High linearity AlGaN/GaN HEMT with double-Vth coupling for millimeter-wave applications

We demonstrated an AlGaN/GaN high electron mobility transistor(HEMT)namely double-Vthcoupling HEMT(DVC-HEMT)fabricated by connecting different threshold voltage(Vth)values including the slant recess element and planar element in parallel along the gate width with N;O plasma treatment on the gate region.The comparative studies of DVC-HEMT and Fin-like HEMT fabricated on the same wafer show significantly improved linearity of transconductance(Gm)and radio frequency(RF)output signal characteristics in DVC-HEMT.The fabricated device shows the transconductance plateau larger than 7 V,which yields a flattened fT/fmax-gate bias dependence.At the operating frequency of 30 GHz,the peak power-added efficiency(PAE)of 41%accompanied by the power density(Pout)of 5.3 W/mm.Furthermore,the proposed architecture also features an exceptional linearity performance with 1-d B compression point(P1 d B)of 28 d Bm,whereas that of the Fin-like HEMT is 25.2 d Bm.The device demonstrated in this article has great potential to be a new paradigm for millimeter-wave application where high linearity is essential.     

Removal of GaN film over AlGaN with inductively coupled BCl3/Ar atomic layer etch

Atomic layer etching(ALE)of thin film GaN(0001)is reported in detail using sequential surface modification by BCl₃ adsorption and removal of the modified surface layer by low energy Ar plasma exposure in a reactive ion etching system.The estimated etching rate of GaN is~0.74 nm/cycle.The GaN is removed from the surface of AlGaN after 135 cycles.To study the mechanism of the etching,the detailed characterization and analyses are carried out,including scanning electron microscope(SEM),x-ray photoelectron spectroscopy(XPS),and atomic force microscope(AFM).It is found that in the presence of GaCl_x after surface modification by BCl₃,the GaCl_x disappears after having exposed to low energy Ar plasma,which effectively exhibits the mechanism of atomic layer etch.This technique enables a uniform and reproducible fabrication process for enhancement-mode high electron mobility transistors with a p-GaN gate.     

Effect of gate length on breakdown voltage in AlGaN/GaN high-electron-mobility transistor

The effects of gate length L_G on breakdown voltage VBRare investigated in AlGaN/GaN high-electron-mobility transistors(HEMTs) with L_G= 1 μm~20 μm. With the increase of L_G, VBRis first increased, and then saturated at LG= 3 μm. For the HEMT with L_G= 1 μm, breakdown voltage VBRis 117 V, and it can be enhanced to 148 V for the HEMT with L-_G= 3 μm. The gate length of 3 μm can alleviate the buffer-leakage-induced impact ionization compared with the gate length of 1 μm, and the suppression of the impact ionization is the reason for improving the breakdown voltage.A similar suppression of the impact ionization exists in the HEMTs with LG 3 μm. As a result, there is no obvious difference in breakdown voltage among the HEMTs with LG= 3 μm~20 μm, and their breakdown voltages are in a range of 140 V–156 V.     

Investigation on threshold voltage of p-channel GaN MOSFETs based on p-GaN/AlGaN/GaN heterostructure

The threshold voltage (V_th) of the p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) is investigated via Silvaco-Atlas simulations. The main factors which influence the threshold voltage of p-channel GaN MOSFETs are barrier height Φ_1,p, polarization charge density σ_b, and equivalent unite capacitance C_oc. It is found that the thinner thickness of p-GaN layer and oxide layer will acquire the more negative threshold voltage V_th, and threshold voltage |V_th| increases with the reduction in p-GaN doping concentration and the work-function of gate metal. Meanwhile, the increase in gate dielectric relative permittivity may cause the increase in threshold voltage |V_th|. Additionally, the parameter influencing output current most is the p-GaN doping concentration, and the maximum current density is 9.5 mA/mm with p-type doping concentration of 9.5×10¹⁶ cm^-3 at V_GS = -12 V and V_DS = -10 V.     

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.     

