatory effects on influenza virus infection in mice and humans. In the present study, we assessed the effect of feeding L. bulgaricus OLL1073R-1 yogurt cultures (YC) on local and systemic humoral immune responses, which were suppressed by oseltamivir treatment, in mice infected with influenza A virus. Yogurt culture (1.14 × 108 cfu/0.4 mL per mouse per day) or sterile water (vehicle) was administered by intragastric gavage for 35 d. At d 22, influenza A virus/Puerto Rico/8/34 (H1N1) (PR8; 0.5 pfu/15 μL per mouse) was instilled intranasally, followed immediately by oral administration of oseltamivir (50 μg/100 μL per mouse, twice daily) or 5% methylcellulose (100 μL/mouse) as a vehicle for 13 d. Titers of anti-PR8-specific IgG and IgA in serum and mucosal secretory IgA (S-IgA) and IgG in bronchoalveolar lavage fluid (BALF) were analyzed by ELISA at 14 d after infection. Oseltamivir significantly suppressed the induction of anti-PR8-specific IgG and IgA in serum and S-IgA and IgG in BALF after infection. Feeding YC mildly but significantly stimulated production of PR8-specific IgA in serum, S-IgA in BALF, and IgG in serum without changing the IgG2a:IgG1 ratio. We analyzed the neutralizing activities against PR8 in serum and BALF and found that oseltamivir also reduced protective immunity, and YC feeding abrogated this effect. The immune-stimulatory tendency of YC on anti-PR8-specific IgA and IgG titers in serum and BALF was also detected in mice re-infected with PR8, but the effect was insignificant, unlike the effect of YC in the initial infection.
Short-channel effects (SCEs) in double-gate silicon carbide junction field-effect transistors (JFETs) fully fabricated by ion implantation are experimentally investigated. The threshold voltage ...shift, drain-induced barrier lowering, and subthreshold slope degradation are clearly observed in the fabricated p- and n-JFETs. The SCEs are quantitatively evaluated by comparing with the theoretical values obtained by solving a 2-D Poisson equation, which shows good agreement with experiments. The dominant parameter for the SCEs in JFETs is the ratio of the channel length (<inline-formula> <tex-math notation="LaTeX">{L} </tex-math></inline-formula>) to the channel thickness (<inline-formula> <tex-math notation="LaTeX">{a} </tex-math></inline-formula>), and the device scaling rule to avoid the SCEs is estimated to be <inline-formula> <tex-math notation="LaTeX">{L}/{a} > {3} </tex-math></inline-formula>.
We demonstrate normally-off 400 °C operation of n-channel and p-channel junction field-effect transistors (JFETs) fabricated by an ion implantation into a common high-purity semi-insulating silicon ...carbide (SiC) substrate. The side-gate structure proposed in this letter has good controllability of threshold voltage. The present results assure the potential of JFET-based complementary logic integrated circuits (ICs) operational at high temperature.
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