11.2 W/mm power density AlGaN/GaN high electron-mobility transistors on a GaN substrate

In this letter,high power density AlGaN/GaN high electron-mobility transistors(HEMTs)on a freestanding GaN substrate are reported.An asymmetricΓ-shaped 500-nm gate with a field plate of 650 nm is introduced to improve microwave power performance.The breakdown voltage(BV)is increased to more than 200 V for the fabricated device with gate-to-source and gate-to-drain distances of 1.08 and 2.92μm.A record continuous-wave power density of 11.2 W/mm@10 GHz is realized with a drain bias of 70 V.The maximum oscillation frequency(f_(max))and unity current gain cut-off frequency(f_(t))of the AlGaN/GaN HEMTs exceed 30 and 20 GHz,respectively.The results demonstrate the potential of AlGaN/GaN HEMTs on freestanding GaN substrates for microwave power applications.

Source-field-plated Ga_2O_3 MOSFET with a breakdown voltage of 550 V

Ga_2O_3 metal–oxide–semiconductor field-effect transistors(MOSFETs) with high-breakdown characteristics were fabricated on a homoepitaxial n-typed β-Ga_2O_3 film, which was grown by metal organic chemical vapor deposition(MOCVD) on an Fedoped semi-insulating(010) Ga_2O_3 substrate. The structure consisted of a 400 nm unintentionally doped(UID) Ga_2O_3 buffer layer and an 80 nm Si-doped channel layer. A high k HfO_2 gate dielectric film formed by atomic layer deposition was employed to reduce the gate leakage. Moreover, a source-connected field plate was introduced to enhance the breakdown characteristics. The drain saturation current density of the fabricated device reached 101 mA/mm at V_(gs) of 3 V. The off-state current was as low as 7.1 ×10^(-11) A/mm, and the drain current I_(ON)/I_(OFF) ratio reached 10~9. The transistors exhibited three-terminal off-state breakdown voltages of 450 and 550 V, corresponding to gate-to-drain spacing of 4 and 8 μm, respectively.

Review on constitutive models of road materials

Road material constitutive model is the essential condition of pavement structure design,calculation,and maintenance decision.Experts have conducted in-depth studies on the problems and proposed many meaningful constitutive models.However,these constitutive models have not been summarized and no uniform conclusion on the applicability of them is reported in the academic circle.The linear elastic model based on Hooke's law is still used in pavement structure design.This paper systematically reviewed the existing constitutive models that reflect the materials'mechanical properties and used for pavement structure calculation.These constitutive models were divided into three categories:linear,nonlinear,and damage models under repeated loads.The applicable conditions,parameter determination methods,advantages,and disadvantages of each model were introduced.Moreover,the model's feasibility in structural design and calculation was analyzed.Follow-up studies should focus on the parameter determination standard of viscoelastic constitutive models,the extension of the 1D constitutive model to 3D state,and the nonlinear constitutive models related to stress-strain state to drive the application of the constitutive models in pavement structure calculation and design.Furthermore,to evaluate the life cycle service performance of road materials and structures,establishing the pavement damage model from the structure perspective will be the future direction.

High-stability _(4)H-SiC avalanche photodiodes for UV detection at high temperatures

In this work,high-stability _[4]H-SiC avalanche photodiodes[APDs]for ultraviolet[UV]detection at high temperatures are fabricated and investigated.With the temperature increasing from room temperature to 150℃,a very small temperature coefficient of 7.4 m V/℃is achieved for the avalanche breakdown voltage of devices.For the first time,the stability of 4H-SiC APDs is verified based on an accelerated aging test with harsh stress conditions.Three different stress conditions are selected with the temperatures and reverse currents of 175℃/100μA,200℃/100μA,and 200℃/500μA,respectively.The results show that our 4H-SiC APD exhibits robust high-temperature performance and can even endure more than120 hours at the harsh aging condition of 200℃/500μA,which indicates that 4H-SiC APDs are very stable and reliable for applications at high temperatures.