940nm垂直腔面发射激光器的设计及制备

利用PICS3D计算得到InGaAs/GaAsP应变补偿量子阱的增益特性,得到量子阱的各项参数,再通过传输矩阵理论和TFCalc膜系设计软件分别仿真出上下分布式布拉格反射镜的白光反射谱.采用金属有机化合物气相沉积技术外延生长了垂直腔面发射激光器结构,之后通过干法刻蚀、湿法氧化以及金属电极等芯片技术制备得到8μm氧化孔径的VCSEL芯片.最终,测试得到其光电特性实现室温下阈值电流和斜效率分别为0.95 mA和0.96 W/A,在6 mA电流和2 V电压下输出功率达到4.75 mW,并测试了VCSEL的高温特性.     

3-D Confocal microscopy track analysis: a promising tool for determining CR-39 response function

A new method based on the use of the confocal microscope is described in order to evaluate the CR-39 response function for Li-7 ions with an incident energy of 10.77 MeV. This method uses the formulations developed by Fromm et al. and considers two etching velocities: V_B represents the bulk etch rate and remains constant, and V_T the track etch rate, which varies along the particle's path. The confocal microscope seems to bring big improvements for track analysis. The first results of V_T versus the particle range are presented and compared with the curves obtained by the sequential etching method. The obtained V_T are plotted and compared to LET, REL₃₅₀ and the cumulative radial dose.     

Bias-dependent timing jitter of 1-GHz sinusoidally gated InGaAs/InP avalanche photodiode

We characterized the dependence of the timing jitter of an InGaAs/InP single-photon avalanche diode on the excess bias voltage(V_(ex)) when operated in 1-GHz sinusoidally gated mode.The single-photon avalanche diode was cooled to-30 degrees Celsius.When the V_(ex) is too low(0.2 V-0.8 V) or too high(3 V-4.2 V),the timing jitter is increased with the V_(ex),particularly at high V_(ex).While at middle V_(ex)(1 V-2.8 V),the timing jitter is reduced.Measurements of the timing jitter of the same avalanche diode with pulsed gating show that this effect is likely related to the increase of both the amplitude of the V_(ex) and the width of the gate-on time.For the 1-GHz sinusoidally gated detector,the best jitter of 93 ps is achieved with a photon detection efficiency of 21.4%and a dark count rate of ~2.08×10~(-5) per gate at the V_(ex) of 2.8 V.To evaluate the whole performance of the detector,we calculated the noise equivalent power(NEP) and the afterpulse probability(P_(ap)).It is found that both NEP and P_(ap) increase quickly when the V_(ex) is above 2.8 V.At ~2.8-V V_(ex),the NEP and P_(ap) are ~2.06×10~(16)W/Hz~(1/2) and 7.11%,respectively.Therefore,the detector should be operated with V_(ex) of 2.8 V to exploit the fast time response,low NEP and low P_(ap).     

Ferroelectric effect and equivalent polarization charge model of PbZr_(0.2)Ti_(0.8)O_3 on AlGaN/GaN MIS-HEMT

PbZr_(0.2)Ti_(0.8)O_3(PZT) gate insulator with the thickness of 30 nm is grown by pulsed laser deposition(PLD) in AlGa N/Ga N metal–insulator–semiconductor high electron mobility transistors(MIS-HEMTs). The ferroelectric effect of PZT Al Ga N/Ga N MIS-HEMT is demonstrated. The polarization charge in PZT varies with different gate voltages. The equivalent polarization charge model(EPCM) is proposed for calculating the polarization charge and the concentration of two-dimensional electron gas(2 DEG). The threshold voltage(Vth) and output current density(IDS) can also be obtained by the EPCM. The theoretical values are in good agreement with the experimental results and the model can provide a guide for the design of the PZT MIS-HEMT. The polarization charges of PZT can be modulated by different gate-voltage stresses and the Vthhas a regulation range of 4.0 V. The polarization charge changes after the stress of gate voltage for several seconds. When the gate voltage is stable or changes at high frequency, the output characteristics and the current collapse of the device remain stable.     

