Low-phase noise AlGaN/GaN FET-based voltage controlled oscillators(VCOs)

The first report of AlGaN/GaN HEMT-based voltage controlled oscillators (VCOs) is presented. Varactor-tuned oscillators implemented using distributed networks oscillate at 6 GHz with high output power (0.5 W), low-phase noise (-92 dBc/Hz SSB noise at 100 kHz offset), and high-tuning bandwidth (10%). The measured phase noise of AlGaN/GaN FETs is compared to the phase noise of GaAs FET and GaAs HBTs at 6 GHz, indicating the AlGaN/GaN FET exhibits equivalent SSB noise to GaAs FETs. These results indicate high power AlGaN/GaN-based VCOs may be used to simplify the line up in a communication radio, while improving the overall efficiency of the radio     

Polarization-Engineered Ga-Face GaN-Based Heterostructures for Normally-Off Heterostructure Field-Effect Transistors

Polarization-engineered Ga-face GaN-based heterostructures with a GaN cap layer and an AlGaN/p-GaN back barrier have been designed for normally-off field-effect transistors (FETs). The simulation results show that an unintentionally doped GaN cap and p-GaN layer in the buffer primarily deplete electrons in the channel and the Al_0.2Ga_0.8N back barrier helps to pinch off the channel. Experimentally, we have demonstrated a normally-off GaN-based field-effect transistor on the designed GaN cap/Al_0.3Ga_0.7N/GaN channel/Al_0.2Ga_0.8N/p-GaN/GaN heterostructure. A positive threshold voltage of 0.2 V and maximum transconductance of 2.6 mS/mm were achieved for 80-μm-long gate devices. The device fabrication process does not require a dry etching process for gate recessing, while highly selective etching of the GaN cap against a very thin Al_0.3GaN_0.7N top barrier has to be performed to create a two-dimensional electron gas for both the ohmic and access regions. A self-aligned, selective etch of the GaN cap in the access region is introduced, using the gate metal as an etch mask. The absence of gate recess etching is promising for uniform and repeatable threshold voltage control in normally-off AlGaN/GaN heterostructure FETs for power switching applications.     

毫米波GaN基HEMT器件材料结构发展的研究

由于GaN材料的高的饱和电子速度和击穿场强,GaN基HEMT已经成为实现毫米波器件的重要选择。回顾了GaN基HEMT器件材料结构的发展历程,就目前GaN基毫米波HEMT器件设计应用存在的短沟道效应和源漏间较大的导通电阻两个主要问题进行了机理分析,并对GaN基HEMT器件的毫米波应用未来发展方向进行了分析。同时从GaN基HEMT器件材料结构设计入手对解决方案进行了探讨性研究。针对面向毫米波应用的GaN基HEMT材料结构,为有效的抑制短沟道效应,可以采用栅凹槽结构加背势垒结构、采用InAlN等新材料,可以有效降低源漏导通电阻。     

Polarization Engineering on Buffer Layer in GaN-Based Heterojunction FETs

To improve the pinched-off characteristics of an AlGaN/GaN heterojunction field effect transistor (HJFET), the conduction band potential of an incorporated AL_xGa_1-xN buffer is designed to be upwardly convex in a band diagram. This approach utilizes the polarization effects specific to GaN-based materials by lowering the Al content x from 30% to 5% almost linearly toward the front side. Fabricated field effect transistors (FETs) adopting the designed buffer have demonstrated the following advanced characteristics in comparison to those of a FET adopting a conventional GaN buffer: less than one-tenth of the buffer leakage current, a gate-to-drain breakdown voltage BV_gd twice or more as high, and remarkably improved carrier confinement and pinched-off behavior. The FETs are operated in an enhancement mode with a gate-to-channel distance thick enough to prevent tunneling current through the gate.     

