In situ Observation of Sodium Dendrite Growth and Concurrent Mechanical Property Measurements Using an Environmental Transmission Electron Microscopy–Atomic Force Microscopy (ETEM-AFM) Platform

Akin to Li, Na deposits in a dendritic form to cause a short circuit in Na metal batteries. However, the growth mechanisms and related mechanical properties of Na dendrites remain largely unknown. Here we report real-time characterizations of Na dendrite growth with concurrent mechanical property measurements using an environmental transmission electron microscopy–atomic force microscopy (ETEM-AFM) platform. In situ electrochemical plating produces Na deposits stabilized with a thin Na2CO3 surface layer (referred to as Na dendrites). These Na dendrites have characteristic dimensions of a few hundred nanometers and exhibit different morphologies, including nanorods, polyhedral nanocrystals, and nanospheres. In situ mechanical measurements show that the compressive and tensile strengths of Na dendrites with a Na2CO3 surface layer vary from 36 to >203 MPa, which are much larger than those of bulk Na. In situ growth of Na dendrites under the combined overpotential and mechanical confinement can generate high stress in these Na deposits. These results provide new baseline data on the electrochemical and mechanical behavior of Na dendrites, which have implications for the development of Na metal batteries toward practical energy-storage applications

Identification of BPIFA1/SPLUNC1 as an epithelium-derived smooth muscle relaxing factor

Asthma is a chronic airway disease characterized by inflammation, mucus hypersecretion and abnormal airway smooth muscle (ASM) contraction. Bacterial permeability family member A1, BPIFA1, is a secreted innate defence protein. Here we show that BPIFA1 levels are reduced in sputum samples from asthmatic patients and that BPIFA1 is secreted basolaterally from healthy, but not asthmatic human bronchial epithelial cultures (HBECs), where it suppresses ASM contractility by binding to and inhibiting the Ca2+ influx channel Orai1. We have localized this effect to a specific, C-terminal α-helical region of BPIFA1. Furthermore, tracheas from Bpifa1−/− mice are hypercontractile, and this phenotype is reversed by the addition of recombinant BPIFA1. Our data suggest that BPIFA1 deficiency in asthmatic airways promotes Orai1 hyperactivity, increased ASM contraction and airway hyperresponsiveness. Strategies that target Orai1 or the BPIFA1 deficiency in asthma may lead to novel therapies to treat this disease

Small- and Large-Signal Analyses of Different Low-Pressure-Chemical-Vapor-Deposition SiNx Passivations for Microwave GaN HEMTs

Three types of SiN x passivation for microwave AlGaN/GaN HEMTs were deposited with low-pressure chemical vapor deposition under different deposition conditions, resulting in different silicon contents. The performance of the HEMTs is comprehensively investigated and compared. Both small- and large-signal analyses, such as generation-recombination (G-R) trap analysis, low-frequency noise characterization, and load-pull measurement, are indispensable to evaluate the effectiveness of a surface passivation. A Si-rich SiN x passivation shows excess G-R centers, whereas a Si-poor SiN x passivation exhibits significant current slump (30%). A bilayer SiN x passivation successfully shows not only a small current slump (9.7%) but also a suppressed G-R trapping/detrapping process. Moreover, the bilayer passivation demonstrates almost 2 orders of magnitude lower gate current noise spectra compared with the single-layer Si-rich SiN x passivation. The capacitance-voltage measurements reveal that the Si-rich SiN x layer removes the deep-level traps at the AlGaN/SiN x interface. Considering both small- and large-signal operations, it is concluded that the bilayer SiN x passivation is a suitable and versatile candidate for microwave GaN devices

High-Performance AlN/GaN MOSHEMTs with Regrown Ohmic Contacts by MOCVD

High-performance AlN/GaN metal oxide-semiconductor heterojunction field-effect transistors (MOSHEMTs) have been fabricated with source/drain (S/D) regrowth technology by metal-organic chemical vapor deposition (MOCVD). The gate and S/D metallization were produced simultaneously employing Ti/Al/Ni/Au. Low S/D contact resistance of 0.33 Omega.mm and interface resistance less than 0 07 Omega.mm were extracted. The fabricated 550-nm gate-length device exhibits a maximum transcondutance (G(m)) of 542 mS/mm and maximum drain current (I-d) of 1120 mA/mm with an on/off state current ratio up to 10(6)

Impact of AlGaN/GaN Interface and Passivation on the Robustness of Low-Noise Amplifiers

Poststress dc characteristics of AlGaN/GaN HEMTs can be used to study the effect of high-power stress on the noise figure (NF) and gain of low-noise amplifiers (LNAs) subjected to large input overdrives. This enables a shift from circuit- to transistor-level measurements to investigate the impact of variations in HEMT design parameters on the robustness (including both recovery time and survivability) by mimicking LNA operation. Using this method, a tradeoff between survivability and recovery time is demonstrated for different AlGaN/GaN interface profiles (sharp interface, standard interface, and AlN interlayer). Furthermore, the impact of different surface passivation schemes (Si-rich, Si-poor, and bilayer SiNx) on robustness is investigated. The bilayer passivation, which features low leakage current and small gain compression under overdrive stress, exhibits relatively weak survivability. The mechanisms influencing the robustness are analyzed based on transistor physics. The short recovery time is mainly due to impeding the injection of hot electrons into surface traps and high reverse current, whereas the survivability is dependent on the local or global peak electrical fields around the gate under high power stress

Low-Leakage-Current AlN/GaN MOSHFETs using Al2O3 for Increased 2DEG

Metal-oxide-semiconductor heterostructure field effect transistors (MOSHFETs) were fabricated with an AlN/GaN heterostructure grown on Si substrates. A 7-nm Al 2O 3 serving as both gate dielectric under the gate electrode and passivation layer in the access region was used. It was found that the Al 2O 3 was superior to SiN x in increasing the 2-D electron gas (2DEG) density and thereby reducing the access resistance. In addition, the off-state leakage current (I off) in these AlN/GaN MOSHFETs was reduced by four orders of magnitude to 7.6 × 10 -5 mA/mm as a result of the Al 2O 3 gate dielectric, compared to that of AlN/GaN HFETs. Meanwhile, the subthreshold slope was improved to a nearly ideal value of 62 mV/dec because of the extremely low I off. The MOSHFETs with 1-μm gate length exhibited good DC characteristics. A maximum drain current of 745 mA/mm and a peak extrinsic transconductance of 280 mS/mm were achieve

Achieving Low-Recovery Time in AlGaN/GaN HEMTs With AlN Interlayer Under Low-Noise Amplifiers Operation

Three transistors with different AlGaN/GaN interface designs (sharp interface, standard interface, and an extra AlN interlayer) were studied in-depth under conditions mimicking low-noise amplifiers (LNAs) operation. A new measurement setup, analog to LNAs operation condition, is established to measure recovery time on device level. For the first time, a direct relationship between the recovery time and the design of AlGaN/GaN interface is revealed in devices with Carbon doping buffer in this letter. An extremely low-recovery time is demonstrated in the transistor with an AlN interlayer. Both transistors without an AlN interlayer exhibit severe gain and drain current degradation after pulsed input stress. The transistor with a sharp interface shows a recovery time around 10 ms, whereas the transistor with a standard interface shows even much longer recovery time. These results imply that AlN interlayer, which can effectively block the injection of hot electrons to AlGaN bulk or surface traps, is highly preferred in systems where LNAs need to function promptly after an input overdrive