AlGaN/GaN和AlGaN SBD的温度特性研究

研究了AlGaN/GaN异质结构上的肖特基接触的基本原理及载流子的高温输运特性.将AlGaN/GaN异质结SBD和AlGaN SBD,在27～250℃进行实验比较.发现随着温度上升,AlGaN SBD的势垒高度下降,理想因子增加,其影响因素包括热电子发射、场发射、隧穿效应及复合电流效应等机制.而AlGaN/GaN异质结SBD由于受到压电极化场和2DEG和的影响,其势垒高度和理想因子随温度的变化趋势与AlGaNSBD相反.实验结果还显示,AlGaN/GaN异质结SBD的反向电流随着温度的上升,呈现先增大后减小的趋势.     

凹槽栅场板结构的AlGaN/GaN HEMT特性分析

研究了无场调制板结构、有场调制板结构但无凹槽栅、结合场调制板结构和凹槽栅工艺三种AlGaN/GaN HEMT的动态I-V特性和微波特性,认为场调制板结构和凹槽栅工艺可以有效改善AlGaN/GaN HEMT器件沟道内电场分布,显著减小电流崩塌现象,提高器件的微波输出功率特性.利用此技术研制的1mm栅宽AlGaN/GaNHEMT输出功率大于10W.     

凹槽栅场板结构的AlGaN/GaN HEMT特性分析

研究了无场调制板结构、有场调制板结构但无凹槽栅、结合场调制板结构和凹槽栅工艺三种AlGaN/GaN HEMT的动态I-V特性和微波特性,认为场调制板结构和凹槽栅工艺可以有效改善AlGaN/GaN HEMT器件沟道内电场分布,显著减小电流崩塌现象,提高器件的微波输出功率特性.利用此技术研制的1mm栅宽AlGaN/GaNHEMT输出功率大于10W.     

势垒材料对GaN基异质结电学特性的影响研究

基于势垒材料分别为Al0.27Ga0.73N和In0.17Al0.83N的GaN基异质结肖特基二极管(SBD),研究了GaN基异质结的漏电流输运机制、二维电子气密度和反向击穿电压等重要电学特性。结果表明,AlGaN/GaN SBD的反向电流主要由Frenkel-Poole(FP)发射机制主导,而InAlN/GaN SBD的反向电流在低电场下表现为FP发射电流,在高电场下则表现为Fowler-Nordheim隧穿电流。InAlN/GaN SBD的异质界面二维电子气密度明显高于AlGaN/GaN SBD,但是InAlN层存在高密度的缺陷,导致InAlN/GaN SBD的反向漏电流较大,且反向击穿电压较低。     

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。     

AlGaN/GaN横向肖特基势垒二极管的仿真与制作

基于GaN横向肖特基势垒二极管(SBD)的频率特性和应用的需要,设计了一种基于AlGaN/GaN异质结的横向SBD.利用Silvaco Atlas软件研究了 AlGaN势垒层的厚度和Al摩尔组分对异质结AlχGaN1-χ/GaN SBD电学性能的影响.仿真结果表明,SBD器件截止频率随Al摩尔组分的增加先增大再减小,当AlχGaN,1-χ层中Al摩尔组分为0.2～0.25,其厚度为20～30nm时,AlGaN/GaN SBD器件的频率特性最好.在仿真的基础上,设计制作出了肖特基接触直径为2 μm的非凹槽和凹槽型AlGaN/GaN横向空气桥SBD.通过直流I-V[测试和射频S参数测试,提取了两种SBD器件的理想因子、串联电阻、结电容、截止频率和品质因子等关键参数,该平面 SBD可应用于片上集成和混合集成的太赫兹电路的设计与制造.     

氮化镓基高电子迁移率晶体管栅电流输运机制研究

制备了与AlGaN/GaN高电子迁移率晶体管栅极结构与性能等效的圆形肖特基二极管结构,测量了器件的变温电流-电压特性,研究其在正向与反向偏压条件下的载流子输运过程。结果表明:(1)正向低偏压线形区的电流主要为缺陷辅助隧穿电流,而体电阻效应显著的高偏压区,经典热发射机制占主导地位;(2)AlGaN势垒层中的极化电场对器件的反向漏电流起重要作用,载流子的主要输运过程为Frenkel-Poole发射机制。     

