Design and fabrication of 10-kV silicon–carbide p-channel IGBTs with hexagonal cells and step space modulated junction termination extension

10-kV 4 H–SiC p-channel insulated gate bipolar transistors(IGBTs) are designed, fabricated, and characterized in this paper. The IGBTs have an active area of 2.25 mm^2 with a die size of 3 mm× 3 mm. A step space modulated junction termination extension(SSM-JTE) structure is introduced and fabricated to improve the blocking performance of the IGBTs.The SiC p-channel IGBTs with SSM-JTE termination exhibit a leakage current of only 50 nA at-10 kV. To improve the on-state characteristics of SiC IGBTs, the hexagonal cell(H-cell) structure is designed and compared with the conventional interdigital cell(I-cell) structure. At an on-state current of 50 A/cm^2, the voltage drops of I-cell IGBT and H-cell IGBT are10.1 V and 8.3 V respectively. Meanwhile, on the assumption that the package power density is 300 W/cm^2, the maximum permissible current densities of the I-cell IGBT and H-cell IGBT are determined to be 34.2 A/cm^2 and 38.9 A/cm^2 with forward voltage drops of 8.8 V and 7.8 V, respectively. The differential specific on-resistance of I-cell structure and H-cell structure IGBT are 72.36 m?·cm^2 and 56.92 m?·cm^2, respectively. These results demonstrate that H-cell structure silicon carbide IGBT with SSM-JTE is a promising candidate for high power applications.

Transmissibility of COVID-19 in 11 major cities in China and its association with temperature and humidity in Beijing, Shanghai, Guangzhou, and Chengdu

Background The new coronavirus disease COVID-19 began in December 2019 and has spread rapidly by human-to-human transmission.This study evaluated the transmissibility of the infectious disease and analyzed its association with temperature and humidity to study the propagation pattern of COVID-19.Methods In this study,we revised the reported data in Wuhan based on several assumptions to estimate the actual number of confirmed cases considering that perhaps not all cases could be detected and reported in the complex situation there.Then we used the equation derived from the Susceptible-Exposed-Infectious-Recovered(SEIR)model to calculate R0 from January 24,2020 to February 13,2020 in 11 major cities in China for comparison.With the calculation results,we conducted correlation analysis and regression analysis between R0 and temperature and humidity for four major cities in China to see the association between the transmissibility of COVID-19 and the weather variables.Results It was estimated that the cumulative number of confirmed cases had exceeded 45000 by February 13,2020 in Wuhan.The average R0 in Wuhan was 2.7,significantly higher than those in other cities ranging from 1.8 to 2.4.The inflection points in the cities outside Hubei Province were between January 30,2020 and February 3,2020,while there had not been an obvious downward trend of R0 in Wuhan.R0 negatively correlated with both temperature and humidity,which was significant at the 0.01 level.Conclusions The transmissibility of COVID-19 was strong and importance should be attached to the intervention of its transmission especially in Wuhan.According to the correlation between R0 and weather,the spread of disease will be suppressed as the weather warms.

Annealing effect on structural and magnetic properties of Tb and Cr co-implanted AIGaN

Unintentionally doped AIGaN layers, which were co.implanted with 400 keV Tb＋ ions and 200keV Cr＋ ions at doses of 1.5×1015cm-2, have been rapid thermally annealed at 800℃ and 900℃ for 5 min in flowing N2, Compared with Tb implanted AIGaN sample, the Tb and Cr co-implanted sample revesls a larger magnetic signal. In this work, the annealing effect on the structural and magnetic properties of Tb and Cr co-implanted AIGaN thin films have been studied. XRD and raman scattering results indicate that no second phase presents in thetin films and mast of the implantation induced defects can be removed by post-implantation annealing. Superconducting quantum interference device （SQUID） measurements show clear room temperature ferromagnetic behavior and an increase in the saturation magnetization as a result of annealing. The saturation magnetization of the 900℃ annealed sample is about 15 times higher than that of the 800℃ annealed sample.