Normally-off AlGaN/GaN heterojunction field-effect transistors with in-situ AlN gate insulator

AlGaN/GaN heterojunction field-effect transistors (HFETs) with p-GaN cap layer are developed for normally-off operation, in which an in-situ grown AlN layer is utilized as the gate insulator. Compared with the SiN_x gate insulator, the AlN/p-GaN interface presents a more obvious energy band bending and a wider depletion region, which helps to positively shift the threshold voltage. In addition, the relatively large conduction band offset of AlN/p-GaN is beneficial to suppress the gate leakage current and enhance the gate breakdown voltage. Owing to the introduction of AlN layer, normally-off p-GaN capped AlGaN/GaN HFET with a threshold voltage of 4 V and a gate swing of 13 V is realized. Furthermore, the field-effect mobility is approximately 1500 cm²·V^-1·s^-1 in the 2DEG channel, implying a good device performance.     

阶梯AlGaN外延新型Al0.25Ga0.75N/GaNHEMTs器件实验研究

本文报道了作者提出的阶梯AlGaN外延层新型AlGaN/GaN HEMTs结构的实验结果. 实验利用感应耦合等离子体刻蚀(ICP)刻蚀栅边缘的AlGaN外延层, 形成阶梯的AlGaN 外延层结构, 获得浓度分区的沟道2DEG, 使得阶梯AlGaN外延层边缘出现新的电场峰, 有效降低栅边缘的高峰电场, 从而优化了AlGaN/GaN HEMTs器件的表面电场分布. 实验获得了阈值电压-1.5 V的新型AlGaN/GaN HEMTs器件. 经过测试, 同样面积的器件击穿电压从传统结构的67 V提高到新结构的106 V, 提高了58%左右; 脉冲测试下电流崩塌量也比传统结构减少了30%左右, 电流崩塌效应得到了一定的缓解.     

High-frequency enhancement-mode millimeterwave AlGaN/GaN HEMT with an fT/fmax over 100 GHz/200 GHz

Ultra-thin barrier (UTB) 4-nm-AlGaN/GaN normally-off high electron mobility transistors (HEMTs) having a high current gain cut-off frequency (f_T) are demonstrated by the stress-engineered compressive SiN trench technology. The compressive in-situ SiN guarantees the UTB-AlGaN/GaN heterostructure can operate a high electron density of 1.27×10¹³cm^-2, a high uniform sheet resistance of 312.8 Ω /□, but a negative threshold for the short-gate devices fabricated on it. With the lateral stress-engineering by full removing in-situ SiN in the 600-nm SiN trench, the short-gated (70 nm) devices obtain a threshold of 0.2 V, achieving the devices operating at enhancement-mode (E-mode). Meanwhile, the novel device also can operate a large current of 610 mA/mm and a high transconductance of 394 mS/mm for the E-mode devices. Most of all, a high f_T/f_max of 128 GHz/255 GHz is obtained, which is the highest value among the reported E-mode AlGaN/GaN HEMTs. Besides, being together with the 211 GHz/346 GHz of f_T/f_max for the D-mode HEMTs fabricated on the same materials, this design of E/D-mode with the realization of f_max over 200 GHz in this work is the first one that can be used in Q-band mixed-signal application with further optimization. And the minimized processing difference between the E- and D-mode designs the addition of the SiN trench, will promise an enormous competitive advantage in the fabricating costs.     

基于原位等离子体氮化及低压化学气相沉积-Si_3N_4栅介质的高性能AlGaN/GaN MIS-HEMTs器件的研究

通过对低压化学气相沉积(LPCVD)系统进行改造,实现在沉积Si_3N_4薄膜前的原位等离子体氮化处理,氮等离子体可以有效地降低器件界面处的氧含量和悬挂键,从而获得了较低的LPCVD-Si_3N_4/GaN界面态,通过这种技术制作的MIS-HEMTs器件,在扫描栅压范围V_(G-sweep)=(-30 V,+24 V)时,阈值回滞为186 mV,据我们所知为目前高扫描栅压V_(G+)(20 V)下的最好结果.动态测试表明,在400 V关态应力下,器件的导通电阻仅仅上升1.36倍(关态到开态的时间间隔为100μs).     

