Explore the vast range of titles available.

Improving a tropical cyclone's forecast and mitigating its destructive potential requires knowledge of various environmental factors that influence the cyclone's path and intensity. Herein, using a combination of observations and model simulations, we systematically demonstrate that tropical cyclone intensification is significantly affected by salinity-induced barrier layers, which are \"quasi-permanent\" features in the upper tropical oceans. When tropical cyclones pass over regions with barrier layers, the increased stratification and stability within the layer reduce storm-induced vertical mixing and sea surface temperature cooling. This causes an increase in enthalpy flux from the ocean to the atmosphere and, consequently, an intensification of tropical cyclones. On average, the tropical cyclone intensification rate is nearly 50% higher over regions with barrier layers, compared to regions without. Our finding, which underscores the importance of observing not only the upper-ocean thermal structure but also the salinity structure in deep tropical barrier layer regions, may be a key to more skillful predictions of tropical cyclone intensities through improved ocean state estimates and simulations of barrier layer processes. As the hydrological cycle responds to global warming, any associated changes in the barrier layer distribution must be considered in projecting future tropical cyclone activity.

Due to the enhanced-mode (E-mode) operation, AlGaN/GaN high-electron-mobility transistors (HEMTs) are considered to be safer for circuit operation. In order to improve the threshold voltage (Vth) of the device, this work provides a hybrid gate structure HEMT by embedding a P-GaN cap on the etched graded AlGaN barrier layer. Through simulation calculations, the P-GaN cap (thickness of P-GaN = 50 nm, concentration of P-type = 2 × 1018 cm−3) and the aluminum (Al) composition (Al:0.3 → 0.24), in the graded AlGaN barrier layer were optimized. Although simulation calculations show that the optimized P-GaN layer can significantly increase the device’s Vth to 8.6 V and transconductance (gm) to 94.7 mS/mm, the device exhibits a lower saturation current (Isat). Therefore, to improve the output characteristics of the devices, the addition of an N-well in the GaN channel layer of such structures was proposed. It can increase the device’s source–drain current while maintaining a steady Vth. Compared with the HEMT structure/combined P-GaN cap with recessed gate and a graded AlGaN barrier layer, the device with the added N-well exhibits a significant improvement of 11.2% in the saturation current (Isat = 718 mA/mm). The results demonstrate that HEMT structures combining recessed gates and P-GaN with N-well have promising applications in next-generation high-power devices.

Previous studies suggest that a thick barrier layer can strengthen surface warming during El Niño and vice versa for La Niña. Here we find barrier layer changes of up to 6 m in the central equatorial Pacific (CEP, 170°E−160°W, 5°S–5°N) in response to a 1°C change in the Niño 3.4 index using 19 years of Argo observations. Our analysis reveals that mixed layer variability due to strong interannual salinity anomalies determines the changes in CEP barrier layer thickness (BLT) during El Niño Southern Oscillation events since isothermal layer variations are minimal there. During El Niño, enhanced rainfall leads to anomalous surface freshening in the CEP especially west of the dateline, while anomalous vertical‐sheared zonal current results in stronger vertical salinity stratification and thicker barrier layer east of the dateline in the CEP. Surface‐layer zonal current advection in general contributes to a thicker barrier layer in the entire CEP.

The tropical Atlantic Ocean receives an important freshwater supply from river runoff and from precipitation in the intertropical convergence zone. It results in a strong salinity stratification that may influence vertical mixing, and thus sea surface temperature (SST) and air–sea fluxes. The aim of this study is to assess the impact of salinity stratification on the tropical Atlantic surface variables. This is achieved through comparison among regional 1/4 ∘ coupled ocean–atmosphere simulations for which the contribution of salinity stratification in the vertical mixing scheme is included or discarded. The analysis reveals that the strong salinity stratification in the northwestern tropical Atlantic induces a significant increase of SST (0.2 ∘ C–0.5 ∘ C) and rainfall (+ 19%) in summer, hereby intensifying the ocean–atmosphere water cycle, despite a negative atmospheric feedback. Indeed, the atmosphere dampens the oceanic response through an increase in latent heat loss and a reduction of shortwave radiation reaching the ocean surface. In winter, the impacts of salinity stratification are much weaker, most probably because of a deeper mixed layer at this time. In the equatorial region, we found that salinity stratification induces a year-round shoaling of the thermocline, reinforcing the cold tongue cool anomaly in summer. The concept of barrier layer has not been identified as relevant to explain the SST response to salinity stratification in our region of interest.

