A study on current collapse in AlGaN/GaN HEMTs induced by bias stress

Drain current collapse in AlGaN/GaN HEMTs has been studied systematically by applying bias stress to the device. The collapse was suppressed by light illumination with energy smaller than the bandgap. The position dependence of the light illumination and the measurement of series source and drain resistances revealed that the collapse was caused by the surface states between the gate and drain electrodes, which captured electrons injected from the gate. The current collapse was eliminated by the passivation of the device surface with Si/sub 3/N/sub 4/ film.     

11.2 W/mm power density AlGaN/GaN high electron-mobility transistors on a GaN substrate

In this letter, high power density AlGaN/GaN high electron-mobility transistors (HEMTs) on a freestanding GaN substrate are reported. An asymmetric Γ-shaped 500-nm gate with a field plate of 650 nm is introduced to improve microwave power performance. The breakdown voltage (BV) is increased to more than 200 V for the fabricated device with gate-to-source and gate-to-drain distances of 1.08 and 2.92 μm. A record continuous-wave power density of 11.2 W/mm@10 GHz is realized with a drain bias of 70 V. The maximum oscillation frequency (f_max) and unity current gain cut-off frequency (f_t) of the AlGaN/GaN HEMTs exceed 30 and 20 GHz, respectively. The results demonstrate the potential of AlGaN/GaN HEMTs on free-standing GaN substrates for microwave power applications.     

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.      

钝化前表面预处理对AlGaN/GaN HEMTs性能的影响

提出一种新的钝化技术--采用盐酸和氢氟酸混合预处理溶液(HF:HCI:H2O=1:4:20)对AIGaN/GaNHEMTs进行表面预处理后冉淀积Si3N4钝化,研究了新型钝化技术对AlGaN/GaN HEMTs性能的影响并分析其机理.与用常规方法钝化的器件相比,经过表面顶处理再钝化,成功地抑制了 AIGaN/GaN HEMTs肖特基特性的恶化,有效地增强抑制电流崩塌效应的能力,将GaN基HEMTs的输出功率密度提高到5.2W/mm,并展现良好的电学可靠性.通过X射线光电子谱(XPS)检测预处理前后的AIGaN表面,观察到经过预处理后的AIGaN表面氧元素的含量大幅度下降.表面氧元素的含量下降,能有效地降低表面态密度和表面电荷陷阱密度,被认为是提高AIGaN/GaN HEMTs性能的主要原因.     

Trapping effects and microwave power performance in AlGaN/GaN HEMTs

The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 μm gate length, an f_T of 30 GHz and an f_max of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 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     

AlGaN/GaN高电子迁移率晶体管大信号I-V特性模型

对非线性电流源Ids(Vgs,Vds)的准确描述是Al GaN/GaN HEMT大信号模型的最重要部分之一.Materka模型考虑了夹断电压与Vds的关系,其模型参数只有三个,但是Ids与Vgs的平方关系不符合实际,计算结果与测量数据有误差.我们在考虑了栅电压与漏电流的关系及不同栅压区漏电流随漏电压斜率改变的基础上,提出了改进的高电子迁移率晶体管(HEMT)的直流特性模型.采用这个模型,计算了Al GaN/GaN HEMT器件的大信号I-V特性,并与实际测量数据进行了比较.实验结果表明改进的模型更精确,Ids与Vgs的呈2.5次方的指数关系.     

Effect of the surface and barrier defects on the AlGaN/GaN HEMT low-frequency noise performance

We report on the effect of Si/sub 3/N/sub 4/ passivation of the surface of AlGaN/GaN transistors on low-frequency noise performance. Low-frequency noise measurements were performed on the device before and after the passivation by a Si/sub 3/N/sub 4/ film. A lower level of the low-frequency noise was observed from the device after the passivation. The passivation layer improved high-frequency, large-signal device performance, but introduced parasitic leakage current from the gate. A lower level of flicker noise is explained by the fact that noise is mostly originated from the fluctuation of sheet charge and mobility in the ungated region of the device due to the defects on the surface and in the barrier of the unpassivated device. Passivation eliminates part of the defects and higher leakage current increases the number of electrons on the surface and in the vicinity of the barrier defects, lowering the contribution to the low-frequency noise according to Hooge's law.     

