Surface Leakage Currents in SiN and Al_2O_3 Passivated AlGaN/GaN High Electron Mobility Transistors

Surface leakage currents of AlGaN/GaN high electron mobility transistors are investigated by utihzing a circular double-gate structure to eliminate the influence of mesa leakage current.Different mechanisms are found under various passivation conditions.The mechanism of the surface leakage current with Al_2O_3 passivation follows the two-dimensional variable range hopping model,while the mechanism of the surface leakage current with SiN passivation follows the Frenkel-Poole trap assisted emission.Two trap levels are found in the trap-assisted emission.One trap level has a barrier height of 0.22 eV for the high electric Geld,and the other trap level has a barrier height of 0.12 eV for the low electric field.     

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.     

AlGaN/GaN肖特基结参数分析与电流运输机理研究

通过对AlGaN/GaN HEMT器件肖特基栅电流输运机理的研究,在变温下采用I-V法对AlGaN/GaN上的Ni/Au肖特基势垒高度和理想因子进行了计算. 通过对不同电流机理的分立研究,得到了更为准确的势垒高度值_b. 通过分析温度在300—550K之间肖特基反向泄漏电流的特性,得出结论：AlGaN材料的表面漏电不是HEMT器件反向泄漏电流的主要来源. 关键词： AlGaN/GaN异质结 肖特基结 理想因子     

具有部分本征GaN帽层新型AlGaN/GaN高电子迁移率晶体管特性分析

为了优化传统Al GaN/GaN高电子迁移率晶体管(high electron mobility transistors,HEMTs)器件的表面电场,提高击穿电压,本文提出了一种具有部分本征GaN帽层的新型Al GaN/GaN HEMTs器件结构.新型结构通过在Al GaN势垒层顶部、栅电极到漏电极的漂移区之间引入部分本征GaN帽层,由于本征GaN帽层和Al GaN势垒层界面处的极化效应,降低了沟道二维电子气(two dimensional electron gas,2DEG)的浓度,形成了栅边缘低浓度2DEG区域,使得沟道2DEG浓度分区,由均匀分布变为阶梯分布.通过调制沟道2DEG的浓度分布,从而调制了Al GaN/GaN HEMTs器件的表面电场.利用电场调制效应,产生了新的电场峰,且有效降低了栅边缘的高峰电场,Al GaN/GaN HEMTs器件的表面电场分布更加均匀.利用ISE-TCAD软件仿真分析得出:通过设计一定厚度和长度的本征GaN帽层,Al GaN/GaN HEMTs器件的击穿电压从传统结构的427 V提高到新型结构的960 V.由于沟道2DEG浓度减小,沟道电阻增加,使得新型Al GaN/GaN HEMTs器件的最大输出电流减小了9.2%,截止频率几乎保持不变,而最大振荡频率提高了12%.     

中子辐照对AlGaN/GaN高电子迁移率晶体管器件电特性的影响

本文采用能量为1 MeV的中子对SiN钝化的AlGaN/GaN HEMT(高电子迁移率晶体管)器件进行了最高注量为1015cm-2的辐照.实验发现:当注量小于1014cm-2时,器件特性退化很小,其中栅电流有轻微变化(正向栅电流IF增加,反向栅电流IR减小),随着中子注量上升,IR迅速降低.而当注量达到1015cm-2时,在膝点电压附近,器件跨导有所下降.此外,中子辐照后,器件欧姆接触的方块电阻退化很小,而肖特基特性退化却相对明显.通过分析发现辐照在SiN钝化层中引入的感生缺陷引起了膝点电压附近漏电流和反向栅泄漏电流的减小.以上结果也表明,SiN钝化可以有效地抑制中子辐照感生表面态电荷,从而屏蔽了绝大部分的中子辐照影响.这也证明SiN钝化的AlGaN/GaN HEMT器件很适合在太空等需要抗位移损伤的环境中应用.     

Fluorine-plasma treated AlGaN/GaN high electronic mobility transistors under off-state overdrive stress

Influences of off-state overdrive stress on the fluorine-plasma treated AlGaN/GaN high-electronic mobility transistors (HEMTs) are experimentally investigated. It is observed that the reverse leakage current between the gate and source decreases after the off-state stress, whereas the current between the gate and drain increases. By analyzing those changes of the reverse currents based on the Frenkel-Poole model, we realize that the ionization of fluorine ions occurs during the off-state stress. Furthermore, threshold voltage degradation is also observed after the off-state stress, but the degradations of AlGaN/GaN HEMTs treated with different F-plasma RF powers are different. By comparing the differences between those devices, we find that the F-ions incorporated in the GaN buffer layer play an important role in averting degradation. Lastly, suggestions to obtain a more stable fluorine-plasma treated AlGaN/GaN HEMT are put forwarded.     

