增强型AlGaN/GaN高电子迁移率晶体管高温退火研究

深入研究了两种增强型AlGaN/GaN高电子迁移率晶体管(HEMT)高温退火前后的直流特性变化.槽栅增强型AlGaN/GaN HEMT在500 ℃ N₂中退火5 min后,阈值电压由0.12 V正向移动到0.57 V,器件Schottky反向栅漏电流减小一个数量级.F注入增强型AlGaN/GaN HEMT在 400 ℃ N₂中退火2 min后,器件阈值电压由0.23 V负向移动到-0.69 V,栅泄漏电流明显增大.槽栅增强型器件退火过程中Schottky有效势垒     

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.     

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.     

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     

An improved EEHEMT model for kink effect on AlGaN/GaN HEMT

In this paper, a new current expression based on both the direct currect(DC) characteristics of the AlGaN/GaN high election mobility transistor(HEMT) and the hyperbolic tangent function tanh is proposed, by which we can describe the kink effect of the AlGaN/GaN HEMT well. Then, an improved EEHEMT model including the proposed current expression is presented. The simulated and measured results of I–V, S-parameter, and radio frequency(RF) large-signal characteristics are compared for a self-developed on-wafer AlGaN/GaN HEMT with ten gate fingers each being 0.4-μm long and 125-μm wide(Such an AlGaN/GaN HEMT is denoted as AlGaN/GaN HEMT(10 × 125 μm)). The improved large signal model simulates the I–V characteristic much more accurately than the original one, and its transconductance and RF characteristics are also in excellent agreement with the measured data.     

Novel attributes of AlGaN/AlN/GaN/SiC HEMTs with the multiple indented channel

In this paper, a high performance AlGaN/AlN/GaN/SiC High Electron Mobility Transistor (HEMT) with the multiple indented channel (MIC-HEMT) is proposed. The main focus of the proposed structure is based on reduction of the space around the gate, stop of the spread of the depletion region around the source–drain, and decrement of the thickness of the channel between the gate and drain. Therefore, the breakdown voltage increases, meanwhile the elimination of the gate depletion layer extension to source/drain decreases the gate–source and gate–drain capacitances. The optimized results reveal that the breakdown voltage and the drain saturation current increase about 178% and 46% compared with a conventional HEMT (C-HEMT), respectively. Therefore, the maximum output power density is improved by factor 4.1 in comparison with conventional one. Also, the cut-off frequency of 25.2 GHz and the maximum oscillation frequency of 92.1 GHz for the MIC-HEMT are obtained compared to 13 GHz and 43 GHz for that of the C-HEMT and the minimum figure noise decreased consequently of reducing the gate–drain and gate–source capacitances by about 42% and 40%, respectively. The proposed MIC-HEMT shows a maximum stable gain (MSG) exceeding 24.1 dB at 3.1 GHz which the greatest gain is yet reported for HEMTs, showing the potential of this device for high power RF applications.     

Optimization of two-dimensional electron gases and – characteristics for AlGaN/GaN HEMT devices

In this paper, we take account of the spontaneous and piezoelectric polarization effect at the heterointerface in the AlGaN/GaN HEMT device, and one-dimensional Schrödinger–Poisson equations are solved self-consistently using a nonuniform mesh; using our findings, the AlGaN/GaN heterostructure conduction band and the two-dimensional electron gas (2DEG) density are investigated. The dependences of the 2DEG characteristics on the Al fraction, the thickness of each layer, the donor concentration and the gate voltage are investigated through simulation. The output characteristics are simulated using a quasi-2D model; a saturation voltage and threshold voltage are also shown. The influence of the spacer layer width on the 2DEG density is calculated for the first time. An explanation and analyses are given.     

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

A reduced surface electric field in 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 source-connected field plate and double field plate, the HEMT with 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× 10¹⁷ cm^-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.     

