蓝宝石衬底上AlGaN/GaN HEMT自热效应研究

建立了包含“自热效应”的A lG aN/G aN HEM T(高电子迁移率晶体管)直流I-V特性解析模型。从理论的角度分析了自热效应对A lG aN/G aN HEM T器件的影响,并同已有的实验结果进行了对比,符合较好。证明基于这种模型的理论分析适于A lG aN/G aN HEM T器件测试及应用的实际情况。     

含有Al组分阶变AlGaN过渡层的Si基AlGaN/GaN HEMT

采用一个AlN缓冲层和两个Al组分阶变的AlGaN过渡层作为中间层,在76.2mm Si衬底上外延生长出1.7μm厚无裂纹AlGaN/GaN异质结材料,利用原子力显微镜、X射线衍射、Hall效应测量和CV测量等手段对材料的结构特性和电学性能进行了表征。材料表面平整光滑,晶体质量和电学性能良好,2DEG面密度为1.12×1013cm-2,迁移率为1 208cm2/(V.s)。由该材料研制的栅长为1μm的AlGaN/GaN HEMT器件,电流增益截止频率fT达到10.4GHz,这些结果表明组分阶变AlGaN过渡层技术可用于实现高性能Si基GaN HEMT。     

Investigation on thermo-mechanical responses in high power multi-finger AlGaN/GaN HEMTs

Both transient temperature and thermal stress responses in high power multi-finger AlGaN/GaN high electron mobility transistors (HEMTs), caused by their self-heating effects, are characterized in the present study. Instead of using commercial software, self-developed algorithm based on hybrid time-domain finite element method (TD-FEM) is applied for thermo-mechanical co-simulation of such 3-D structure. The temperature-dependent properties of most constitutive parameters of all materials involved, in particular for electrical conductivities, thermal conductivities, thermal expansion coefficients, and Young’s modulus, are described by several sets of nonlinear polynomial expressions. The algorithm accuracy is validated in detail, with good agreement achieved in comparison with the commercial software COMSOL. It is believed that this study will be useful for effectively evaluating the reliability and lifetime of AlGaN/GaN HEMTs and their monolithic microwave integrated circuits (MMIC) used in high power communication systems and radars.     

一种考虑自热效应的AlGaN/GaN HEMT大信号模型

当AlGaN/GaN HEMT输出高功率密度时,器件沟道温度的升高将引起电流的下降(自热效应).提出了一种针对AlGaN/GaN HEMT改进的大信号等效电路模型,考虑了HEMT自热效应,建立了一种改进的大信号I-V特性模型,仿真结果与测试结果符合较好,提高了大信号模型的精度.     

Thermal stability investigations of AlGaN/GaN HEMTs with various high work function gate metal designs

The operation of high power RF transistor generates a huge amount of heat and thermal effect is a major consideration for improving the efficiency of power transistors. AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have been studied extensively because of their high thermal conductivity. This study comprehensively investigates the DC, low frequency noise, microwave and RF power performance of Al_0.27Ga_0.73N/GaN HEMTs on silicon substrates at temperatures from room temperature to 100 °C using high work function metals such as palladium (Pd) and iridium (Ir) gate metals. Although the conventional Ni gate exhibited a good metal work function with AlGaN, which is beneficial for increasing the Schottky barrier height of HEMTs, the diffusion of Ni metal toward the AlGaN and GaN layers influences the DC and RF stability of the device at high temperatures or over long-term operation. Pd and Ir exhibited less diffusion at high temperature than Ni, resulting in less degradation of device characteristics after high temperature operation.     

Post-process thermal treatment for microwave power improvement of AlGaN/GaN HEMTs

The effects of post-process rapid thermal annealing (RTA) treatment after device fabrication on direct current, microwave and power performances of AlGaN/GaN high electron mobility transistors (HEMTs) with a gate-length of 0.2 μm were fully investigated. By 3 min post-process RTA treatment at 350 °C under N₂ atmosphere, the direct current (DC), radio frequency (RF) small signal and power performances of AlGaN/GaN HEMTs have been much improved. The output power, power gain and power added efficiency (PAE) of GaN HEMT device with gate wide of 1 mm increase from 37.09 dBm, 6.09 dB and 42.79% to 38.22 dBm, 7.22 dB and 67.3%. The post-process RTA after device fabrication has two merits. On the one hand, it improves passivation effect of SiN_x dielectric layer on AlGaN/GaN HEMT surface, suppressing RF current dispersion. On the other hand, it helps recover dry-etch damage at the Schottky metal/AlGaN interface, leading to reduction of reverse Schottky leakage current.     

Electron transport in passivated AlGaN/GaN/Si HEMTs

AlGaN/GaN/Si high electron mobility transistors (HEMTs) grown by molecular beam epitaxy are investigated using direct-current and radio-frequency measurements. As has been found, the maximum of drain current achieves 881 mA/mm with an extrinsic current gain cutoff frequency of 37 GHz for a 0.25 µm gate length. Pulsed characteristics also showed a reduction of trapping centers that improves the quality of the epilayers.     