Ni/Au Schottky gate oxidation and BCB passivation for high-breakdown-voltage AlGaN/GaN HEMT

A new AlGaN/GaN high electron mobility transistor (HEMT) employing Ni/Au Schottky gate oxidation and benzocyclobutene (BCB) passivation is fabricated in order to increase a breakdown voltage and forward drain current. The Ni/Au Schottky gate metal with a thickness of 50/300 nm is oxidized under oxygen ambient at 500 C and the highly resistive NiO is formed at the gate edge. The leakage current of AlGaN/GaN HEMTs is decreased from 4.94 μA to 3.34 nA due to the formation of NiO. The BCB, which has a low dielectric constant, successfully passivates AlGaN/GaN HEMTs by suppressing electron injection into surface states. The BCB passivation layer has a low capacitance, so BCB passivation increases the switching speed of AlGaN/GaN HEMTs compared with silicon nitride passivation, which has a high dielectric constant. The forward drain current of a BCB-passivated device is 199 mA /mm, while that of an unpassivated device is 172 mA /mm due to the increase in two-dimensional electron gas (2DEG) charge.     

High power-added-efficiency AlGaN/GaN HEMTs fabricated by atomic level controlled etching

An atomic-level controlled etching(ACE)technology is invstigated for the fabrication of recessed gate AlGaN/GaN high-electron-mobility transistors(HEMTs)with high power added efficiency.We compare the recessed gate HEMTs with conventional etching(CE)based chlorine,Cl₂-only ACE and BCl³/Cl₂ACE,respectively.The mixed radicals of BCl₃/Cl₂were used as the active reactants in the step of chemical modification.For ensuring precise and controllable etching depth and low etching damage,the kinetic energy of argon ions was accurately controlled.These argon ions were used precisely to remove the chemical modified surface atomic layer.Compared to the HEMTs with CE,the characteristics of devices fabricated by ACE are significantly improved,which benefits from significant reduction of etching damage.For BCl₃/Cl₂ACE recessed HEMTs,the load pull test at 17 GHz shows a high power added efficiency(PAE)of 59.8%with an output power density of 1.6 W/mm at Vd=10 V,and a peak PAE of 44.8%with an output power density of 3.2 W/mm at Vd=20 V in a continuous-wave mode.     

基于GaN同质衬底的高迁移率AlGaN/GaNHEMT材料

研究了表面预处理对GaN同质外延的影响,获得了高电子迁移率AlGaN/GaN异质结材料.通过NH_3/H_2混合气体与H_2交替通入反应室的方法对GaN模板和GaN半绝缘衬底进行高温预处理.研究结果表明,NH_3/H_2能够抑制GaN的分解,避免粗糙表面,但不利于去除表面的杂质,黄光带峰相对强度较高;H_2促进GaN分解,随时间延长GaN分解加剧,导致模板表面粗糙不平,AlGaN/GaN HEMT材料二维电子气迁移率降低.采用NH_3/H_2混合气体与H_2交替气氛模式处理模板或衬底表面,能够清洁表面,去除表面杂质,获得平滑的生长表面和外延材料表面,有利于提高AlGaN/GaN HEMT材料电学性能.在GaN衬底上外延AlGaN/GaN HEMT材料,2DEG迁移率达到2113 cm~2/V·s,电学性能良好.     

A novel Si-rich SiN bilayer passivation with thin-barrier AlGaN/GaN HEMTs for high performance millimeter-wave applications