沥青路面结构内部的力学响应特征及分析

为了揭示沥青路面结构内部的力学响应特征及变化规律,为研发适合我国国情和公路建设特点的沥青路面结构设计方法提供可靠的实测数据,依托我国足尺路面试验环道RIOHTrack,开展了沥青路面结构内部应力应变等力学响应的持续跟踪观测,分析了不同影响因素下力学响应行为的变化规律.结果表明,沥青路面结构内部的力学响应具有明显的温度依赖性和荷载依赖性,且随着温度、荷载的变化呈现显著的非线性变化特征,并表现出以年为周期的交替波动变化;温度变化除引起力学响应数值的增减以外,还会引起某些位置处力学响应方向的改变,出现压缩到拉伸或者拉伸到压缩的状态转换,应变与温度的关系曲线存在拉伸-压缩转换点和应变0值线,出现压缩-拉伸转换的深度位于24~36 cm之间,随着温度的升高,压缩-拉伸转换的位置会渐渐背离路表而向下移动;沥青层层底应力、应变响应随着荷载水平的增加而显著增大,并表现出明显的非线性变化趋势,尤其是在高温、重载时非线性现象更加明显.研究成果可为完善沥青路面设计及分析方法提供一定的数据支撑.

基于RIOHTrack足尺加速加载试验的长寿命沥青路面行为研究进展

采用足尺加速加载试验模拟和研究路面长期服役性能,是世界公认的最为高效、对自然和荷载耦合作用仿真度最高的技术手段,是路面设计理论发展的重要支撑.面向新一代长寿命路面技术目标,充分发挥足尺加速加载试验的优势,深入揭示荷载与环境耦合作用下的复杂路面结构行为机理,是革新路面设计方法的有效途径之一.第S54次香山科学会议"中国长寿命路面关键科学问题及技术前沿"的议题之一"路面足尺试验的目标与科学问题",即立足国内外足尺加速加载试验研究的主要经验与贡献,对研究目标和科学问题进行了讨论,进一步明确了该领域的发展方向和研究重点.本文通过对我国首条足尺加速加载路面试验环道RIOHTrack(Research Institute of Highway MOT Track)设计理念及研究进展的介绍,论述了长期持续观测积累的科学数据对于认识、发现路用性能演化规律的重要性,以及由现象学层面特征和差异出发,遵循现象发现-理论解析-实践验证的路面工程研究方法论,在非线性力学分析体系、多元仿真模型构建等研究方面取得的主要进展.

Coeffect of trapping behaviors on the performance of GaN-based devices

Trap-induced current collapse has become one of the critical issues hindering the improvement of Ga Nbased microwave power devices. It is difficult to study the behavior of each trapping effect separately with the experimental measurement. Transient simulation is a useful technique for analyzing the mechanism of current collapse. In this paper, the coeffect of surface-and bulk-trapping behaviors on the performance of Al Ga N/Ga N HEMTs is investigated based on the two-dimensional（2 D） transient simulation. In addition, the mechanism of trapping effects is analyzed from the aspect of device physics. Two simulation models with different types of traps are used for comparison, and the simulated results reproduced the experimental measured data. It is found that the final steady-state current decreases when both the surface and bulk traps are taken into account in the model.However, contrary to the expectation, the total current collapse is dramatically reduced（e.g. from 18% to 4% for the 90 nm gate-length device）. The results suggest that the surface-related current collapse of Ga N-based HEMTs may be mitigated in some degree due to the participation of bulk traps with short time constant. The work in this paper will be helpful for further optimization design of material and device structures.

Large-area 4H-SiC avalanche photodiodes with high gain and low dark current for visible-blind ultraviolet detection

In this Letter, we report large-area（600 μm diameter） 4H-SiC avalanche photodiodes（APDs） with high gain and low dark current for visible-blind ultraviolet detection. Based on the separate absorption and multiplication structure, 4H-SiC APDs passivated with SiNxinstead of SiO2 are demonstrated for the first time, to the best of our knowledge. Benefitting from the SiNx passivation, the surface leakage current is effectively suppressed. At room temperature, high multiplication gain of 6.5 × 10^5 and low dark current density of 0.88 μA∕cm^2 at the gain of 1000 are achieved for our devices, which are comparable to the previously reported small-area Si C APDs.

High-performance 4H-SiC p-i-n ultraviolet avalanche photodiodes with large active area

Ultraviolet(UV) detectors with large photosensitive areas are more advantageous in low-level UV detection applications. In this Letter, high-performance 4 H-SiC p-i-n avalanche photodiodes(APDs) with large active area(800 μm diameter) are reported. With the optimized epitaxial structure and device fabrication process,a high multiplication gain of 1.4 × 10^6 is obtained for the devices at room temperature, and the dark current is as low as ~10 p A at low reverse voltages. In addition, record external quantum efficiency of 85.5% at 274 nm is achieved, which is the highest value for the reported Si C APDs. Furthermore, the rejection ratio of UV to visible light reaches about 10^4. The excellent performance of our devices indicates a tremendous improvement for largearea SiC APD-based UV detectors. Finally, the UV imaging performance of our fabricated 4 H-SiC p-i-n APDs is also demonstrated for system-level applications.