Low damage, Cl/sub 2/-based gate recess etching for 0.3-/spl mu/m gate-length AlGaN/GaN HEMT fabrication

The traditional dry etching for GaN using the Ar/Cl/sub 2/ mixture gas in the reactive ion etching system has been developed. In order to reduce the surface damage, the additional CH/sub 4/ gas is introduced. However, this approach still has the problems of the residual surface damage and low etching selectivity between the AlGaN and GaN materials. Therefore, the following rapid thermal annealing (RTA) at 700/spl deg/C is necessary to recover the surface properties. In this study, we proposed the Ar/Cl/sub 2//CH/sub 4//O/sub 2/ for the GaN gate-recess etching in AlGaN/GaN HEMTs fabrication, which achieves a low surface damage and a high etching selectivity simultaneously. The 0.3 /spl mu/m gate-length AlGaN/GaN HEMTs present a transconductance of 230 mS/mm, an f/sub T/ of 48 GHz, and f/sub max/ of 60 GHz, respectively.     

SiN钝化前的NF3等离子体处理对AlGaN/GaN HEMT性能的影响

研究了SiN钝化前利用感应耦合等离子体（ICP）对AlGaN/GaN HEMT表面进行NF3等离子体处理对器件性能的影响. 结果表明,运用低能量的NF3等离子体处理钝化前的AlGaN/GaN HEMT表面能有效抑制器件电流崩塌,而器件直流及微波小信号特性则未受影响. 微波功率测试表明,经过6min NF3等离子体处理的AlGaN/GaN HEMT在2GHz, 30V工作电压下达到6.15W/mm的输出功率密度,而未经过处理的器件只达到1.82W/mm的输出功率密度.     

Transient pulsed analysis on GaN HEMTs at cryogenic temperatures

A pulsed measurement of AlGaN/GaN high electron mobility transistors (HEMTs) current-voltage (I-V) output characteristics from 100 to 300 K temperatures has been systematically investigated, and a significant kink is clearly observed, which is more severe at cryogenic temperatures. By comparing the pulsed and dc I-V curves, the kink effect is more significant in the pulsed mode evaluation, which indicates a time constant related mechanism involved in the carrier transport. Moreover, a weak impact ionization by monitoring the gate current in the on-state of device has also been observed, and it is more significant at cryogenic temperatures.     

Pulsed measurements and circuit modeling of weak and strong avalanche effects in GaAs MESFETs and HEMTs

An extensive characterization of the on-state breakdown characteristics of GaAs based MESFETs and HEMTs has been carried out by means of DC and pulsed measurements and of circuit simulations. A computer-controlled, three-terminal Transmission Line Pulse (TLP) system with 50-100 ns pulse width and sub-ns risetime has been developed, which allows automated pulsed measurements of device I-V characteristics. The TLP system has been adopted for nondestructive measurements of the on-state breakdown characteristics of GaAs MESFETs and HEMTs up to unprecedented values of gate current density (I/sub G//W=30 mA/mm has been reached), in strong avalanche conditions. The device behavior in strong avalanche conditions is dominated by a parasitic bipolar effect (PBE) similarly to SOI and bulk Si MOSFETs. By taking into account this and other parasitic effects, an equivalent circuit model, suitable for SPICE simulations has been developed. The proposed model is capable of predicting the exact behavior of the gate and drain currents in both weak and strong avalanche conditions.     

Thermal modeling and measurements of AlGaN/GaN HEMTs including thermal boundary resistance

In this paper an analysis of thermal behavior of microwave power AlGaN/GaN HEMTs has been carried out through pulsed current–voltage (PIV) measurements and small signal [S] parameters. A special care about trapping effects has been followed where it is shown that the thermal resistance of the device can be accurately determined provided that some assumptions on the trapping behavior of the device are verified. The values obtained have been checked by three dimensional finite element (3D-FE) simulations. Finally, the thermal boundary resistance (TBR) between GaN/SiC has been extracted and compared to literature. The results we have obtained are in line with what can be found. In addition to this first set of results, a second 3D-FE analysis of a larger transistor has been performed and checked thanks to liquid crystal measurements.     