We demonstrate a novel Si-rich SiN bilayer passivation technology for AlGaN/GaN high electron mobility transistors (HEMTs) with thin-barrier to minimize surface leakage current to enhance the breakdown voltage. The bilayer SiN with 20-nm Si-rich SiN and 100-nm Si$_{3}$N$_{4}$ was deposited by plasma-enhanced chemical vapor deposition (PECVD) after removing 20-nm SiO$_{2}$ pre-deposition layer. Compared to traditional Si$_{3}$N$_{4}$ passivation for thin-barrier AlGaN/GaN HEMTs, Si-rich SiN bilayer passivation can suppress the current collapse ratio from 18.54% to 8.40%. However, Si-rich bilayer passivation leads to a severer surface leakage current, so that it has a low breakdown voltage. The 20-nm SiO$_{2}$ pre-deposition layer can protect the surface of HEMTs in fabrication process and decrease Ga-O bonds, resulting in a lower surface leakage current. In contrast to passivating Si-rich SiN directly, devices with the novel Si-rich SiN bilayer passivation increase the breakdown voltage from 29 V to 85 V. Radio frequency (RF) small-signal characteristics show that HEMTs with the novel bilayer SiN passivation leads to $f_{\rm T}/f_{\rm max}$ of 68 GHz/102 GHz. At 30 GHz and $V_{\rm DS} = 20$ V, devices achieve a maximum $P_{\rm out}$ of 5.2 W/mm and a peak power-added efficiency (PAE) of 42.2%. These results indicate that HEMTs with the novel bilayer SiN passivation can have potential applications in the millimeter-wave range.     

Photoresponse and trap characteristics of transparent AZO-gated AlGaN/GaN HEMT

AZO-gated and Ni/Au-gated AlGaN/GaN HEMTs are fabricated successfully,and an excellent transparency of AZOgated electrode is achieved.After a negative gate bias stress acts on two kinds of the devices,their photoresponse characteristics are investigated by using laser sources with different wavelengths.The effect of photoresponse on AZO-gated electrode device is more obvious than on Ni/Au-gated electrodes device.The electrons are trapped in the AlGaN barrier of AZO-gated HEMT after it has experienced negative gate bias stress,and then the electrons can be excited effectively after it has been illuminated by the light with certain wavelengths.Furthermore,the trap state density D_T and the time constantτ_T of the AZO-gated Schottky contact are extracted by fitting the measured parallel conductance in a frequency range from10 kHz to 10 MHz.The constants of the trap range from about 0.35 μs to 20.35 μs,and the trap state density increased from1.93×l0~(13)eV 1·cm~2 at an energy of 0.33 eV to 3.07×10~(11) eV~1·cm~2 at an energy of 0.40 eV.Moreover,the capacitance and conductance measurements are used to characterize the trapping effects under different illumination conditions in AZO-gated HEMTs.Reduced deep trap states' density is confirmed under the illumination of short wavelength light.     

具有p-GaN岛状埋层耐压结构的横向AlGaN/GaN高电子迁移率晶体管

GaN基高电子迁移率晶体管(HEMT)相对较低的击穿电压严重限制了其大功率应用.为了进一步改善器件的击穿特性,通过在n-GaN外延缓冲层中引入六个等间距p-GaN岛掩埋缓冲层(PIBL)构成p-n结,提出一种基于p-GaN埋层结构的新型高耐压AlGaN/GaN HEMT器件结构.Sentaurus TCAD仿真结果表明,在关态高漏极电压状态下,p-GaN埋层引入的多个反向p-n结不仅能够有效调制PIBL AlGaN/GaN HEMT的表面电场和体电场分布,而且对于缓冲层泄漏电流有一定的抑制作用,这保证了栅漏间距为10μm的PIBL HEMT能够达到超过1700 V的高击穿电压(BV),是常规结构AlGaN/GaN HEMT击穿电压(580 V)的3倍.同时,PIBL结构AlGaN/GaN HEMT的特征导通电阻仅为1.47 m?·cm~2,因此获得了高达1966 MW·cm~(-2)的品质因数(FOM=BV~2/R_(on,sp)).相比于常规的AlGaN/GaN HEMT,基于新型p-GaN埋岛结构的HEMT器件在保持较低特征导通电阻的同时具有更高的击穿电压,这使得该结构在高功率电力电子器件领域具有很好的应用前景.     

