Recess-free enhancement-mode AlGaN/GaN RF HEMTs on Si substrate

Enhancement-mode(E-mode)GaN-on-Si radio-frequency(RF)high-electron-mobility transistors(HEMTs)were fabri-cated on an ultrathin-barrier(UTB)AlGaN(<6 nm)/GaN heterostructure featuring a naturally depleted 2-D electron gas(2DEG)channel.The fabricated E-mode HEMTs exhibit a relatively high threshold voltage(VTH)of+1.1 V with good uniformity.A maxi-mum current/power gain cut-off frequency(fT/fMAX)of 31.3/99.6 GHz with a power added efficiency(PAE)of 52.47%and an out-put power density(Pout)of 1.0 W/mm at 3.5 GHz were achieved on the fabricated E-mode HEMTs with 1-μm gate and Au-free ohmic contact.

The low-temperature mobility of two-dimensional electron gas in AlGaN/GaN heterostructures

To reveal the internal physics of the low-temperature mobility of two-dimensional electron gas （2DEG） in Al- GaN/GaN heterostructures, we present a theoretical study of the strong dependence of 2DEG mobility on Al content and thickness of AlGaN barrier layer. The theoretical results are compared with one of the highest measured of 2DEG mobility reported for AlGaN/GaN heterostructures. The 2DEG mobility is modelled as a combined effect of the scat- tering mechanisms including acoustic deformation-potential, piezoelectric, ionized background donor, surface donor, dislocation, alloy disorder and interface roughness scattering. The analyses of the individual scattering processes show that the dominant scattering mechanisms are the alloy disorder scattering and the interface roughness scattering at low temperatures. The variation of 2DEG mobility with the barrier layer parameters results mainly from the change of 2DEG density and distribution. It is suggested that in AlGaN/GaN samples with a high Al content or a thick AlGaN layer, the interface roughness scattering may restrict the 2DEG mobility significantly, for the AlGaN/GaN interface roughness increases due to the stress accumulation in AlGaN layer.

Influence of drain bias on the electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

Using measured capacitance-voltage curves and current-voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances, the influence of drain bias on the electron mobility is investigated. It is found that below the knee voltage the longitudinal optical （LO） phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance （here 4/5）, and the polarization Coulomb field scattering is dominant for the sample with a small ratio （here 1/5）. However, the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance （here 20 μm） compared with the one with a large distance （here 100 μm）. This is due to the induced strain in the AlGaN layer caused by the drain bias.

The effects of vicinal sapphire substrates on the properties of AlGaN/GaN heterostructures

A1GaN/GaN heterostructures on vicinal sapphire substrates and just-oriented sapphire substrates （0001） are grown by the metalorganic chemical vapor deposition method. Samples are studied by high-resolution x-ray diffraction, atomic force microscopy, capacitance-voltage measurement and the Van der Panw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Fur- thermore, the electrical properties are also improved when the sample is grown on the vicinal substrate. This is due to the fact that the use of vicinal substrate can promote the step-flow mode of crystal growth, so many macro-steps are formed during crystal growth, which causes a reduction of threading dislocations in the crystal and an improvement in the electrical properties of the AlGaN/GaN heterostructure.

Determination of the relative permittivity of the AlGaN barrier layer in strained AlGaN/GaN heterostructures

Using the measured capacitance voltage curves and the photocurrent spectrum obtained from the Ni Schottky contact on a strained Al0.3Ga0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving SchrSdinger＇s and Poisson＇s equations. It is shown that the calculated values of the relative permittivity are different from those formerly reported, and reverse biasing the Ni Schottky contact has an influence on the value of the relative permittivity. As the reverse bias increases from 0 V to -3 V, the value of the relative permittivity decreases from 7.184 to 7.093.

Influence of thermal stress on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on A1GaN/GaN heterostructures have been fabricated. The samples are then thermally treated in a furnace with N2 ambient at 600℃ for different times （0.5, 4.5, 10.5, 18, 33, 48 and 72 h）. Current-voltage （I-V） and capacitance-voltage （C-V） relationships are measured, and SchrSdinger＇s and Poisson＇s equations are self- consistently solved to obtain the characteristic parameters related to A1GaN/GaN heterostructure $chottky contacts： the two-dimensional electron gas （2DEG） sheet density, the polarization sheet charge density, the 2DEG distribution in the triangle quantum well and the Schottky barrier height for each thermal stressing time. Most of the above parameters reduce with the increase of stressing time, only the parameter of the average distance of the 2DEG from the A1CaN/GaN interface increases with the increase of thermal stressing time. The changes of the characteristic parameters can be divided into two stages. In the first stage the strain in the A1GaN barrier layer is present. In this stage the characteristic parameters change rapidly compared with those in the second stage in which the AlGaN barrier layer is relaxed and no strain is present.