Characteristics of AlGaN/GaN/AlGaN double heteroj unction HEMTs with an improved breakdown voltage

正We studied the performance of AlGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with an AlGaN buffer layer,which leads to a higher potential barrier at the backside of the twodimensional electron gas channel and better carrier confinement.This,remarkably,reduces the drain leakage current and improves the device breakdown voltage.The breakdown voltage of AlGaN/GaN double heterojunction HEMTs (~ 100 V) was significantly improved compared to that of conventional AlGaN/GaN HEMTs(~50 V) for the device with gate dimensions of 0.5 x 100μm and a gate-drain distance of 1μm.The DH-HEMTs also demonstrated a maximum output power of 7.78 W/mm,a maximum power-added efficiency of 62.3%and a linear gain of 23 dB at the drain supply voltage of 35 V at 4 GHz.     

The Effect of an Fe-doped GaN Buffer on off-State Breakdown Characteristics in AlGaN/GaN HEMTs on Si Substrate

An Fe-doped GaN buffer layer was employed in the growth of AlGaN/GaN high-electron mobility transistors (HEMTs) on Si substrates. In order to investigate the effects of an Fe-doped GaN buffer on OFF-state breakdown characteristics, HEMT devices with an Fe-doped GaN buffer on Si substrates were fabricated along with conventional devices utilizing an unintentionally doped GaN buffer on Si substrates. The device characteristics were compared. While HEMT devices with the conventional structure showed an extremely unstable OFF-state breakdown behavior due to punchthrough to the Si substrate, it was demonstrated that an Fe-doped GaN buffer layer on a Si substrate successfully suppressed the premature failure caused by Si-induced breakdown. As a result, the AlGaN/GaN HEMTs with an Fe-doped GaN buffer on Si substrates exhibited much more consistent and enhanced breakdown voltages, when compared with the conventional devices. Consequently, it is highly desirable that AlGaN/GaN HEMTs on Si substrates have an Fe-doped GaN buffer layer in order to achieve stable and robust OFF-state breakdown characteristics     

InGaN背势垒AlGaN/GaN HEMT中的电流崩塌研究

Current collapses were studied,which were observed in AlGaN/GaN high electron mobility transistors(HEMTs) with and without InGaN back barrier(BB) as a result of short-term bias stress.More serious drain current collapses were observed in InGaN BB AlGaN/GaN HEMTs compared with the traditional HEMTs.The results indicate that the defects and surface states induced by the InGaN BB layer may enhance the current collapse.The surface states may be the primary mechanism of the origination of current collapse in AlGaN/GaN HEMTs for short-term direct current stress.     

La2O3 gate dielectrics for AlGaN/GaN HEMT

The annealing temperature dependent electrical characteristics of La₂O₃ gate dielectrics for W gated AlGaN/GaN high electron mobility transistors (HEMTs) have been characterized. The threshold voltage (V_th) has been found to shift to positive direction with higher temperature annealing, exceeding those of Schottky HEMTs, presumably attributed to the presence of negative fixed charges at the interface between La₂O₃ and AlGaN layers. At a high temperature annealing over 500 °C, a high dielectric constant (k-value) of 27 has been achieved with poly-crystallization of the La₂O₃ film, which is useful to limit the reduction in gate capacitance. A high k-value for La₂O₃ gate dielectrics and the presence of negative charges at the interface are attractive for AlGaN/GaN HEMTs with low gate leakage and normally-off operation.     

AlGaN/GaN高电子迁移率晶体管小信号等效电路的参数提取

本文在高电子迁移率晶体管(HEMT)小信号等效电路模型的基础上,考虑了AlGaN/GaN HEMT的结构特性,具体分析了寄生参数和本征参数的提取方法.采用这些方法,实际测量了5～10 GHz频率下HEMT器件的小信号S参数并提取了它的电学参数,S参数的计算值与实际测量值进行了比较.实验结果表明此方法简单易行,较为精确.     

多栅指AlGaN/GaN HEMT器件自热效应研究

Self-heating in multi-finger AlGaN/GaN high-electron-mobility transistors(HEMTs) is investigated by measurements and modeling of device junction temperature under steady-state operation.Measurements are carried out using micro-Raman scattering to obtain the detailed and accurate temperature distribution of the device.The device peak temperature corresponds to the high field region at the drain side of gate edge.The channel temperature of the device is modeled using a combined electro-thermal model considering 2DEG transport characteristics and the Joule heating power distribution.The results reveal excellent correlation to the micro-Raman measurements, validating our model for the design of better cooled structures.Furthermore,the influence of layout design on the channel temperature of multi-finger AlGaN/GaN HEMTs is studied using the proposed electro-thermal model, allowing for device optimization.     

Improved temperature model of AlGaN/GaN HEMT and device characteristics at variant temperature

In this paper, an improved temperature model for AlGaN/GaN high electron mobility transistor (HEMT) is presented. Research is being conducted for a high-performance building block for high frequency applications that combine lower costs with improved performance and manufacturability. The effects of channel conductance in the saturation region and the parasitic resistance due to the undoped GaN buffer layer have been included. The effect of both spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN heterointerface has been incorporated. The proposed model is used to determine the transfer characteristics, output current-voltage characteristics and small-signal microwave parameters of HEMTs. The investigated temperature range is from 100–600 K. The small signal microwave parameters have been evaluated to determine the unity current gain cut-off frequency (f _T ). High f _T (10–70 GHz) values and high current levels (～550 mA/mm) are achieved for a 1 μm AlGaN/GaN HEMTs. A custom DC measurement system is used to facilitate the DC characterization of the unpackaged GaN HEMT test device. The calculated critical parameters and the simulation results suggest that the performance of the proposed device degrades at elevated temperatures.     

