国外GaN功率器件衬底材料和外延技术研发现状

GaN基功率器件的衬底和外延技术的发展对于器件性能的提升和成本的降低起着非常重要的作用.介绍了国外SiC基、Si基以及新型金刚石基GaN功率器件衬底材料和GaN外延技术的研发现状.重点讨论了大尺寸衬底技术(6英寸SiC衬底、8英寸Si衬底)、GaN HEMT与Si CMOS器件异质集成技术以及金刚石基GaN HEMT材料集成技术的研发进展.分析了GaN功率器件材料技术的发展趋势,认为更大尺寸更高质量衬底和外延材料制作、外延技术的改进、金刚石等新型衬底材料研发以及GaN基材料与Si材料的异质集成技术等将是未来研究的重点.     

AlGaN/GaN HEMT高温特性的研究进展

回顾了在高温条件下A1GaN/GaN HEMT器件特性的研究进展.发现2DEG的高温特性是影响器件高温性能的根本内在因素,且外延材料的缺陷、衬底及其器件的封装形式也影响器件的高温特性.最后总结了适合高温下工作的AlGaN/GaN HEMT的改进方法.     

金刚石衬底GaN HEMT研究进展

SiC衬底GaN高电子迁移率晶体管(HEMT)综合了AlGaN/GaN异质结优异的输运特性与SiC衬底高导热性能,在高频、宽带、高效、大功率应用领域表现出显著的性能优势。但GaN外延材料中存在高密度的缺陷,影响了导电沟道的散热,散热问题成为影响GaN HEMT性能进一步发挥的主要障碍。本文分析了GaN外延材料高缺陷密度形成的原因,介绍了近年来国外正在开展的基于转移技术金刚石衬底GaN HEMT技术,解决GaN HEMT散热问题的研究进展。研究结果表明,基于转移技术的金刚石衬底GaN HEMT有望成为继SiC衬底GaN HEMT之后的下一代固态微波功率器件主导型器件技术。     

垂直结构GaN基LEDs电流分布计算分析

电流分布是影响大功率LEDs器件性能的重要因素,与传统结构GaN基器件比较,垂直结构器件通过采用上下电极分布,明显改善了LEDs器件内部电流分布均匀性。通过理论分析与数值计算,建立起了垂直结构GaN基LEDs电流分布模型,研究了垂直结构GaN基LEDs电流分布及I-V特性。结果表明,与传统平面结构比较,垂直结构GaN基LEDs的电流分布均匀性得到了明显改善,同时正向电压降低约7%。最后,通过晶片键合与激光剥离技术,制备了垂直结构GaN基LEDs,测试结果表明,实验结果和理论计算值相吻合。该结果对GaN基LEDs器件的优化设计具有重要指导意义。     

AlGaN/GaN SBD的正向导通特性研究

主要研究了横向AlGaN/GaN异质结肖特基势垒二极管(简称SBD)的正向导通特性,设计制备了基于蓝宝石衬底和硅衬底的不同器件结构的AlGaN/GaN SBD器件。测量结果表明,通过适当改变肖特基-欧姆电极布局,以及在导电衬底上施加相应的偏压,可以有效改善器件的正向导通特性。实验所制备的肖特基电极半径为120μm、肖特基-欧姆电极间距为25μm的基于Al2O3衬底的AlGaN/GaN SBD器件,实现了正向导通电流0.05A@2V(Ron=9.13mΩ·cm2)、反向饱和漏电流为10-6 A的性能。对制备的硅基AlGaN/GaN SBD器件的测试发现,通过外加衬底偏压能够有效改善其正向导通特性。     

溅射AlN技术对GaN基LED性能的影响

研究了在图形蓝宝石衬底(PSS)上利用磁控溅射制备AlN薄膜的相关技术,随后通过采用金属有机化学气相沉积(MOCVD)在相关AlN薄膜上生了长GaN基LED.通过一系列对比实验,分析了AlN薄膜的制备条件对GaN外延层晶体质量的影响,研究了AlN薄膜溅射前N2预处理功率和溅射后热处理温度对GaN基LED性能的作用机制.实验结果表明:AlN薄膜厚度的增加,导致GaN缓冲层成核密度逐渐升高和GaN外延膜螺位错密度降低刃位错密度升高;N2处理功率的提升会加剧衬底表面晶格损伤,在GaN外延膜引入更多的螺位错;AlN热处理温度的升高粗化了表面并提高了GaN成核密度,使得GaN外延膜螺位错密度降低刃位错密度升高;而这些GaN外延膜位错密度的变化又进一步影响到LED的光电特性.     