Previous studies have confirmed the diverse spatiotemporal characteristics of Atlantic Niño events. Our research further reveals the crucial preparatory role of equatorial western Atlantic barrier layers (BL) and the triggering effect of westerly wind bursts (WWB) on different varieties of Atlantic Niño. Strong easterly winds typically facilitate the formation of thick BL by deepening isothermal layer depth in the western Atlantic through horizontal transport. The existence of BL accumulates the necessary heat for the onset of Atlantic Niño. Additionally, the timing of BL occurrences, the presence of easterly wind anomalies preceding WWB, and the duration of westerly wind anomalies jointly contribute to Atlantic Niño diversity. Persistent westerly wind anomalies following strong easterly winds often lead to Atlantic Niño events lasting over 6 months, while short‐lived events occur when westerly wind anomalies cease shortly after their onset. Plain Language Summary The tropical Atlantic Ocean experiences significant year‐to‐year climate variability known as Atlantic Niño or Niña, similar to the warm and cold phases of El Niño–Southern Oscillation (ENSO) in the Pacific. Atlantic Niño has a considerable impact on local rainfall and cyclone activity. However, each instance of Atlantic Niño has unique spatiotemporal development characteristics, which can be classified into four varieties. While previous research has demonstrated that distinct preconditions give rise to different varieties of Atlantic Niño events, there hasn't been any investigation into the common factors among them. The salinity stratified isothermal layer between the base of the mixed layer and the top of the thermocline is referred to as the barrier layer (BL), which is a common feature of tropical western Pacific and Atlantic. The BL in the tropical western Pacific has been confirmed to facilitate the accumulation of heat in the upper ocean and can provide favorable thermal conditions for the onset of El Niño events. Our study reveals the key role of BL induced heat accumulation in various Atlantic Niño onsets. This suggests that anomalies of BL can be reliable indicators for predicting the onset of Atlantic Niño events. Key Points The role of barrier layers in heat buildup is confirmed during the development of the four varieties of Atlantic Niño The heat buildup caused by barrier layers, combined with zonal wind events, regulates the diversity of Atlantic Niño The sustainability of westerly wind anomalies links to the strength of Atlantic Niño events

As the only commercialized thermoelectric for low-grade waste heat recovery applications, Bi 2 Te 3 -based devices commonly use nickel as the electrode. The long-term chemical stability of the Bi 2 Te 3 /Ni junction, particularly for the hot side, is one of the major concerns for Bi 2 Te 3 -based and other thermoelectrics because of the formation of Ni-Te intermediate compounds. The utilization of diffusion barrier layers has been proven to be an effective solution and the barriers should have both a good chemical inertness and a slow diffusion. In this work, Ti was screened out from 13 metals in total as an effective barrier material between p-type Bi 0.5 Sb 1.5 -Te 3 thermoelectric materials and Ni electrodes, because of its low diffusion coefficient and long-term interfacial stability. The fabricated p-type Bi 0.5 Sb 1.5 Te 3 /Ti/Ni single-leg devices show a conversion efficiency over 6%, at a temperature difference of 200 K, without observable degradation for 1860 cycles of measurements lasting for 10 days. This study offers a useful strategy for making efficient and durable thermoelectric devices.

To enhance the DC and RF performance of AlGaN/GaN HEMTs, a novel device structure was proposed and investigated through simulation. The key innovation of this new structure lies in the incorporation of an Al0.7In0.15Ga0.15N back-barrier layer and an N-type locally doped AlGaN barrier layer (BD-HEMT), based on conventional device architecture. The Al0.7In0.15Ga0.15N back-barrier layer effectively confines electrons within the channel, thereby increasing the electron concentration. Simultaneously, the N-type locally doped AlGaN barrier layer introduced beneath the gate supplies additional electrons to the channel, further enhancing the electron density. These modifications collectively lead to improved DC and RF characteristics of the device. Compared to the conventional AlGaN/GaN HEMT, BD-HEMT achieves a 24.8% increase in saturation drain current and a 10.4% improvement in maximum transconductance. Furthermore, the maximum cutoff frequency and maximum oscillation frequency are enhanced by 14.8% and 21.2%, respectively.