Degradation mechanism of two-dimensional electron gas density in high Al-content AlGaN/GaN heterostructures

This paper finds that the two-dimensional electron gas density in high Al-content A1GaN/GaN heterostructures exhibits an obvious time-dependent degradation after the epitaxial growth. The degradation mechanism was investigated in depth using Hall effect measurements,high resolution x-ray diffraction,scanning electron microscopy,x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy.The results reveal that the formation of surface oxide is the main reason for the degradation,and the surface oxidation always occurs within the surface hexagonal defects for high Al-content AlGaN/GaN heterostructures.

Electron mobility in the linear region of an AlGaN/AlN/GaN heterostructure field-effect transistor

We simulate the current-voltage （I-V） characteristics of AlGaN/AlN/GaN heterostructure field-effect transistors （HFETs） with different gate lengths using the quasi-two-dimensional （quasi-2D） model. The calculation results obtained using the modified mobility model are found to accord well with the experimental data. By analyzing the variation of the electron mobility for the two-dimensional electron gas （213EG） with the electric field in the linear region of the AlGaN/AlN/GaN HFET I-V output characteristics, it is found that the polarization Coulomb field scattering still plays an important role in the electron mobility of AlGaN/AlN/GaN HFETs at the higher drain voltage and channel electric field. As drain voltage and channel electric field increase, the 2DEG density reduces and the polarization Coulomb field scattering increases, as a result, the 2DEG electron mobility decreases.

Fabrication of GaN-Based Heterostructures with an InA1GaN/AlGaN Composite Barrier

CaN-based heterostructures with an InAlCaN/AlCaN composite barrier on sapphire （0001） substrates are grown by a low-pressure metal organic chemical vapor deposition system. Compositions of the InAiGaN layer are determined by x-ray photoelectron spectroscopy, structure and crystal quality of the heterostruetures are identified by high resolution x-ray diffraction, surface morphology of the samples are examined by an atomic force microscope, and Hall effect and capacitance-voltage measurements are performed at room temperature to evaluate the electrical properties of heterostructures. The Al/In ratio of the InAlGaN layer is 4.43, which indicates that the InAlCaN quaternary layer is nearly lattice-matched to the CaN channel. Capacitance-voltage results show that there is no parasitic channel formed between the InAIGaN layer and the AlCaN layer. Compared with the InAl- CaN/CaN heterostructure, the electrical properties of the InAlCaN/AlGaN/GaN heterostructure are improved obviously. Influences of the thickness of the AlGaN layer on the electrical properties of the heterostructures are studied. With the optimal thickness of the AlGaN layer to be 5 nm, the 2DEG mobility, sheet density and the sheet resistance of the sample is 1889.61 cm2/V.s, 1.44 × 10^13 cm-2 and as low as 201.1 Ω/sq, respectively.

Abnormal phenomenon of source-drain current of AlGaN/GaN heterostructure device under UV/visible light irradiation

We report an abnormal phenomenon that the source-drain current(I_(D))of AlGaN/GaN heterostructure devices decreases under visible light irradiation.When the incident light wavelength is 390 nm,the photon energy is less than the band gaps of GaN and AlGaN whereas it can causes an increase of ID.Based on the UV light irradiation,a decrease of I_(D) can still be observed when turning on the visible light.We speculate that this abnormal phenomenon is related to the surface barrier height,the unionized donor-like surface states below the surface Fermi level and the ionized donor-like surface states above the surface Fermi level.For visible light,its photon energy is less than the surface barrier height of the AlGaN layer.The electrons bound in the donor-like surface states below the Fermi level are excited and trapped by the ionized donor-like surface states between the Fermi level and the conduction band of AlGaN.The electrons trapped in ionized donor-like surface states show a long relaxation time,and the newly ionized donor-like surface states below the surface Fermi level are filled with electrons from the two-dimensional electron gas(2DEG)channel at AlGaN/GaN interface,which causes the decrease of ID.For the UV light,when its photon energy is larger than the surface barrier height of the AlGaN layer,electrons in the donor-like surface states below the Fermi level are excited to the conduction band and then drift into the 2DEG channel quickly,which cause the increase of ID.