蓝宝石衬底AlGaN/GaN HFET功率特性

通过台面隔离与注入隔离结合的方法,解决了由于GaN外延材料缓冲层质量差造成的AlGaN/GaN HFET击穿电压低的问题.通过优化的500℃/50s栅退火条件,改善Ni-AlGaN/GaN二极管特性,理想因子和势垒高度分别优化到1.5和0.87eV.通过改进AlGaN/GaN HFET制备工艺,得到提高器件输出功率的优化工艺,采用这一工艺制备的蓝宝石衬底总栅宽为1mm,器件在频率为8GHz时输出功率达到4.57W,功率附加效率为55.1%.     

蓝宝石衬底AlGaN/GaN HFET功率特性

通过台面隔离与注入隔离结合的方法,解决了由于GaN外延材料缓冲层质量差造成的AlGaN/GaN HFET击穿电压低的问题.通过优化的500℃/50s栅退火条件,改善Ni-AlGaN/GaN二极管特性,理想因子和势垒高度分别优化到1.5和0.87eV.通过改进AlGaN/GaN HFET制备工艺,得到提高器件输出功率的优化工艺,采用这一工艺制备的蓝宝石衬底总栅宽为1mm,器件在频率为8GHz时输出功率达到4.57W,功率附加效率为55.1%.     

Fabrication and Emission Properties of a n-ZnO/p-GaN Heterojunction Light-Emitting Diode

报道了n-ZnO/p-GaN异质结构发光二极管的制备及其发光特性.采用金属有机气相外延技术在Mg掺杂p型GaN衬底上外延n型ZnO薄膜以形成p-n结.实验发现在一定配比的HF酸和NH4Cl溶液中,腐蚀深度和腐蚀时间呈线性关系,并且二氧化硅和ZnO的腐蚀速率得到很好的控制,这对器件制备的可靠性非常重要.电流-电压(I-V)特性测试显示该器件结构具有明显的整流特性.室温下,在正反向偏压状态下都可用肉眼观察到电致发光现象.同时,通过与光致发光谱进行比较,对电致发光谱中发光峰的起源和发光机制进行了探讨.     

基于AlN和GaN形核层的AlGaN/GaN HEMT外延材料和器件对比

使用金属有机物化学气相淀积(MOCVD)方法在蓝宝石衬底上分别采用AlN和GaN作为形核层生长了AlGaN/GaN高电子迁移率晶体管(HEMT)外延材料,并进行了器件制备和性能分析.通过原子力显微镜(AFM)、高分辨率X射线双晶衍射仪(HR-XRD)和二次离子质谱仪(SIMS)等仪器对两种样品进行了对比分析,结果表明采用AlN形核层的GaN外延材料具有更低的位错密度,且缓冲层中氧元素的拖尾现象得到有效地抑制.器件直流特性显示,与基于GaN形核层的器件相比,基于AlN形核层的器件泄漏电流低3个数量级.脉冲Ⅰ-Ⅴ测试发现基于GaN形核层的HEMT器件受缓冲层陷阱影响较大,而基于AlN形核层的HEMT器件缓冲层陷阱作用不明显.     

Power and linearity characteristics of GaN MISFETs on sapphire substrate

The improvement of device performance arising from the adoption of a MIS gate structure in GaN field-effect transistor (FET) is presented. GaN MISFET/MESFET devices were fabricated on sapphire substrate with and without the insertion of a thin SiN layer on device surface. The MISFET device showed improved device characteristic due to significant reduction in device gate leakage with respect to the standard MESFET structure. Measured power and linearity performance showed promising results. Under single-tone testing at 4 GHz, device yielded saturated output power 6.2 W/mm with 55% peak power added efficiency. When tested with two-tone signal device maintained a carrier to third order intermodulation ratio of 30 dBc up to power levels of 1.8 W/mm with 40% power added efficiency.     

GaN HEMT器件稳态热特性试验研究

大功率GaN HEMT器件在工作时较高的热流密度引发器件高温,而高温会显著影响器件性能及可靠性。从不同器件结构设计出发,结合器件热量传递理论,建立了器件热阻模型;采用高速红外热像仪试验分析了器件结构对GaN HEMT器件稳态热特性的影响,定量给出了不同总栅宽、不同单指栅宽、不同栅间距在不同功率密度下的稳态温升。相关结果可用于研发阶段器件结温的快速评估。     

GaN Surface-Emitting Laser With Monolithic Cavity-Folding Mirrors

We report on the demonstration of a GaN-based surface-emitting laser (SEL) in a folded-cavity (FC) scheme. Two 45deg-angled sidewall deflectors were monolithically integrated at both the edges of an otherwise conventional InGaN multiple-quantum-well edge-emitting laser structure. Despite a low effective mirror reflectivity of R(<1%), the optically pumped broad-area laser structure produced laser oscillation in the vertical direction with a low threshold pump intensity of ~350kW/cm². Computer simulations on the FC, which was based on the finite-difference time-domain method, not only quantified the cavity quality of our FC-SEL device but also suggested methods to lower the laser threshold     