GaN基肖特基势垒二极管的漏电流传输与退化机制

通过比较反向偏压下AlGaN/GaN异质结肖特基势垒二极管(SBD)和GaN SBD的电流特性、电场分布和光发射位置,研究了GaN基SBD的漏电流传输与退化机制。结果表明,AlGaN/GaN SBD退化前后漏电流均由Frenkel-Poole(FP)发射机制主导,而GaN SBD低场下为FP发射电流,高场下则为Fowler-Nordheim(FN)隧穿电流。电场模拟和光发射测试结果表明,引起退化的主要原因是高电场,由于结构不同,两种SBD的退化机制和退化位置并不相同。根据实验结果,提出了一种高场FN隧穿退化模型,该模型强调应力后三角势垒变薄导致FN隧穿增强是GaN SBD退化的内在机制。     

GaN HEMT器件60Co-γ辐照效应研究

研究了总剂量4 Mrad(Si) ⁶⁰Co-γ辐照对AlGaN/GaN HEMT器件的影响，器件在辐照过程中采用不同的加电方式。辐照过程会增加器件的栅泄漏电流，但辐照终止后电流快速恢复至初始状态。对比辐照前后的直流性能，器件的肖特基势垒高度、阈值电压、漏电流、跨导等未出现退化问题。实验结果表明，基于自主开发的GaN标准工艺平台所制备AlGaN/GaN HEMT器件，具有稳定可靠的⁶⁰Co-γ抗辐照能力。     

GaN基SBD功率器件研究进展

作为第三代宽禁带半导体器件,GaN基肖特基势垒二极管(SBD)功率器件具有耐高温、耐高压和导通电阻小等优良特性,在功率器件方面具有显著的优势。概述了基于功率应用的GaN SBD功率器件的研究进展。根据器件结构,介绍了基于材料特性的GaN SBD和基于AlGaN/GaN异质结界面特性的GaN异质结SBD。根据器件结构对开启电压的影响,对不同阳极结构器件进行了详细的介绍。阐述了不同的肖特基金属的电学特性和热稳定性。分析了表面处理,包括表面清洗、表面等离子体处理和表面钝化对器件漏电流的影响。介绍了终端保护技术,尤其是场板技术对击穿电压的影响。最后探讨了GaN基SBD功率器件未来的发展趋势。     

Comparison of electrical characteristics between AlGaN/GaN and lattice-matched InAlN/GaN heterostructure Schottky barrier diodes

Lattice-matched Pt/Au–In_0.17Al_0.83N/GaN hetreojunction Schottky barrier diodes (SBDs) with circular planar structure have been fabricated. The electrical characteristics of InAlN/GaN SBD, such as two-dimensional electron gas (2DEG) density, turn-on voltage, Schottky barrier height, reverse breakdown voltage and the forward current-transport mechanisms, are investigated and compared with those of a conventional AlGaN/GaN SBD. The results show that, despite the higher Schottky barrier height, more dislocations in InAlN layer causes a larger leakage current and lower reverse breakdown voltage than the AlGaN/GaN SBD. The emission microscopy images of past-breakdown device suggest that a horizontal premature breakdown behavior attributed to the large leakage current happens in the InAlN/GaN SBD, differing from the vertical breakdown in the AlGaN/GaN SBD.     

采用场板和边缘终端技术的大电流Ni/4H-SiC SBDs

研究了4H-SiC低缺陷密度外延层的制造和Ni/SiC肖特基势垒二极管的正、反向电学特性。采用了偏8°4H-SiC衬底上台阶控制外延方法进行同质外延,外延温度1580℃,最后得到了低缺陷密度的3英寸外延片。采用了原子力显微镜和扫描电子显微镜进行了测试。在外延片上进行的Ni/4H-SiC肖特基势垒二极管的制造,采用了B+离子注入形成的一个非晶区域作为边缘终端,然后使用经过1000℃下退火10min的PECVD生长的SiO2作为场板介质。最终得到的Ni/4H-SiC肖特基势垒二极管的理想因子为1.03,势垒高度为1.6eV,在反向偏压1102V时,漏电流密度只有1.15×10-3A/cm2。在正向压降3.5V时得到了7.47A的大电流输出,特征导通电阻为6.22Ω.cm2。     

Schottky barriers on n-GaN grown on SiC

Characteristics of Schottky barriers fabricated on n-type GaN were investigated. The barriers were formed by vacuum thermal evaporation of Cr, Au, and Ni. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the barriers were measured in a wide temperature and current density range. Fundamental parameters (barrier height and built-in potential) of the Schottky barriers were determined. The dependence of the barrier ideality factor on doping concentration in GaN was measured. Correlation between the barrier height and metal work function was observed. The electron affinity for GaN was determined using both C-V and I-V characteristics. The current flow mechanism through the barriers is discussed.     

