Methylmercury formation is mainly driven by microbial-mediated process. The mechanism of microbial mercury methylation has become a crucial research topic for understanding methylation in the environment. Pioneering studies of microbial mercury methylation are focusing on functional strain isolation, microbial community composition characterization, and mechanism elucidation in various environments. Therefore, the functional genes of microbial mercury methylation, global isolations of Hg methylation strains, and their methylation potential were systematically analyzed, and methylators in typical environments were extensively reviewed. The main drivers (key physicochemical factors and microbiota) of microbial mercury methylation were summarized and discussed. Though significant progress on the mechanism of the Hg microbial methylation has been explored in recent decade, it is still limited in several aspects, including (1) molecular biology techniques for identifying methylators; (2) characterization methods for mercury methylation potential; and (3) complex environmental properties (environmental factors, complex communities, etc.). Accordingly, strategies for studying the Hg microbial methylation mechanism were proposed. These strategies include the following: (1) the development of new molecular biology methods to characterize methylation potential; (2) treating the environment as a micro-ecosystem and studying them from a holistic perspective to clearly understand mercury methylation; (3) a more reasonable and sensitive inhibition test needs to be considered. Key points: • Global Hg microbial methylation is phylogenetically and functionally discussed. • The main drivers of microbial methylation are compared in various condition. • Future study of Hg microbial methylation is proposed. Graphical

The PNP bipolar junction transistors (BJTs) are susceptible to surface recombination in both the emitter–base depletion and neutral base regions when their passivation oxide layers are damaged by ionization irradiation, leading to an increase of base current. The previous model of excess base current proposed in 2015 omits the damage in the neutral base, and the impact of oxide charges, which results in an underestimation of bias sensitivity of excess base current. This work presents a physics-based improvement of the previous model, and it provides a much better fit to the experimental data for the bias dependence of excess base current for the 2N2907 vertical and gate controlled lateral PNP BJTs subjected to γ-ray irradiation. The model suggests that the damage in the neutral base can be significant for a lateral PNP with wide base, operated at high bias voltage, and exposed to a low dose. Furthermore, the model provides a simple and practical method to separate and estimate the total oxide charge and increased interface trap densities from excess base current readings on irradiated PNP BJTs. The extracted values of interface trap densities show less dependence on E-B bias voltage than with previous models, which provides evidence of its improved precision.

Drift BJTs, characterized by a graded doping profile in the base region, are susceptible to the recombination in both the base and emitter-base depletion regions when they are damaged by atomic displacement, leading to gain degradation. Previous model of the gain degradation of BJTs (i.e., Messenger-Spratt model) adopts the assumptions of the neutral base and ideal depletion, which are not valid for the drift BJTs. A drift BJT has an extra build-in electric field which breaks the neutral base condition, and the excess recombination rate is reduced and depends on the drift parameters. Furthermore, the non-ideal depletion effects can suppress the excess recombination in the junction region, which will also reduce the susceptibility of the displacement damage. This work presents a physics-based improvement of the previous model, and provides a much better fit to the experimental data of the 3CK3B drift BJTs subjected to Deuterium-Tritium 14 MeV neutron irradiation. The new model also suggests that the ideal factor of excess base current induced by displacement damage approaches to 1.33 for the BJTs technologies with narrow base or operated under low bias conditions.

The MOS-controlled Thyristor (MCT) has been characterized by MOS-gating, high current rise rate, and high blocking capabilities. The anode short MCT (AS-MCT) is distinguished from conventional MCT by an anode-short structure, which forms an extracting path for electron current at the gate-ground and develops a normally-off characteristic. As a composite structure made of metal-oxide-silicon and bipolar junction transistors, AS-MCT is susceptible to displacement damage. For the first time, this work reports the displacement damage effects on AS-MCT with the fast neutron flux in the range of 3:1×109–5:5×1013 cm–2. Both the transfer and forward conductive characteristics are degraded subsequent to neutron exposures. When the neutron flux surpasses a critical value, the function failure of AS-MCT occurs, i.e., the AS-MCT cannot be latched up and enter into the thyristor-like conductivity mode and instead remains in the insulated gate bipolar transistorlike conductivity mode. For the forward blocking characteristics (open gate), both the anode leakage current and forward blocking voltage increases subsequent to neutron exposures. To note, a novel phenomenon is observed for AS-MCT following neutron exposures, i.e., the forward current-voltage curve exhibiting ”double snap-back effect”. This paper proposes, from a device physics perspective, the mechanism behind the characteristics degradation from displacement damage in AS-MCT. The dependencies of the key parameters on neutron flux and the failure neutron flux are analytically modeled. Our models provide an excellent fit to the experimental data of the XND1 AS-MCT subjected to fission neutrons.