开态应力下电压和电流对AlGaN/GaN高电子迁移率晶体管的退化作用研究

通过采集等功率的两种不同开态直流应力作用下AlGaN/GaN高电子迁移率晶体管(HEMTs)漏源电流输出特性、源区和漏区大信号寄生电阻、转移特性、阈值电压随应力时间的变化, 并使用光发射显微镜观察器件漏电流情况, 研究了开态应力下电压和电流对AlGaN/GaN高电子迁移率晶体管的退化作用. 结果表明, 低电压大电流应力下器件退化很少, 高电压大电流下器件退化较明显. 高电压是HEMTs退化的主要因素, 栅漏之间高电场引起的逆压电效应对参数的永久性退化起决定性作用. 除此之外, 器件表面损坏部位的显微图像表明低电压大电流下器件失效是由于局部电流密度过高, 出现热斑导致器件损伤引起的.     

Measurement of bulk etch rate of LR115 detector with atomic force microscopy

Equations for calculating track parameters have been proposed, which invariably involve the track etch rate V_t and the bulk etch rate V_b. The present study measured V_b for the LR115 solid-state nuclear track detector using atomic force microscopy (AFM). The detectors were partially masked using rubber cement and then etched in 2.5 N NaOH solution at 60°C for time periods ranging from 5 to 40 min. The rubber cement was then peeled off and cross-sectional images of the LR115 detectors were obtained by AFM. V_b has been found to have different values below and beyond the etching time of about 13.5 min, with the values of 0.0555 and 0.0875 μm min⁻¹, respectively. The increase in V_b with the etching time can be explained by a diffusion-etch model, in which the additional damage of the detector material is due to those etchant ions diffused into the detector over time. Now that V_b has been determined, this can be combined with the track etch rate V_t to calculate track parameters.     

Improved device performance of recessed-gate AlGaN/GaN HEMTs by using in-situ N2O radical treatment

The N₂O radicals in-situ treatment on gate region has been employed to improve device performance of recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs). The samples after gate recess etching were treated by N₂O radicals without physical bombardment. After in-situ treatment (IST) processing, the gate leakage currents decreased by more than one order of magnitude compared to the sample without IST. The fabricated HEMTs with the IST process show a low reverse gate current of 10^-9 A/mm, high on/off current ratio of 10⁸, and high f_T×L_g of 13.44 GHz· μm. A transmission electron microscope (TEM) imaging illustrates an oxide layer with a thickness of 1.8 nm exists at the AlGaN surface. X-ray photoelectron spectroscopy (XPS) measurement shows that the content of the Al-O and Ga-O bonds elevated after IST, indicating that the Al-N and Ga-N bonds on the AlGaN surface were broken and meanwhile the Al-O and Ga-O bonds formed. The oxide formed by a chemical reaction between radicals and the surface of the AlGaN barrier layer is responsible for improved device characteristics.     

基于电感耦合等离子体的InP基半导体材料干法刻蚀的研究

研究了基于电感耦合等离子体(ICP)刻蚀系统的InP基半导体材料的干法刻蚀.采用Cl2/Ar/H:混合刻蚀气体,分别研究了氯气体积分数和ICP功率与刻蚀速率之间的关系,及镍、二氧化硅和二者结合型掩膜版的适用范围.获得有效的刻蚀速率为450～1 200 nm/min,InP对金属镍的选择性刻蚀比值为175～190.掩膜版...     

Enhancement of fMAX of InP-based HEMTs by double-recessed offset gate process

A double-recessed offset gate process technology for InP-based high electron mobility transistors (HEMTs) has been developed in this paper. Single-recessed and double-recessed HEMTs with different gate offsets have been fabricated and characterized. Compared with single-recessed devices, the maximum drain-source current (I_D,max) and maximum extrinsic transconductance (g_m,max) of double-recessed devices decreased due to the increase in series resistances. However, in terms of RF performance, double-recessed HEMTs achieved higher maximum oscillation frequency (f_MAX) by reducing drain output conductance (g_ds) and drain to gate capacitance (C_gd). In addition, further improvement of f_MAX was observed by adjusting the gate offset of double-recessed devices. This can be explained by suppressing the ratio of C_gd to source to gate capacitance (C_gs) by extending drain-side recess length (L_rd). Compared with the single-recessed HEMTs, the f_MAX of double-recessed offset gate HEMTs was increased by about 20%.     