AlGaN/GaN HEMTs on SiC for high power broadband applications up to 40 GHz

An overview of properties and recent achievements for AlGaN/GaN high electron mobility transistors (HEMT) on semi-insulating SiC substrate is given towards high power and broadband applications up to a frequency of 40 GHz. Starting from epitaxial growth and process technology we present state-of-the-art power results obtained at the Fraunhofer Institute (IAF) from AlGaN/GaN HEMTs on SiC. Further, a one-stage 16 GHz MMIC power amplifier circuit with 1.6 W output power is presented. This result represents the first AlGaN/GaN MMIC on SiC fabricated in Europe.     

Investigation of Self-Heating Effects in Submicrometer GaN/AlGaN HEMTs Using an Electrothermal Monte Carlo Method

An electrothermal Monte Carlo (MC) method is applied in this paper to investigate electron transport in submicrometer wurtzite GaN/AlGaN high-electron mobility transistors (HEMTs) grown on various substrate materials including SiC, Si, GaN, and sapphire. The simulation method is an iterative technique that alternately runs an MC electronic simulation and solves the heat diffusion equation using an analytical thermal resistance matrix method. Results demonstrate how the extent of the thermal droop in the I_d-V_ds characteristics and the device peak temperature depend upon both the biasing conditions and the substrate material type. Polarization effects are considered in the simulations, as they greatly influence electron transport in GaN/AlGaN HEMTs by creating a highly concentrated two-dimensional electron gas (2DEG) at the GaN/AlGaN interface. It is shown that a higher 2DEG density provides the devices with a better current handling capability but also increases the importance of the thermal effects     

Stability of submicron AlGaN/GaN HEMT devices irradiated by gamma rays

AlGaN/GaN high electron mobility transistors (HEMTs) with 0.75 μm gate-length and incorporated C-doped GaN buffer layers have been exposed to gamma radiation. The devices have been irradiated to cumulative doses up to 10⁷ rad. The effect of gamma irradiation on the direct current (DC) and low-frequency noise properties of these devices have been investigated in reference to the unexposed device. The DC and noise characteristics show deteriorating device performance upon the gamma exposure. However, some DC parameters, such as transconductance, tended to recover after the irradiation. The gate leakage current and low-frequency noise power spectra indicate this trend even couple of months after the irradiation.     

Effect of a trace of water vapor on Ohmic contact formation for AlGaN/GaN epitaxial wafers

The authors observed that a trace of water vapour can have a significant degradation effect on Ohmic contact formation for AlGaN/GaN high electron mobility transistors (HEMTs). The degradation effect due to a trace of water vapour is less serious for n-GaN. This is a more serious problem for AlGaN/GaN HEMT than AlGaAs/GaAs HEMT or InP based HEMT because of the higher temperature needed for Ohmic contact annealing when AlGaN or GaN are involved. Using cooling water with a temperature slightly higher than the ambient temperature during rapid thermal annealing (RTA) appears to be the best approach.     

Seamless On-Wafer Integration of Si(100) MOSFETs and GaN HEMTs

The first on-wafer integration of Si(100) MOSFETs and AlGaN/GaN high electron mobility transistors (HEMTs) is demonstrated. To enable a fully Si-compatible process, we fabricated a novel Si(100)-GaN-Si(100) virtual substrate through a wafer bonding and etch-back technique. The high thermal stability of nitride semiconductors allowed the fabrication of Si MOSFETs on this substrate without degrading the performance of the GaN epilayers. After the Si devices were fabricated, the nitride epilayer is exposed, and the nitride transistors are processed. By using this technology, GaN and Si devices separated by less than 5 mum from each other have been fabricated, which is suitable for building future heterogeneous integrated circuits.     

MOCVD-Grown AlGaN Buffer GaN HEMTs With V-Gates for Microwave Power Applications

We report the performance of AlGaN buffer GaN high-electron mobility transistors (HEMTs) grown by metal–organic chemical vapor deposition. GaN HEMTs on high-quality AlGaN buffer were grown on SiC substrates. The incorporation of an AlGaN buffer into the GaN HEMT significantly improves channel confinement and suppresses the short-channel effect. Advanced deep-recess V-gate structures were employed to optimize the device for better microwave power performance. With a 10-nm GaN channel layer sandwiched between the AlGaN barrier and buffer, excellent power performance was achieved. The output power density is 13.1 W/mm, and the associated power-added efficiency is 72% at 4-GHz frequency and 48-V drain bias. This power performance is comparable to the state-of-the-art GaN HEMTs grown on GaN buffers, indicating that the AlGaN buffer in our optimized device structure does not introduce any noticeable trapping.