Planar integration of E/D-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment

A planar-fabrication technology for integrating enhancement/depletion (E/D)-mode AlGaN/GaN high-electron mobility transistors (HEMTs) has been developed. The technology relies heavily on CF/sub 4/ plasma treatment, which is used in two separate steps to achieve two objectives: 1) active device isolation and 2) threshold-voltage control for the enhancement-mode HEMT formation. Using the planar process, depletion- and enhancement-mode AlGaN/GaN HEMTs are integrated on the same chip, and a direct-coupled FET logic inverter is demonstrated. Compared with the devices fabricated by a standard mesa-etching technique, the HEMTs by a planar process have comparable dc and RF characteristics with no obvious difference in the device isolation. The device isolation by a plasma treatment remains the same after 400 /spl deg/C annealing, indicating a good thermal stability. At a supply voltage (V/sub DD/) of 3.3 V, the E/D-mode inverters show an output swing of 2.85 V, with the logic-low and logic-high noise margins at 0.34 and 1.47 V, respectively.     

一种紧凑的GaN高电子迁移率晶体管大信号模型拓扑

GaN高电子迁移率晶体管（HEMT）以其复杂的器件特性使其大信号建模变得十分困难,尽管EEHEMT、Angelov等模型结构曾经成功应用于GaAs HEMT/MESFET的大信号模型,但当它们被用于GaN HEMT建模时却不再准确和完备.面向GaN HEMT器件的大信号模型,本文提出了一种紧凑的模型拓扑,此模型拓扑综合了GaN HEMT器件的直流电压-电流（I-V）特性、非线性电容、寄生参数、栅延迟漏延迟与电流崩塌、自热效应以及噪声等特性.经验证此模型拓扑在仿真中具有很好的收敛性,适用于GaN HEMT器件的大信号模型的建立,满足GaN基微波电路设计对器件模型的需求.     

Microwave noise performance of AlGaN-GaN HEMTs with small DC power dissipation

We report low microwave noise performance of discrete AlGaN-GaN HEMTs at DC power dissipation comparable to that of GaAs-based low-noise FETs. At 1-V source-drain (SD) bias and DC power dissipation of 97 mW/mm, minimum noise figures (NF/sub min/) of 0.75 dB at 10 GHz and 1.5 dB at 20 GHz were achieved, respectively. A device breakdown voltage of 40 V was observed. Both the low microwave noise performance at small DC power level and high breakdown voltage was obtained with a shorter SD spacing of 1.5 /spl mu/m in 0.15-/spl mu/m gate length GaN HEMTs. By comparison, NF/sub min/ with 2 /spl mu/m SD spacing was 0.2 dB greater at 10 GHz.     

DC and RF Characteristics of AlGaN/GaN/InGaN/GaN Double-Heterojunction HEMTs

We present the detailed dc and radio-frequency characteristics of an Al_0.3Ga_0.7N/GaN/In_0.1Ga_0.9 N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In_0.1Ga_0.9N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In_0.1Ga_0.9N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF₄-based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1times100 mum E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs     

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。     

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.     

9.4-W/mm power density AlGaN-GaN HEMTs on free-standing GaN substrates

High power microwave AlGaN-GaN high electron-mobility transistors (HEMTs) on free-standing GaN substrates are demonstrated for the first time. Measured gate leakage was -2.2 /spl mu/A/mm at -20 V and -10 /spl mu/A/mm at -45 V gate bias. When operated at a drain bias of 50 V, devices showed a record continuous-wave output power density of 9.4 W/mm at 10 GHz with an associated power-added efficiency of 40%. Long-term stability of device RF operation was also examined. Under room conditions, devices driven at 25 V and 3-dB gain compression remained stable in 200 h, degrading only by 0.18 dB in output power. Such results illustrate the potential of GaN substrate technology in supporting reliable, high performance AlGaN-GaN HEMTs for microwave power applications.