Low-thermal-budget Au-free ohmic contact to an ultrathin barrier AlGaN/GaN heterostructure utilizing a micro-patterned ohmic recess

A pre-ohmic micro-patterned recess process,is utilized to fabricate Ti/Al/Ti/TiN ohmic contact to an ultrathin-barrier(UTB)AlGaN/GaN heterostructure,featuring a significantly reduced ohmic contact resistivity of 0.56Ω·mm at an alloy temperature of 550℃.The sheet resistances increase with the temperature following a power law with the index of +2.58,while the specific contact resistivity decreases with the temperature.The contact mechanism can be well described by thermionic field emission(TFE).The extracted Schottky barrier height and electron concentration are 0.31 eV and 5.52×10^(18) cm^(−3),which suggests an intimate contact between ohmic metal and the UTB-AlGaN as well as GaN buffer.A good correlation between ohmic transfer length and the micro-pattern size is revealed,though in-depth investigation is needed.A preliminary CMOS-process-compatible metal-insulator-semiconductor high-mobility transistor(MIS-HEMT)was fabricated with the proposed Au-free ohmic contact technique.

面向下一代GaN功率技术的超薄势垒AlGaN/GaN异质结功率器件

AlGaN/GaN异质结型功率电子器件具有高工作温度、高击穿电压、高电子迁移率等优点,在推动下一代功率器件小型化、智能化等方面具有很大的材料和系统优势。从5种实现增强型GaN基功率电子器件的方法入手,重点介绍了采用超薄势垒AlGaN(小于6 nm)/GaN异质结实现无需刻蚀AlGaN势垒层的GaN基增强型器件的物理机理和实现方法。同时介绍了在超薄势垒AlGaN/GaN异质结构上实现增强型/耗尽型绝缘栅高电子迁移率晶体管单片集成的研究进展,进一步论证了在大尺寸Si基AlGaN/GaN超薄势垒平台上同片集成射频功率放大器、整流二极管、功率三极管等器件的可行性,为Si基GaN射频器件、功率器件、驱动和控制电路的单片集成奠定了技术基础。

Ka波段InAlN/GaN/AlGaN双异质结场效应晶体管

InAlN/GaN/AlGaN双异质结材料可以兼顾较高的二维电子气浓度和较好的载流子限域性,适宜制备Ka波段及以上微波功率器件。使用金属有机物气相外延方法,在SiC衬底上生长高性能InAlN/GaN/AlGaN双异质结构材料并制备了栅长为0.2μm的异质结场效应晶体管。测试结果表明,在栅压+2 V时器件的最大电流密度为1.12 A/mm,直流偏置条件VDS为+15 V和VGS为-1.6 V时,最高输出功率密度为2.1 W/mm,29 GHz下实现功率附加效率(ηPAE)为22.3%,截止频率fT达到60 GHz,最高振荡频率fmax为105 GHz。就功率密度和功率附加效率而言,是目前报道的Ka波段InAlN/GaN/AlGaN双异质结场效应晶体管研究的较好结果。

AlGaN/GaN纳米异质结构中的二维电子气密度研究

本文设计了纳米线核壳AlGaN/GaN异质结构,研究了势垒层厚度、Al组分、掺杂浓度对平面和纳米线异质结构中二维电子气(2DEG)浓度的影响规律。结果表明,随着势垒层厚度的逐渐增大,两种结构中2DEG浓度增速逐渐减缓,当达到40 nm后,由于表面态电子完全发射,2DEG浓度逐渐稳定不变。随着Al组分的增加,极化效应逐渐增强,使得两种结构在异质界面处的2DEG浓度都逐渐增加。当掺杂浓度逐渐提高时,两者在异质界面处电势差增大,势阱加深,束缚电子能力加强,最终导致2DEG浓度逐渐增加,当掺杂浓度增加到2.0×10^(18)cm^(-3)后,2DEG面密度达到最大值。与平面结构相比,纳米线结构可以实现更高的Al组分,在高Al组分之下,2DEG面密度最高可达5.13×10^(13)cm^(-2),相比于平面结构有较大的提高。