阶梯AlGaN外延新型Al_(0.25)Ga_(0.75)N/GaNHEMTs击穿特性分析

为了优化AlGaN/GaN HEMTs器件表面电场,提高击穿电压,本文首次提出了一种新型阶梯AlGaN/GaN HEMTs结构.新结构利用AlGaN/GaN异质结形成的2DEG浓度随外延AlGaN层厚度降低而减小的规律,通过减薄靠近栅边缘外延的AlGaN层,使沟道2DEG浓度分区,形成栅边缘低浓度2DEG区,低的2DEG使阶梯AlGaN交界出现新的电场峰,新电场峰的出现有效降低了栅边缘的高峰电场,优化了AlGaN/GaN HEMTs器件的表面电场分布,使器件击穿电压从传统结构的446 V,提高到新结构的640 V.为了获得与实际测试结果一致的击穿曲线,本文在GaN缓冲层中设定了一定浓度的受主型缺陷,通过仿真分析验证了国际上外延GaN缓冲层时掺入受主型离子的原因,并通过仿真分析获得了与实际测试结果一致的击穿曲线.     

Impact of oxygen in electrical properties and hot-carrier stress-induced degradation of GaN high electron mobility transistors

The role of the oxygen in AlGaN/GaN high electron mobility transistors (HEMTs) before and after semi-on state stress was discussed. Comparing with the electrical characteristics of the devices in vacuum, air, and oxygen atmosphere, it is revealed that the oxygen has significant influence on the electric characteristics and the hot-carrier-stress-induced degradation of the device. Comparing with the situation in vacuum, the gate leakage increased an order of magnitude in oxygen and air atmosphere. Double gate structure was used to separate the barrier leakage and surface leakage of AlGaN/GaN HEMT it is found that surface leakage is the major influencing factor in gate leakage of SiN-passivated devices before and after semi-on state stress. During semi-on state stress in the oxygen atmosphere, the electric-field-driven oxidation process promoted the oxidation of the nitride layer, and the oxidation layer in the SiN/AlGaN interface leads to the decreasing of the surface leakage.     

Geometric structure of N2Oq+ (q = 5, 6) studied by Ne8+ ion-induced Coulomb explosion imaging

We report the study on the complete three-body Coulomb explosion (CE) of N₂O^q+ (q = 5, 6) induced by 56-keV/u Ne⁸⁺ ion collision with N₂O gaseous molecule. Six CE channels for N₂O⁵⁺ and seven for N₂O⁶⁺ are identified by measuring three ionic fragments and the charge-changed projectile in quadruple coincidence. Correspondingly the kinetic energy release (KER) and momentum correlation angle (MCA) distributions of three ionic fragments for each of the CE channels are also deduced. Numerical computation is presented to reconstruct the geometric structure of N₂O^q+ prior to dissociation based on the measured KER and MCA. The N-N and N-O bond lengths and the N-N-O bond angles of N₂O^q+ for each of the channels are determined.     

Effect of heavy ion irradiation on the interface traps of AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) were irradiated with heavy ions at various fluences. After irradiation by 2.1 GeV¹⁸¹ Ta³²⁺ ions, the electrical characteristics of the devices significantly decreased. The threshold voltage shifted positively by approximately 25% and the saturation currents decreased by approximately 14%. Defects were induced in the band gap and the interface between the gate and barrier acted as tunneling sites, which increased the gate current tunneling probability. According to the pulsed output characteristics, the amount of current collapse significantly increased and more surface state traps were introduced after heavy ion irradiation. The time constants of the induced surface traps were mainly less than 10 μs.     

DLTS characterization of silicon nitride passivated AlGaN/GaN heterostructures

Passivating the ungated surface of AlGaN/GaN HEMTs with silicon nitride (SiN) is effective in improving the microwave output power performances of these devices. However, very little information is available about surface states in GaN-based HEMTs after SiN passivation. In this work we investigate AlGaN/GaN HEMTs structures having either metal–semiconductor or metal–SiN–semiconductor gate contacts. In short gate devices conductance DLTS measurements point out a hole-like peak that shows an anomalous behaviour and can be ascribed to surface states in the access regions of the device. In insulated gate HEMTs a band of levels is detected and ascribed to surface states, whose energy ranges from 0.14 to 0.43 eV. Capacitance–voltage measurements allow us to point out the existence of a second band of interface states deeper in energy than the former one. This band is responsible for slow transients observed in the characteristics of the insulated gate FAT-HEMT.     

