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.     

Transient simulation and analysis of current collapse due to trapping effects in AlGaN/GaN high-electron-mobility transistor

In this paper, two-dimensional(2D) transient simulations of an Al Ga N/Ga N high-electron-mobility transistor(HEMT)are carried out and analyzed to investigate the current collapse due to trapping effects. The coupling effect of the trapping and thermal effects are taken into account in our simulation. The turn-on pulse gate-lag transient responses with different quiescent biases are obtained, and the pulsed current–voltage(I–V) curves are extracted from the transients. The experimental results of both gate-lag transient current and pulsed I–V curves are reproduced by the simulation, and the current collapse due to the trapping effect is explained from the view of physics based on the simulation results. In addition, the results show that bulk acceptor traps can influence the gate-lag transient characteristics of Al Ga N/Ga N HEMTs besides surface traps and that the thermal effect can accelerate the emission of captured electrons for traps. Pulse transient simulation is meaningful in analyzing the mechanism of dynamic current collapse, and the work in this paper will benefit the reliability study and model development of Ga N-based devices.     

Radio-Frequency Performance of Epitaxial Graphene Field-Effect Transistors on Sapphire Substrates

We report dc and the first-ever measured small signal rf performance of epitaxial graphene field-effect transistors （GFETs）, where the epitaxial graphene is grown by chemical vapor deposition （CVD） on a 2-inch c-plane sapphire substrate. Our epitaxial graphene material has a good flatness and uniformity due to the low carbon concentration during the graphene growth. With a gate length Lg = 100 nm, the maximum drain source current Ids and peak transconductance gm reach 0.92 A/mm and 0.143 S/mm, respectively, which are the highest results reported for GFETs directly grown on sapphire. The extrinsic cutoff frequency （fT） and maximum oscillation frequency （fmax） of the device are 12 GHz and 9.5 GHz, and up to 32 GHz and 21.5 GHz after de-embedding, respectively. Our work proves that epitaxial graphene on sapphire substrates is a promising candidate for rf electronics.     

Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance-voltage curves,the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode,and also much lower than the theoretical values.Moreover,by fitting the measured forward I–V curves,the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode.As a result,the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer,which is attributed to the more dislocations.     

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 SiN_xinstead of SiO_2 are demonstrated for the first time, to the best of our knowledge. Benefitting from the SiN_x 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.     

Influence of temperature on strain-induced polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

Electron mobility scattering mechanism in AlN/GaN heterostuctures is investigated by temperature-dependent Hall measurement, and it is found that longitudinal optical phonon scattering dominates electron mobility near room temperature while the interface roughness scattering becomes the dominant carrier scattering mechanism at low temperatures(～100 K).Based on measured current–voltage characteristics of prepared rectangular AlN/GaN heterostructure field-effect transistor under different temperatures, the temperature-dependent variation of electron mobility under different gate biases is investigated. The polarization Coulomb field(PCF) scattering is found to become an important carrier scattering mechanism after device processing under different temperatures. Moreover, it is found that the PCF scattering is not generated from the thermal stresses, but from the piezoelectric contribution induced by the electrical field in the thin AlN barrier layer. This is attributed to the large lattice mismatch between the extreme thinner AlN barrier layer and GaN, giving rise to a stronger converse piezoelectric effect.     

两种纤维状矿物作为汽车刹车片增强材料的实验研究

以改性针状硅灰石和纤维状海泡石作为复合增强体制备了多种刹车片试样,在综合分析各刹车片的冲击强度及定速摩擦试验数据的基础上,优选出了最佳刹车片配方,并针对基于优选配方的刹车片试样进行了卡斯试验及台架摩擦试验测定.结果表明,以1:6(质量分数)的硬脂酸改性细粒(0.018mm,颗粒长径比L∶D>12)针状硅灰石和硬脂酸改性粗粒(0.28mm,L∶D>50)纤维状海泡石作为复合增强体所制备的刹车片的综合性能最佳.基于优选配方所研制的19号刹车片试样的各项性能指标符合汽车刹车片材料国家标准,完全可以替代以石棉摩擦材料为增强材料的刹车片.     

基于脉冲方法的超短栅长GaN基高电子迁移率晶体管陷阱效应机理

陷阱效应导致的电流崩塌是制约GaN基微波功率电子器件性能提高的一个重要因素,研究深能级陷阱行为对材料生长和器件开发具有非常重要的意义.随着器件频率的提升,器件尺寸不断缩小,对小尺寸器件中深能级陷阱的表征变得越发困难.本文制备了超短栅长（L_g=80 nm）的AlGaN/GaN金属氧化物半导体高电子迁移率晶体管（MOSHEMT）,并基于脉冲I-V测试和二维数值瞬态仿真对器件的动态特性进行了深入研究,分析了深能级陷阱对AlGaN/GaN MOSHEMT器件动态特性的影响以及相关陷阱效应的内在物理机制.结果表明,AlGaN/GaN MOSHEMT器件的电流崩塌随着栅极静态偏置电压的增加呈非单调变化趋势,这是由栅漏电注入和热电子注入两种陷阱机制共同作用的结果.根据研究结果推断,可通过改善栅介质的质量以减小栅漏电或提高外延材料质量以减少缺陷密度等措施达到抑制陷阱效应的目的,从而进一步抑制电流崩塌.