AlGaN插入层对InAlN/AlGaN/GaN异质结散射机制的影响

InAlN/AlN/GaN异质结中,名义上的AlN插入层实为Ga含量很高的AlGaN层, Al, Ga摩尔百分比决定了电子波函数与隧穿几率,因此影响与InAlN/AlGaN势垒层有关的散射机制.本文通过求解薛定谔-泊松方程与输运方程,研究了AlGaN层Al摩尔百分含量对InAlN组分不均匀导致的子带能级波动散射、导带波动散射以及合金无序散射三种散射机制的影响.结果显示:当Al含量由0增大到1,子带能级波动散射强度与合金无序散射强度先增大后减小,导带波动散射强度单调减小;在Al含量为0.1附近的小组分范围内,合金无序散射是限制迁移率的主要散射机制,该组分范围之外,子带能级波动散射是限制迁移率的主要散射机制;当Al摩尔百分含量超过0.52,三种散射机制共同限制的迁移率超过无插入层结构的迁移率, AlGaN层显示出对迁移率的提升作用.

Comparison of GaN-Based Light-Emitting Diodes by Using the AlGaN Electron-Blocking Layer and InAlN Electron-Blocking Layer

Optical properties of GaN-based light-emitting diodes(LEDs)are studied numerically by using AlGaN and InAlN electron-blocking layers(EBLs).Through the simulations of emission spectra,carrier concentration distribution,energy band,electrostatic field,internal quantum efficiency and output power,the results show that the LEDs with design of the InAlN EBL structure have a better performance over the original LEDs using an AlGaN EBL.The spectrum intensity and output power are enhanced significantly,and the efficiency droop of internal quantum efficiency is improved effectively with this design of InAlN EBL structure.It is proved that the strengths of carrier confinement and electron leakage current play a critical role in the performance of luminescence in LEDs.

基于FC技术的AlGaN/GaN HEMT

采用FC技术将管芯倒扣至AlN基板散热的AlGaN/GaN HEMTs,并通过热阻模型分析了FC方式的散热机理.从测试结果看,器件的热阻可大幅降到14 .9K·mm/W,直流特性明显增加,饱和电流提高33%.表明采用FC技术有效改善了器件散热,而且引入的寄生电感较小,可获得更大输出功率.如果进一步完善频率特性的优化,可以加快FC技术的AlGaN/GaN大功率HEMT器件的实用化进程.

高击穿电压AlGaN/GaN HEMT电力开关器件研究进展

作为第三代宽禁带半导体材料的典型代表,GaN材料在各个应用领域的研究工作都受到了高度的重视。概述了基于AlGaN/GaN HEMT结构的新型高压、高频、低损耗电力开关器件的最新研究进展。从器件的结构特征入手,详细介绍了改善器件击穿特性的途径、高频开关特性的研究情况、Si衬底上AlGaN/GaN HEMT结构材料的生长、增强型器件的制备技术和功率集成电路的研究等几个国际上的热点问题。最后,对该项研究面临的问题及未来的发展趋势做了展望。

AlGaN/GaN异质结辐射感生界面态电荷对二维电子气输运的影响

根据荷电中心与自由载流子间的库仑散射作用 ,给出了异质结辐射感生界面态电荷对二维电子气 (2DEG)迁移率的散射模型 .计算了在不同沟道电子面密度下 ,界面态电荷密度与其所限制的迁移率之间的关系 .运用马德森定则分析了辐射感生界面态电荷散射对总迁移率的影响 .分析表明 ,辐射感生界面态电荷在累积到一定量后 ,会显著影响迁移率 ,一定程度上提高

高性能1mm AlGaN/GaN功率HEMTs研制

报道了基于蓝宝石衬底的高性能 1mmAlGaN/GaNHEMTs功率器件 .为了提高微波功率器件性能 ,采用新的欧姆接触和新型空气桥方案 .测试表明 ,器件电流密度为 0 784A/mm ,跨导 197mS/mm ,击穿电压大于 4 0V ,截止态漏电较小 ,1mm栅宽器件的单位截止频率达到 2 0GHz,最大振荡频率为 2 8GHz ,功率增益为 11dB ,功率密度为 1 2W/mm ,PAE为 32 % 。