Novel Cs-Free GaN Photocathodes

We report on a novel GaN photocathode structure that eliminates the use of Cs for photocathode activation. Development of such a photocathode structure promises reduced cost and complexity of the device, potentially with stable operation for a longer time. Device simulation studies suggest that deposition of Si delta-doped GaN on p-GaN templates induces sharp downward energy band bending at the surface, assisting in achieving effective negative electron affinity for GaN photocathodes without the use of Cs. A series of experiments has been performed to optimize the quality of the Si delta-doped layer to minimize the emission threshold of the device. This report includes significant observations relating the dependence of device properties such as emission threshold, quantum efficiency, and surface morphology on the Si incorporation in the Si delta-doped layer. An optimum Si incorporation has been observed to provide the minimum emission threshold of 4.1 eV for the discussed Cs-free GaN photocathodes. Photoemission (PE), atomic force microscopy (AFM), and secondary-ion mass spectroscopy (SIMS) have been performed to study the effect of growth conditions on device performance.     

Reliability issues of GaN based high voltage power devices

GaN based power devices for high efficiency switching applications in modern power electronics are rapidly moving into the focus of world wide research and development activities. Due to their unique material properties GaN power devices are distinguished by featuring high breakdown voltages, low on-state resistances and fast switching properties at the same time. Finally, these properties are the consequences of extremely high field and current densities that are possible per unit device volume or area. Therefore, in order to obtain very high performance, the material itself is stressed significantly during standard device operation and any imperfection may lead to wear out and reliability problems. Thus material quality, the specific epitaxial design as well as the device topology will directly influence device performance, reliability and mode of degradation. The paper will mainly discuss those degradation mechanisms that are especially due to the specific material combinations used in GaN based high voltage device technology such as epitaxial layer design, chip metallization, passivation schemes and general device topology and layout. It will then discuss technological ways towards engineering reliability into these devices. Generally, device designs are required that effectively minimize high field regions in the internal device or shift them towards less critical locations. Furthermore, an optimized thermal design in combination with suitable chip mounting technologies is required to enable maximum device performance.     

GaN and AlGaN high-voltage rectifiers grown by metal-organic chemical-vapor deposition

In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5 V at 100 A/cm² are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm², and a forward-voltage drop of 3.5 V at a current density of 100 A/cm². It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge terminations in mesageometry GaN Schottky rectifiers has also been studied.     

蓝宝石衬底AlGaNöGaN 功率HEM Ts 研制

基于蓝宝石衬底的高微波特性 Al Ga N/Ga N HEMTs功率器件 ,器件采用了新的欧姆接触和新型空气桥方案。测试表明 ,器件电流密度 0 .784A/mm,跨导 1 97m S/mm,关态击穿电压 >80 V,截止态漏电很小 ,栅宽 1 mm的器件的单位截止频率 ( f T)达到 2 0 GHz,最大振荡频率 ( fmax) 2 8GHz,2 GHz脉冲测试下 ,栅宽 0 .75 mm器件 ,功率增益1 1 .8d B,输出功率 3 1 .2 d Bm,功率密度 1 .75 W/mm。     

蓝宝石衬底AlGaN/GaN共栅共源微波HEMTs器件

报道了蓝宝石衬底AlGaN/GaN共栅共源器件的制备与特性.该器件包括栅长为0.8μm共源器件与栅长为1μm的共栅器件.实验表明,共栅器件的第二栅压会显著影响器件饱和电流与跨导特性,从而控制功率增益.与共源器件相比,共栅共源器件表现出稍低的 f T、较低的反馈、显著增加的功率资用增益及较高的端口阻抗.     

Trade-offs and challenges of short channel design on millimetre-wave power performance of GaN HFETs

For the application of undoped AlGaN/GaN HFETs to Ka-band millimetre(mm)-wave high frequency power performance, the maximum frequency of oscillation, f/sub max/, was found to be seriously limited by gate resistance and output conductance with the gate length down to 0.1 /spl mu/m. This makes it difficult for devices to achieve both high f/sub T/ and f/sub max/ at the same time. However, the technology of field-plate gate, to increase device breakdown voltage, will add extra gate capacitance. It makes the optimum gate structure design more important. The influence of gate metal thickness and gate length on f/sub max/ based on the lumped small signal circuit model analysis and the possibility to obtain high f/sub T/ and f/sub max/ simultaneously for the GaN material structure is discussed for application to the Ka-band mm-wave operating system.     

基于垂直结构的有机薄膜晶体管

介绍了基于垂直结构的有机薄膜晶体管的器件结构、工作原理以及相关的研究进展。通过对垂直结构的有机薄膜晶体管的特性分析,探讨了采用这种结构的有机薄膜晶体管驱动有机发光二极管的可行性。此项研究为发展新型全有机的柔性显示器提供了新的思路。