High Al-content AlGaN/GaN MODFETs for ultrahigh performance

The use of an AlGaN layer with high Al mole-fraction is proposed to increase the equivalent figures of merit of the AlGaN/GaN MODFET structure. It is shown that the room temperature mobility has little degradation with increasing Al mole-fraction up to 50%. 0.7-μm gate-length Al_0.5Ga_0.5N/GaN MODFETs by optical lithography exhibit a current density of 1 A/mm and three-terminal breakdown voltages up to 200 V. These devices on sapphire substrates without thermal management also show CW power densities of 2.84 and 2.57 W/mm at 8 and 10 GHz, respectively, representing a marked performance improvement for GaN-based FETs     

Breakdown Voltage Enhancement of AlGaN/GaN High-Electron-Mobility Transistors via Selective-Area Growth for Ohmic Contacts over Ion Implantation

Selective-area growth (SAG) based on plasma-assisted molecular-beam epitaxy (PAMBE) was shown to facilitate improvement of Ohmic contacts and direct-current (DC) characteristics for GaN-based field-effect transistors (FETs) over the widely accepted ion-implantation technique. Twofold improvements in breakdown voltage were also demonstrated for samples grown on both sapphire and silicon substrates. An AlGaN/GaN high-electron-mobility transistor (HEMT) fabricated with PAMBE-SAG exhibited a low specific contact resistivity of 5.86 × 10⁻⁷ Ω cm², peak drain current of 420 mA/mm, and high breakdown voltage of 77 V. These results demonstrate that PAMBE-SAG is suited to fabricating HEMTs for high-power applications.     

12 GHz $F_{rm MAX}$ GaN/AlN/AlGaN Nanowire MISFET

GaN/AlN/AlGaN/GaN nanowire metal–insulator–semiconductor field-effect transistors (MISFETs) have been fabricated for the first time with submicrometer gate lengths. Their microwave performances were investigated. An intrinsic current-gain cutoff frequency $(F_{T})$ of 5 GHz as well as an intrinsic maximum available gain $(F_{rm MAX})$ cutoff frequency of 12 GHz have been obtained for the first time and associated with a gate length of 0.5 $muhbox{m}$. These results show the great potentiality of GaN-based nanowire FETs for microwave applications.      

GaN基光电子材料及器件的研究

GaN材料是性能优越的化合物半导体,与其相关的合金材料可以制作出高亮度蓝光、绿光发光二极管、紫外探测器、半导体蓝色激光器(LD)等具有重要应用价值的光电子器件,因而备受重视,本文综述了GaN基光电子器件研究开发现状及其应用前景。     

GaN based transistors for high power applications

Unique properties of GaN and related semiconductors make them superior for high-power applications. The maximum density of the two-dimensional electron gas at the GaN/AlGaN heterointerface or in GaN/AlGaN quantum well structures can exceed 2×10¹³ cm⁻², which is more than an order of magnitude higher than for traditional GaAs/AlGaAs heterostructures. The mobility-sheet carrier concentration product for these two dimensional systems might also exceed that for GaAs/AlGaAs heterostructures and can be further enhanced by doping the conducting channels and by using “piezoelectric” doping. Current densities over 20 A mm⁻¹ can be reached in GaN-based high electron mobility transistors (HEMTs). These high current values can be combined with very high breakdown voltages in high-power HEMTs, which are expected to reach several thousand volts. Recent Monte Carlo simulations point to strong ballistic and overshoot effects in GaN and related materials, which should be even more pronounced than in GaAs-based compounds but at much higher electric fields. This should allow us to achieve faster switching, minimizing the power dissipation during switching events. Self-heating, which is unavoidable in power devices, raises operating temperatures of power devices well above the ambient temperature. For GaN-based devices, the use of SiC substrates allows to combine the best features of both GaN and SiC technologies; and GaN/SiC-based semiconductors and heterostructures should find numerous applications in power electronics.     

氮化镓功率半导体器件技术

作为第三代半导体材料的典型代表,宽禁带半导体氮化镓(GaN)具有许多硅材料所不具备的优异性能,是高频、高压、高温和大功率应用的优良半导体材料,在民用和军事领域具有广阔的应用前景。随着GaN技术的进步,特别是大直径硅(Si)基GaN外延技术的逐步成熟并商用化,GaN功率半导体技术有望成为高性能低成本功率技术解决方案,从而受到国际著名半导体厂商和研究单位的关注。总结了GaN功率半导体器件的最新研究,并对GaN功率器件发展所涉及的器件击穿机理与耐压优化、器件物理与模型、电流崩塌效应、工艺技术以及材料发展等问题进行了分析与概述。     