AlGaN/GaN 高电子迁移率晶体管漏电流退化机理研究

本文首先制备了与AlGaN/GaN高电子迁移率晶体管 (HEMT) 结构与特性等效的AlGaN/GaN异质结肖特基二极管, 采用步进应力测试比较了不同栅压下器件漏电流的变化情况, 然后基于电流-电压和电容-电压测试验证了退化前后漏电流的传输机理, 并使用失效分析技术光发射显微镜 (EMMI) 观测器件表面的光发射, 研究了漏电流的时间依赖退化机理. 实验结果表明: 在栅压高于某临界值后, 器件漏电流随时间开始增加, 同时伴有较大的噪声. 将极化电场引入电流与电场的依赖关系后, 器件退化前后的 log(I_FT/E)与√E 都遵循良好的线性关系, 表明漏电流均由电子Frenkel-Poole (FP) 发射主导. 退化后 log(I_FT/E)与√E 曲线斜率的减小, 以及利用EMMI在栅边缘直接观察到了与缺陷存在对应关系的“热点”, 证明了漏电流退化的机理是: 高电场在AlGaN层中诱发了新的缺陷, 而缺陷密度的增加导致了FP发射电流I_FT的增加. 关键词： AlGaN/GaN 高电子迁移率晶体管 漏电流 退化机理     

An AlGaN/GaN HEMT with a reduced surface electric field and an improved breakdown voltage

A reduced surface electric field in an AlGaN/GaN high electron mobility transistor(HEMT) is investigated by employing a localized Mg-doped layer under the two-dimensional electron gas(2-DEG) channel as an electric field shaping layer.The electric field strength around the gate edge is effectively relieved and the surface electric field is distributed evenly as compared with those of HEMTs with conventional source-connected field plate and double field plate structures with the same device physical dimensions.Compared with the HEMTs with conventional sourceconnected field plates and double field plates,the HEMT with a Mg-doped layer also shows that the breakdown location shifts from the surface of the gate edge to the bulk Mg-doped layer edge.By optimizing both the length of Mg-doped layer,L m,and the doping concentration,a 5.5 times and 3 times the reduction in the peak electric field near the drain side gate edge is observed as compared with those of the HEMTs with source-connected field plate structure and double field plate structure,respectively.In a device with V GS = -5 V,L m = 1.5 μm,a peak Mg doping concentration of 8×10 17cm-3 and a drift region length of 10 μm,the breakdown voltage is observed to increase from 560 V in a conventional device without field plate structure to over 900 V without any area overhead penalty.     

氩、氢混合等离子体处理对GaAs表面性质及发光特性的影响

介绍了一种氩、氢混合等离子体清洗GaAs基片的实验工艺,深入研究了氩、氢等离子体清洗GaAs表面污染物和氧化层,并活化表面性能的基本原理,同时讨论了气体流量、溅射功率和清洗时间等不同溅射参数对等离子体清洗效果的影响。结果表明,在氩气和氢气流量分别为10 cm³/min和30 cm³/min,溅射功率为20 W,清洗时间为15 min的条件下,GaAs样品的光致发光强度提高达139.12%,样品表面的As-O键和Ga-O键基本消失。     

AlGaN表面坑状缺陷及GaN缓冲层位错缺陷对AlGaN/GaN HEMT电流崩塌效应的影响

利用金属有机气相外延(MOVPE)技术生长了具有不同AlGaN表面坑状缺陷和GaN缓冲层位错缺陷密度的AlGaN/GaN 高电子迁移率晶体管(HEMT)样品,并对比研究了两种缺陷对器件栅、漏延迟电流崩塌效应的影响.栅延迟测试表明,AlGaN表面坑状缺陷会引起栅延迟电流崩塌效应和源漏电阻的增加,而且表面坑状缺陷越多,栅延迟电流崩塌程度和源漏电阻的增加越明显.漏延迟测试显示,AlGaN表面坑状缺陷对漏延迟电流崩塌影响不大,而GaN缓冲层位错缺陷主要影响漏延迟电流崩塌.研究结果表明,AlGaN表面坑状缺陷和Ga 关键词： AlGaN/GaN HEMT 电流崩塌 坑状缺陷 位错缺陷