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.     

Comparison of electrical characteristic between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance–voltage curves,the electrical characteristics of AlN/GaN Schottky diode,such as Schottky barrier height,turn-on voltage,reverse breakdown voltage,ideal factor,and the current-transport mechanism,are analyzed and then compared with those of an AlGaN/GaN diode by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the dislocation-governed tunneling is dominant for both AlN/GaN and AlGaN/GaN Schottky diodes.However,more dislocation defects and a thinner barrier layer for AlN/GaN heterostructure results in a larger tunneling probability,and causes a larger leakage current and lower reverse breakdown voltage,even though the Schottky barrier height of AlN/GaN Schottky diode is calculated to be higher that of an AlGaN/GaN diode.     

The degradation mechanism of an AlGaN/GaN high electron mobility transistor under step-stress

Step-stress experiments are performed in this paper to investigate the degradation mechanism of an AlGaN/GaN high electron mobility transistor(HEMT).It is found that the stress current shows a recoverable decrease during each voltage step and there is a critical voltage beyond which the stress current starts to increase sharply in our experiments.We postulate that defects may be randomly induced within the AlGaN barrier by the high electric field during each voltage step.But once the critical voltage is reached,the trap concentration will increase sharply due to the inverse piezoelectric effect.A leakage path may be introduced by excessive defect,and this may result in the permanent degradation of the AlGaN/GaN HEMT.     

Al2O3绝缘层的AlGaN/GaN MOSHEMT器件研究

在研制AlGaN/GaN HEMT器件的基础上,采用ALD法制备了Al₂O₃ AlGaN/GaN MOSHEMT器件.通过X射线光电子能谱测试表明在AlGaN/GaN异质结材料上成功淀积了Al₂O₃薄膜.根据对HEMT和MOSHEMT器件肖特基电容、器件输出以及转移特性的测试进行分析发现：所制备的Al₂O₃薄膜与AlGaN外延层间界面态密度较小,因而MOSHEMT器件呈现出较 关键词： 2O₃')" href="#">Al₂O₃ ALD GaN MOSHEMT     

In-situ SiN combined with etch-stop barrier structure for high-frequency AlGaN/GaN HEMT

The etch-stop structure including the in-situ SiN and AlGaN/GaN barrier is proposed for high frequency applications.The etch-stop process is realized by placing an in-situ SiN layer on the top of the thin AlGaN barrier.F-based etching can be self-terminated after removing SiN,leaving the AlGaN barrier in the gate region.With this in-situ SiN and thin barrier etch-stop structure,the short channel effect can be suppressed,meanwhile achieving highly precisely controlled and low damage etching process.The device shows a maximum drain current of 1022 mA/mm,a peak transconductance of 459 mS/mm,and a maximum oscillation frequency(fmax)of 248 GHz.     

The degradation and recovery properties of AlGaN/GaN high-electron mobility transistors under direct current reverse step voltage stress

Direct current(DC) reverse step voltage stress is applied on the gate of an AlGaN/GaN high-electron mobility transistor(HEMT).Experiments show that parameters degenerate under stress.Large-signal parasitic source/drain resistance(RS/RD) and gate-source forward I-V characteristics are recoverable after breakdown of the device under test(DUT).Electrons trapped by both the AlGaN barrier trap and the surface state under stress lead to this phenomenon,and surface state recovery is the major reason for the recovery of device parameters.     

Mechanism and control of current transport in GaN and AlGaN Schottky barriers for chemical sensor applications

Strong interests are recently emerging for development of integrated high-performance chemical sensor chips. In this paper, the present status of understanding and controlling the current transport in the GaN and AlGaN Schottky diodes is discussed from the viewpoint of chemical sensor applications. For this purpose, a series of works recently carried out by our group are reviewed in addition to a general discussion. First, current transport in GaN and AlGaN Schottky barriers is discussed, introducing the thin surface barrier (TSB) model to explain the anomalously large leakage currents. Following this, attempts to reduce the leakage currents are presented and discussed. Then, as an example of gas-phase sensors using Schottky barriers, a Pd/AlGaN/GaN Schottky diode hydrogen sensor developed recently by our group is presented with a discussion on the sensing mechanism and related current transport. On the other hand, in liquid-phase sensors, contact is made between liquid and semiconductor which is regarded as a kind of Schottky barrier by electrochemists. As one of such liquid-phase sensors, open-gate AlGaN/GaN heterostructure field effect transistor (HFET) pH sensor developed recently by our group is presented. Finally, a brief summary is given together with some remarks for future research.     