Gallium-nitride-based materials for blue to ultravioletoptoelectronics devices

Breakthroughs in materials growth have enabled extremely high-efficiency blue and green GaN light emitting diodes (LEDs) to be achieved for the first time. Blue LEDs with external quantum efficiency exceeding 9% have enabled hybrid GaN/phosphor white lamp LEDs. GaN LEDs complete the primary color spectrum and have enabled bright and reliable full-color solid-state displays to be realized. Recently, room-temperature operation of continuous wave current-injection blue-violet lasers emitting at 417 nm has further increased the possible applications for GaN-based opto-electronic devices. In this paper, we review the key technologies for GaN-based materials and devices. Developments in the methods for thin-film deposition by metalorganic chemical vapor deposition and molecular beam epitaxy and resulting film properties are highlighted     

Progress in Group Ⅲ nitride semiconductor electronic devices

Recently there has been a rapid domestic development in groupⅢnitride semiconductor electronic materials and devices.This paper reviews the important progress in GaN-based wide bandgap microelectronic materials and devices in the Key Program of the National Natural Science Foundation of China,which focuses on the research of the fundamental physical mechanisms of group III nitride semiconductor electronic materials and devices with the aim to enhance the crystal quality and electric performance of GaN-based electronic materials, develop new GaN heterostructures,and eventually achieve high performance GaN microwave power devices.Some remarkable progresses achieved in the program will be introduced,including those in GaN high electron mobility transistors(HEMTs) and metal-oxide-semiconductor high electron mobility transistors(MOSHEMTs) with novel high-k gate insulators,and material growth,defect analysis and material properties of InAlN/GaN heterostructures and HEMT fabrication,and quantum transport and spintronic properties of GaN-based heterostructures,and highelectric -field electron transport properties of GaN material and GaN Gunn devices used in terahertz sources.     

High-Speed GaN-Based Green Light-Emitting Diodes With Partially n-Doped Active Layers and Current-Confined Apertures

We demonstrate a high-speed GaN-based green light-emitting diode for plastic optical fiber (POF) communication applications. By using a combination of n-type doping and undoped In_xGa_1-xN/GaN based multiple quantum wells (MQWs), and a 76-mum-diameter current-confined aperture structure, we can obtain an extremely high electrical-to-optical (E-O) 3 dB bandwidth (~330 MHz), which is limited by the spontaneous recombination lifetime of the MQWs. A reasonable coupled power (-264 muW) can be simultaneously achieved for a 2 mm in diameter POF with a 0.5 numerical aperture (NA).     

GaN肖特基SIT的Monte Carlo模拟

用全带多粒子Monte Carlo模拟方法研究了GaN基肖特基势垒静电感应晶体管(SIT)的特性,给出了器件的电势、电场强度和电子浓度分布的Monte Carlo模拟结果。模拟得到的SIT输出特性曲线呈现非饱和特性,即类三极管特性。当VGS=0,VDS=35 V时,漏源电流为47 A/cm,跨导为300 mS/mm,电流截至频率为150 GHz。结果表明该器件具有大电流、高跨导和高频工作的潜力。     

氮化物半导体电子器件的若干研究进展

近年来,氮化物半导体电子器件和材料研究有了重大的进展。在国家自然科学基金资助下,西安电子科技大学、北京大学和中科院微电子所完成了国家自然科学基金重点项目《GaN宽禁带微电子材料和器件重大基础问题研究》。致力于通过氮化物电子材料和器件的基础物理机理研究提高GaN电子材料的结晶质量和电学性能、发展新结构GaN异质结材料研究,获得高性能的GaN HEMT微波功率器件。本文主要介绍该项目在GaN微波功率HEMT和新型高k栅介质MOS-HEMT、InAlN/GaN材料的生长和物性缺陷分析以及HEMT器件研制、GaN异质结的量子输运和自旋性质研究以及GaN材料高场输运性质和耿氏器件等几个方面取得的研究进展。     