Switching Characteristics of GaN HFETs in a Half Bridge Package for High Temperature Applications

AlGaN/GaN heterojunction field effect transistors (HFETs) are expected to be a good candidate for power switching application at high temperatures. We designed and fabricated a discrete HFET package and a half bridge module using the AlGaN/GaN HFETs and SiC Schottky barrier diodes (SBDs) for high temperature applications. The half bridge module exhibited good reliability after 250 C and 400 h high temperature storage. Switching characteristics of the AlGaN/GaN HFET were investigated. Qg x Ron, which shows a figure of merit of switching operation, was more than 10 times better than commercial Si MOSFETs. The switching characteristics of the HFET showed no significant degradation up to 225 C.     

Current transport mechanisms and trap state investigations in (Ni/Au)-AlN/GaN Schottky barrier diodes

The current transport mechanisms in (Ni/Au)-AlN/GaN Schottky barrier diodes (SBDs) were investigated by the use of current-voltage characteristics in the temperature range of 80-380 K. In order to determine the true current transport mechanisms for (Ni/Au)-AlN/GaN SBDs, by taking the J_s(tunnel), E₀, and R_s as adjustable fit parameters, the experimental J-V data were fitted to the analytical expressions given for the current transport mechanisms in a wide range of applied biases and at different temperatures. Fitting results show the weak temperature dependent behavior in the saturation current and the temperature independent behavior of the tunneling parameters in this temperature range. Therefore, it has been concluded that the mechanism of charge transport in (Ni/Au)-AlN/GaN SBDs, along the dislocations intersecting the space charge region, is performed by tunneling.In addition, in order to analyze the trapping effects in (Ni/Au)-AlN/GaN SBDs, the capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics were measured in the frequency range 0.7-50 kHz. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming the models in which traps are located at the heterojunction interface. The density (D_t) and time constants (τ_t) of the trap states have been determined as a function of energy separation from the conduction-band edge (E_c − E_t) as D_t≅(5-8)×10¹², respectively.     

Electrical and Optical Characterization of Ni/Al0.3Ga0.7N/GaN Schottky Barrier Diodes

Ni/AlGaN/GaN Schottky barrier diodes were characterized by electrical and optical measurements. Analysis of temperature-dependent (80?K to 550?K) current–voltage characteristics considering various transport mechanisms shows that the tunneling current dominates in the samples investigated. Thermionic emission current, extracted from the total current by a fitting procedure, yielded an effective barrier height of 1.36?eV to 1.39?eV at 300?K, and its slight decrease with increased temperature. This result shows that significantly lower barrier heights reported before (0.73?eV to 0.96?eV) follow from an assumption that the measured and thermionic currents are equal. The barrier height of 1.66?eV extracted from photoemission measurements confirms that electrically evaluated barrier heights are underestimated. The tunneling current contribution is considered to be dislocation governed, and a dislocation density of about 2?×?10⁸?cm^?2 is calculated.     

采用场板和B+离子注入边缘终端技术的Ti/4H-SiC肖特基势垒二极管

采用自主外延的4H-SiC外延片,利用PECVD生长的SiO2做场板介质,B+离子注入边缘终端技术,制造了Ti/4H-SiC肖特基势垒二极管.测试结果表明,Ti/4H-SiC肖特基势垒二极管的理想因子n=1.08,势垒高度(ψe)=1.05eV,串联电阻为6.77mΩ·cm2,正向电压为4V时,电流密度达到430A/cm2.反向击穿电压大于1.1kV,室温下,反向电压为1.1kV时,反向漏电流为5.96×10-3 A/cm2.     

Degradation of AlGaN/GaN HEMTs under elevated temperature lifetesting

Elevated temperature lifetesting was performed on 0.25 μm AlGaN/GaN HEMTs grown by MOCVD on 2-in. SiC substrates. A temperature step stress (starting at T_a of 150 °C with a step of 15 °C; ending at T_a of 240 °C; 48 h for each temperature cycle) was employed for the quick reliability evaluation of AlGaN/GaN HEMTs. It was found that the degradation of AlGaN/GaN HEMTs was initiated at ambient temperature of 195 °C. The degradation characteristics consist of a decrease of drain current and transconductance, and an increase of channel-on-resistance. However, there is no noticeable degradation of the gate diode (ideality factor, barrier height, and reverse gate leakage current). The FIB/STEM technique was used to examine the degraded devices. There is no detectable ohmic metal or gate metal interdiffusion into the epitaxial materials. Accordingly, the degradation mechanism of AlGaN/GaN HEMTs under elevated temperature lifetesting differs from that observed in GaAs and/or InP HEMTs. The reliability performance was also compared between two vendors of AlGaN/GaN epilayers. The results indicate that the reliability performance of AlGaN/GaN HEMTs could strongly depend on the material quality of AlGaN/GaN epitaxial layers on SiC substrates.