AlGaN/GaN HEMTs were fabricated using two special gate metals, Hf and W, as gate Schottky contact materials. Based on the measured I-V data and two-dimensional scattering theory, the electron mobility corresponding to polarization Coulomb field (PCF) scattering and other scattering mechanisms was calculated. The additional polarization charge (APC) due to the inverse piezoelectric effect (IPE) was also calculated. The differences in the effects of two special gate metals, Hf and W, on the PCF scattering intensity are analyzed from the aspects of Young’s modulus and work function of the metal. It was found that compared with W metal, Hf metal has a smaller Young’s modulus and weaker ability to resist elastic deformation, which makes the AlGaN barrier layer in contact with Hf more easily deformed due to IPE, resulting in more APC under the Hf metal Schottky contact. On the other hand, Hf has a smaller work function compared to W metal, so the 2DEG density is higher. The influence of special gate metals Hf and W on the PCF scattering intensity is the result of the combined effect of these two factors. This study is of great significance for understanding the electron transport mechanisms of AlGaN/GaN HEMTs with special gate metals such as Hf and W, and for further improving the electrical performance and stability of the devices.

In order to improve the stability of Bi2Te3-based thermoelectric devices, we attempted to prepare a barrier layer via diffusion welding using Co powder as the barrier materials. The diffusion welding process was conducted at 674 K for a duration of 5 min. The optimal connection was achieved at the interfaces of Co/Bi0.4Sb1.6Te3 and Co/Bi2Te2.7Se0.3. The results showed that the Co/Bi0.4Sb1.6Te3 joint had diffusion thickness of 1.6 µm, contact resistivity of 0.83 µΩ cm2, and bonding strength of 4.86 MPa for the barrier layer. Similarly, the Co/Bi2Te2.7Se0.3 joint had diffusion thickness of 1.6 µm, contact resistivity of 0.75 µΩ cm2, and bonding strength of 3.99 MPa for the barrier layer. The thermoelectric device fabricated through this process exhibited a hot–cold cycle number of 35,000, which was significantly higher than the device with a Ni-based barrier layer under similar experimental conditions. Thus, it indicates that Co is a promising material for the preparation of barrier layers in Bi2Te3-based thermoelectric devices.

This work investigates the impact of barrier layer thickness on DC and RF performance of a GaN HEMT device, targeting the low noise high gain application. An optimisation workflow based on the barrier layer thickness and Al mole fraction is presented for improving the RF metrics of a GaN HEMT. AlGaN/GaN HEMTs with a gate length of 400 nm were fabricated with 22% Al content and a barrier layer thickness of 23 and 20 nm, respectively. TCAD simulation studies were carried out for different barrier thickness and Al mole fraction in accordance with the fabricated devices. Increasing the barrier thickness increases the 2-DEG density which increases the maximum drain current and results in a negative shift in the threshold voltage. With a thin barrier layer, the AlGaN/GaN HEMTs exhibit a higher transconductance due to improved gate action. The fabricated devices were investigated with the help of small-signal equivalent circuit, which demonstrate higher capacitances associated with a thin barrier layer. Apart from DC characteristics and small-signal performance, the intrinsic gain ( g m / g d ratio), noise performance, and large-signal performance of the device has been investigated which provides a great contribution in creating a design subspace for a specific application (depending on the performance requirement). A thin barrier layer improves the intrinsic gain of the GaN HEMT device by 74% due to a higher transconductance and comparatively lower output conductance values. An increase in Al mole fraction increases the transconductance but is dominated by an increase in the output conductance, which in turn reduces the intrinsic gain of the device. An in-depth analysis is presented by investigating and optimising the trade-offs with barrier layer thickness and Al mole fraction towards the noise performance of the devices at microwave C- and X-band.