GaN转移电子器件的性能与基本设计

基于GaN转移电子器件最基本的工作模式--畴渡越时间模式,计算了GaN转移电子器件的理想最高振荡频率,得到该类型微波转移电子器件的最高振荡频率可达4.7THz,接近GaAs转移电子器件最高振荡频率（0.6THz）的8倍. 从理论上计算出GaN转移电子器件的理想最大输出功率,结果表明GaN转移电子器件在功率输出方面具有很大优势. 最后还讨论了GaN转移电子器件在畴渡越时间模式下,能够产生稳定Gunn振荡的两个基本条件,即电子浓度N与器件有源区长度L乘积要大于该器件的设计标准（(NL)0=6.3E12cm-2) 及有源区的掺杂浓度N要小于临界掺杂浓度Ncrit(3.2E17cm-3) . 本工作揭示出GaN转移电子器件在高频率和大功率输出方面都具有重要优势,作为大功率THz微波信号源将具有广阔的应用前景.     

提高GaN功率器件开关频率的技术途径

GaN基增强型高速开关器件是提升X波段微系统集成放大器工作效率的核心器件.介绍了凹槽栅结构、F-注入等制作GaN基增强型器件的关键技术,同时分析了场板、介质栅等对器件击穿特性的影响.针对影响GaN基功率器件开关特性的主要因素,重点分析了提高增强型GaN基功率器件开关频率的主要技术途径.减小器件的接触电阻、沟道方块电阻可以降低器件电阻对频率的影响.小栅长器件中栅电容较低,电子的沟道渡越时间较短,也可以提高器件的频率特性.此外,由于GaN基的功率器件频率高,设计应用在GaN器件上的栅驱动电路显得尤为重要.     

GaN转移电子器件的性能与基本设计

基于GaN转移电子器件最基本的工作模式--畴渡越时间模式,计算了GaN转移电子器件的理想最高振荡频率,得到该类型微波转移电子器件的最高振荡频率可达4.7THz,接近GaAs转移电子器件最高振荡频率(0.6THz)的8倍.从理论上计算出GaN转移电子器件的理想最大输出功率,结果表明GaN转移电子器件在功率输出方面具有很大优势.最后还讨论了 GaN转移电子器件在畴渡越时间模式下,能够产生稳定Gunn振荡的两个基本条件,即电子浓度N与器件有源区长度L乘积要大于该器件的设计标准((NL)0=6.3×1012cm2)及有源区的掺杂浓度N要小于临界掺杂浓度Norit(3.2×1017cm-3).本工作揭示出GaN转移电子器件在高频率和大功率输出方面都具有重要优势,作为大功率THz微波信号源将具有广阔的应用前景.     

Nano-Processing Techniques Applied in GaN-Based Light-Emitting Devices With Self-Assembly Ni Nano-Masks

We have developed a simple method to fabricate nanoscale masks by using self-assembly Ni clusters formed through a rapid thermal annealing (RTA) process. The density and dimensions of the Ni nano-masks could be precisely controlled. The nano-masks were successfully applied to GaN-based light-emitting diodes (LEDs) with nano-roughened surface, GaN nanorods, and GaN-based nanorod LEDs to enhance light output power or change structure properties. The GaN-based LED with nano-roughened surface by Ni nano-masks and excimer laser etching has increased 55% light output at 20 mA when compared to that without the nano-roughened process. The GaN nanorods fabricated by the Ni nano-masks and ICP-RIE dry etching showed 3.5 times over the as-grown sample in photoluminescence (PL) intensity. The GaN-based nanorod LEDs assisted by photo-enhanced chemical (PEC) wet oxidation process were also demonstrated. The electroluminescence (EL) intensity of the GaN-based nanorod LED with PEC was about 1.76 times that of the as-grown LED. The fabrication, structure properties, physical features, and the optical and electrical properties of the fabricated devices will be discussed.     

AlGaN/GaN hybrid MOS-HEMT analytical mobility model

Since it is naturally normally-off, the hybrid AlGaN/GaN MOS-HEMT has a tremendous potential for an advanced GaN-based power switch. An analytical model for the hybrid AlGaN/GaN HEMT on-resistance is presented in this paper. The methodology presented here can aid the designers to understand the physics and to electrically characterize the new generation of GaN based devices. The models proposed here can also easily be implemented in TCAD simulation packages where models for GaN devices are not mature.