The Transport Mechanisms of Reverse Leakage Current in Ultraviolet Light-Emitting Diodes

The transport mechanisms of the reverse leakage current in the UV light-emitting diodes(380 nm) are investigated by the temperature-dependent current-voltage measurement first.Three possible transport mechanisms,the space-limited-charge conduction,the variable-range hopping and the Poole-Frenkel emission,are proposed to explain the transport process of the reverse leakage current above 295 K,respectively.With the in-depth investigation,the former two transport mechanisms are excluded.It is found that the experimental data agree well with the Poole-Frenkel emission model.Furthermore,the activation energies of the traps that cause the reverse leakage current are extracted,which are 0.05 eV,0.09 eV,and O.lleV,respectively.This indicates that at least three types of trap states are located below the bottom of the conduction band in the depletion region of the UV LEDs.     

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.     

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.     

Analytical modeling and simulation of subthreshold behavior in nanoscale dual material gate AlGaN/GaN HEMT

A two-dimensional (2-D) analytical model for a Dual Material Gate (DMG) AlGaN/GaN High Electron Mobility Transistor (HEMT) has been developed to demonstrate the unique attributes of this device structure in suppressing short channel effects (SCEs). The model accurately predicts the channel potential, electric field variation along the channel, and sub-threshold drain current, taking into account the effect of lengths of the two gate metals, their work functions, barrier layer thicknesses, and applied drain biases. It is seen that the SCEs and hot carrier effects in DMG AlGaN/GaN HEMT are suppressed due to the work function difference of the two metal gates, thereby screening the drain potential variations by the gate near the drain. Besides, a more uniform electric field along the channel leads to improved carrier transport efficiency. The accuracy of the results obtained from our analytical model has been verified using ATLAS device simulations.     

Effects of the strain relaxation of an AlGaN barrier layer induced by various cap layers on the transport properties in AlGaN/GaN heterostructures

The strain relaxation of an AlGaN barrier layer may be influenced by a thin cap layer above, and affects the transport properties of AlGaN/GaN heterostructures. Compared with the slight strain relaxation found in AlGaN barrier layer without cap layer, it is found that a thin cap layer can induce considerable changes of strain state in the AlGaN barrier layer. The degree of relaxation of the AlGaN layer significantly influences the transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. It is observed that electron mobility decreases with the increasing degree of relaxation of the AlGaN barrier, which is believed to be the main cause of the deterioration of crystalline quality and morphology on the AlGaN/GaN interface. On the other hand, both GaN and AlN cap layers lead to a decrease in 2DEG density. The reduction of 2DEG caused by the GaN cap layer may be attributed to the additional negative polarization charges formed at the interface between GaN and AlGaN, while the reduction of the piezoelectric effect in the AlGaN layer results in the decrease of 2DEG density in the case of AlN cap layer.     

AlGaN/GaN MIS-HEMT using NbAlO dielectric layer grown by atomic layer deposition

We present an AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MIS-HEMT) with an NbAlO high-k dielectric deposited by atomic layer deposition (ALD).Surface morphology of samples are observed by atomic force microscopy (AFM),indicating that the ALD NbAlO has an excellent-property surface.Moreover,the sharp transition from depletion to accumulation in capacitance-voltage (C-V)curse of MIS-HEMT demonstrates the high quality bulk and interface properties of NbAlO on AlGaN.The fabricated MIS-HEMT with a gate length of 0.5 μm exhibits a maximum drain current of 960 mA/mm,and the reverse gate leakage current is almost 3 orders of magnitude lower than that of reference HEMT.Based on the improved direct-current operation,the NbAlO can be considered to be a potential gate oxide comparable to other dielectric insulators.     

生物分子膜门电极AlGaN/GaN高电子迁移率晶体管(HEMT)生物传感器研究

设计并制作了结构尺寸为毫米量级的AlGaN/GaN高电子迁移率晶体管(HEMT)生物传感器,采用数值分析的方法分析了器件传感区域长度与宽度比值及待测物调控二维电子气(2DEG)距离与感测信号之间的关系,给出了结构尺寸为毫米量级的AlGaN/GaN HEMT生物传感器的设计依据,以不同浓度的前列腺特异性抗原(PSA)为待测物,对制作的AlGaN/GaN HEMT生物传感器进行了初步测量,测试结果表明,在50 mV的电压下,毫米量级的AlGaN/GaN HEMT生物传感器的对PSA的探测极限低于0.1 pg/ml.实验表明毫米量级的AlGaN/GaN HEMT生物传感器具有灵敏度高,易于集成等优点,具备良好的应用前景.