Operational improvement of AlGaN/GaN HEMT on SiC substrate with the amended depletion region

In this paper, a high performance AlGaN/GaN High Electron Mobility Transistor (HEMT) on SiC substrates is presented to improve the electrical operation with the amended depletion region using a multiple recessed gate (MRG–HEMT). The basic idea is to change the gate depletion region and a better distribution of the electric field in the channel and improve the device breakdown voltage. The proposed gate consists of lower and upper gate to control the channel thickness. Also, the charge of the depletion region will change due to the optimized gate. In addition, a metal between the gate and drain including the horizontal and vertical parts is used to better control the thickness of the channel. The breakdown voltage, maximum output power density, cut-off frequency, maximum oscillation frequency, minimum noise figure, maximum available gain (MAG), and maximum stable gain (MSG) are some parameters for designers which are considered and are improved in this paper.     

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.     

AlGaN/GaN异质结Ni/Au肖特基表面处理及退火研究

采用O₂等离子体及HF溶液对AlGaN/GaN异质结材料进行表面处理后，Ni/Au肖特基接触特性比未处理有了明显改善，反向泄漏电流减小3个数量级.对制备的肖特基接触进行200—600℃ 5min的N₂气氛退火，发现退火冷却后肖特基反向泄漏电流随退火温度增大进一步减小.N₂气中600℃退火后肖特基二极管C-V特性曲线在不同频率下一致性变好，这表明退火中Ni向材料表面扩散减小了表面陷阱密度；C-V特性曲线随退火温度增大向右移动，从二维电子气耗尽电压绝对值减小反映了肖特基势垒的提高. 关键词： AlGaN/GaN 肖特基接触 表面处理 退火     

Breakdown mechanisms in AlGaN/GaN high electron mobility transistors with different GaN channel thickness values

In this paper,the off-state breakdown characteristics of two different AlGaN/GaN high electron mobility transistors（HEMTs）,featuring a 50-nm and a 150-nm GaN thick channel layer,respectively,are compared.The HEMT with a thick channel exhibits a little larger pinch-off drain current but significantly enhanced off-state breakdown voltage(SV_off).Device simulation indicates that thickening the channel increases the drain-induced barrier lowering（DIBL） but reduces the lateral electric field in the channel and buffer underneath the gate.The increase of BV_off in the thick channel device is due to the reduction of the electric field.These results demonstrate that it is necessary to select an appropriate channel thickness to balance DIBL and BV_off in AlGaN/GaN HEMTs.     

场板结构AlGaN/GaN HEMT的电流崩塌机理

在不同的漏偏压下,研究了钝化和不同场板尺寸AlGaN/GaN HEMT对电流崩塌的抑制能力.实验结果表明,钝化器件对电流崩塌的抑制能力随着漏偏压的升高而显著下降;在高漏偏压下,场板的尺寸对器件抑制崩塌的能力有较大影响,而合适尺寸的场板结构在各个漏偏压下都能够很好的抑制电流崩塌.深入分析发现,场板结构不仅能够抑制虚栅的充电过程,而且提供了放电途径,有利于虚栅的放电,从而抑制电流崩塌.在此基础上,通过建立场板介质对虚栅放电的模型,解释了高漏偏压下场板的尺寸对器件抑制崩塌的能力有较大影响的原因. 关键词： AlGaN/GaN HEMT 场板 电流崩塌     

场板结构AlGaN/GaN HEMT的电流崩塌机理

在不同的漏偏压下，研究了钝化和不同场板尺寸AlGaN/GaN HEMT对电流崩塌的抑制能力.实验结果表明，钝化器件对电流崩塌的抑制能力随着漏偏压的升高而显著下降；在高漏偏压下，场板的尺寸对器件抑制崩塌的能力有较大影响，而合适尺寸的场板结构在各个漏偏压下都能够很好的抑制电流崩塌.深入分析发现，场板结构不仅能够抑制虚栅的充电过程，而且提供了放电途径，有利于虚栅的放电，从而抑制电流崩塌.在此基础上，通过建立场板介质对虚栅放电的模型，解释了高漏偏压下场板的尺寸对器件抑制崩塌的能力有较大影响的原因.     