单晶金刚石氢终端场效应晶体管特性

基于微波等离子体化学气相淀积生长的单晶金刚石制作了栅长为2μm的耗尽型氢终端金刚石场效应晶体管,并对器件特性进行了分析.器件的饱和漏电流在栅压为-6 V时达到了96 mA/mm,但是在-6 V时栅泄漏电流过大.在-3.5 V的安全工作栅压下,饱和漏电流达到了77 mA/mm.在器件的饱和区,宽5.9 V的栅电压范围内,跨导随着栅电压的增加而近线性增大到30 mS/mm.通过对器件导通电阻和电容-电压特性的分析,氢终端单晶金刚石的二维空穴气浓度达到了1.99×10~(13)cm~(-2),并且迁移率和载流子浓度均随着栅压向正偏方向的移动而逐渐增大.分析认为,沟道中高密度的载流子、大的栅电容以及迁移率的逐渐增加是引起跨导在很大的栅压范围内近线性增加的原因.     

Migration of weakly bonded oxygen atoms in a-IGZO thin films and the positive shift of threshold voltage in TFTs

Amorphous indium-gallium-zinc oxide (a-IGZO) thin films are prepared by pulsed laser deposition and fabricated into thin-film transistor (TFT) devices. In-situ x-ray photoelectron spectroscopy (XPS) illustrates that weakly bonded oxygen (O) atoms exist in a-IGZO thin films deposited at high O₂ pressures, but these can be eliminated by vacuum annealing. The threshold voltage (V_th) of the a-IGZO TFTs is shifted under positive gate bias, and the V_th shift is positively related to the deposition pressure. A temperature variation experiment in the range of 20 K-300 K demonstrates that an activation energy of 144 meV is required for the V_th shift, which is close to the activation energy required for the migration of weakly bonded O atoms in a-IGZO thin films. Accordingly, the V_th shift is attributed to the acceptor-like states induced by the accumulation of weakly bonded O atoms at the a-IGZO/SiO₂ interface under positive gate bias. These results provide an insight into the mechanism responsible for the V_th shift of the a-IGZO TFTs and help in the production of reliable designs.     

Chemical reactions of chlorine on a vicinal Si(100) surface studied by ESDIAD

The reaction of Cl₂ on a vicinal Si(100) surface has been studied by ESDIAD. This type of surface possesses two types of sites: pairs of dangling bonds on Si---Si terrace dimers, and single dangling bonds on the 2-atom layer high steps. The reactivity of these two sites is compared. For low coverages the step dangling bonds are identified as the preferred Cl bonding site after 673 K-annealing. Upon higher temperature annealing and SiCl₂(g) desorption, the terrace-site Cl species are depleted more rapidly than the step-site Cl species. Extensive isothermal etching under a continuous Cl₂ flux at 800 K is found to produce a disordered surface structure. Heating to 1123 K causes a reordering of the surface.     

Direct simulation Monte Carlo analysis of flows and etch rate in aninductively coupled plasma reactor

The direct simulation Monte Carlo (DSMC) method was employed to predict the etch rate distribution on Si wafer. The etchant is assumed to be Cl. The production rate of Cl due to electron impact was obtained separately by preprocessing an inductively coupled chlorine plasma by use of the particle-in-cell/Monte Carlo method. Under the condition of constant total pressure, the etch rate increases with the mass flow rate of source gas Cl₂. The density of the etch product SiCl₂ rapidly decreases with increasing the flow rate. The density of the etchant hardly depends on the flow rate. The recombination 2Cl→Cl₂ on the inner walls of etching apparatus has a large effect on the etch rate; recombination probability of 0.1 results in 50% reduction of the etch rate. The etch rate distribution becomes more uniform when the reaction probability at the wafer surface is reduced     

国产30/600μm掺镱光纤实现4.1kW激光输出

采用化学气相沉积结合气相/液相复合掺杂方式制备30/600 μm掺镱双包层光纤,石英纤芯中的掺杂组分为Yb₂O₃, Al₂O₃,P₂O₅。基于976 nm发光二极管反向抽运方式,构建全光纤化的主控振荡器功率放大器结构对增益光纤进行测试。实验中,种子源功率为189 W,当泵浦总功率为4747 W时,激光输出功率为4120 W,放大级光光效率为85%,3 dB带宽为1.6 nm。激光器连续工作1 h,激光功率稳定在4100 W,未发生明显的功率衰退现象。     

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.