高击穿电压的AlGaN/GaN FP-HEMT研究与分析

就蓝宝石衬底上制备的AlGaN/GaN场板(field plate)HEMT器件、常规HEMT器件性能进行了分析对比.结果证明两种结构的器件直流参数变化不大，但是采用场板后器件的击穿电压从52V提高到了142V.在此基础上利用Sivaco软件对两种器件进行模拟仿真，深入分析了FP对器件击穿电压的影响.     

AlGaN/GaN槽栅HEMT模拟与实验研究

分析了栅槽深度对AlGaN/GaN HEMT特性的影响,并对不同栅槽深度的器件特性进行了模拟,得到了器件饱和电流、最大跨导和阈值电压随栅槽深度的变化规律.当槽栅深度增大,器件饱和电流逐渐下降,而最大跨导逐渐增大,阈值电压向X轴正方向移动.研制出不同栅槽深度的蓝宝石衬底AlGaN/GaN HEMT,用实验数据验证了得到的不同栅槽深度器件特性变化规律.从刻蚀损伤和刻蚀引入界面态的角度分析了模拟与实验规律产生差别的原因. 关键词： 高电子迁移率晶体管 AlGaN/GaN 槽栅器件     

Investigation of AlGaN/GaN fluorine plasma treatment enhancement-mode high electronic mobility transistors by frequency-dependent capacitance and conductance analysis

This paper reports fluorine plasma treatment enhancement-mode HEMTs (high electronic mobility transistors) EHEMTs and conventional depletion-mode HEMTs DHEMTs fabricated on one wafer using separate litho-photography technology. It finds that fluorine plasma etches the AlGaN at a slow rate by capacitance--voltage measurement. Using capacitance--frequency measurement, it finds one type of trap in conventional DHEMTs with τ_T=(0.5-6) ms and D_T= (1 - 5) × 10¹³ cm^-2·eV^-1. Two types of trap are found in fluorine plasma treatment EHEMTs, fast with τ_T(f)=(0.2-2) μs and slow with τ_T(s)=(0.5-6) ms. The density of trap states evaluated on the EHEMTs is D_T(f)=(1 - 3) × 10¹² cm^-2·eV^-1 and D_T(s)=(2 - 6) × 10¹² cm^-2·eV^-1 for the fast and slow traps, respectively. The result shows that the fluorine plasma treatment reduces the slow trap density by about one order, but introduces a new type of fast trap. The slow trap is suggested to be a surface trap, related to the gate leakage current.     

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 Al_0.3Ga_0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving Schr?dinger'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.     

Analysis of the modulation mechanisms of the electric field and breakdown performance in AlGaN/GaN HEMT with a T-shaped field-plate

A novel Al Ga N/Ga N high electron mobility transistor(HEMT) with a source-connected T-shaped field-plate(ST-FP HEMT) is proposed for the first time in this paper. The source-connected T-shaped field-plate(ST-FP) is composed of a source-connected field-plate(S-FP) and a trench metal. The physical intrinsic mechanisms of the ST-FP to improve the breakdown voltage and the FP efficiency and to modulate the distributions of channel electric field and potential are studied in detail by means of two-dimensional numerical simulations with Silvaco-ATLAS. A comparison to the HEMT and the HEMT with an S-FP(S-FP HEMT) shows that the ST-FP HEMT could achieve a broader and more uniform channel electric field distribution with the help of a trench metal, which could increase the breakdown voltage and the FP efficiency remarkably. In addition, the relationship between the structure of the ST-FP, the channel electric field, the breakdown voltage as well as the FP efficiency in ST-FP HEMT is analyzed. These results could open up a new effective method to fabricate high voltage power devices for the power electronic applications.     

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

AlGaN/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 Pauw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Furthermore, 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.