GaN HEMT reliability

This paper reviews the experimental evidence behind a new failure mechanism recently identified in GaN high-electron mobility transistors subject to electrical stress. Under high voltage, it has been found that electrically active defects are generated in the AlGaN barrier or at its surface in the vicinity of the gate edge. These defects reduce the drain current, increase the parasitic resistance and provide a path for excess gate current. There is mounting evidence for the role of the inverse piezoelectric effect in introducing mechanical stress in the AlGaN barrier layer and eventually producing these defects. The key signature of this mechanism is a sudden and non-reversible increase in the gate leakage current of several orders of magnitude. This degradation mechanism is voltage driven and characterized by a critical voltage below which degradation does not occur. This hypothesis suggests several paths to enhance the electrical reliability of GaN HEMTs which are borne out by experiments.     

Critical Voltage for Electrical Degradation of GaN High-Electron Mobility Transistors

We have found that there is a critical drain-to-gate voltage beyond which GaN high-electron mobility transistors start to degrade in electrical-stress experiments. The critical voltage depends on the detailed voltage biasing of the device during electrical stress. It is higher in the OFF state and high-power state than at V_DS = 0. In addition, as |V_GS| increases, the critical voltage decreases. We have also found that the stress current does not affect the critical voltage although soft degradation at low voltages takes place at high stress currents. All of our findings are consistent with a degradation mechanism based on crystallographic-defect formation due to the inverse piezoelectric effect. Hot-electron-based mechanisms seem to be in contradiction with our experimental results.     

Stress-related hydrogen degradation of 0.1-/spl mu/m InP HEMTs and GaAs PHEMTs

Hydrogen degradation of III-V field-effect transistors (FETs) is a serious reliability concern. Previous work has shown that threshold-voltage shifts induced by H/sub 2/ exposure in 1-/spl mu/m-channel InP high-electron mobility transitors (HEMTs) can be attributed to compressive stress in the gate due to the formation of TiH/sub x/ in Ti/Pt/Au gates. The compressive stress affects the device characteristics through the piezoelectric effect. This paper examined the H/sub 2/ sensitivity of 0.1-/spl mu/m strained-channel InP HEMTs and GaAs pseudomorphic HEMTs. After exposure to H/sub 2/, the threshold voltage V/sub T/ of both types of devices shifted positive. This positive shift in V/sub T/ is predicted by a model for hydrogen-induced piezoelectric effect. In situ V/sub T/ measurements reveal distinct time dependences of the V/sub T/ shifts, which are also consistent with stress-related phenomena.     

High voltage degradation of GaN High Electron Mobility Transistors on silicon substrate

We have electrically stressed GaN High Electron Mobility Transistors on Si substrate at high voltages. We observe a pattern of device degradation that differs markedly from previous reports in GaN-on-SiC HEMTs. Similarly to these devices, the gate leakage current of GaN-on-Si HEMTs increases by several orders of magnitude at a certain critical voltage and this increase is irreversible. However, in contrast with devices on SiC, the critical voltage varies substantially across the wafer, even over short distances, with values as high as 75 V being observed. In addition, for voltages below the critical voltage, we observe a prominent degradation in the drain current and the source and drain resistances, something not observed in devices on SiC. This degradation is almost completely recoverable under UV illumination. We attribute these results to the high mismatch that exists between GaN and Si that leads to a large concentration of electrically active traps and a lower and non-uniform initial strain in the AlGaN barrier. This is evidenced by observed correlations between threshold voltage and maximum drain current in fresh devices and their corresponding critical voltages.     

AlGaN/GaN High Electron Mobility Transistor degradation under on- and off-state stress

AlGaN/GaN High Electron Mobility Transistors (HEMTs) with various gate lengths have been step-stressed under both on- and off-state conditions. On-state, high power stress tests were performed on 0.17 μm gate length HEMTs and a single 5 μm spaced TLM pattern. Significant degradation of the submicron HEMTs as compared to the excellent stability of the TLM patterns under the same stress conditions reveal that the Schottky contact is the source of degradation. Off-state stress showed a linear relationship between the critical degradation voltage and gate length, though two dimensional ATLAS/Blaze simulations show that the maximum electric field is similar for all gate lengths. Additionally, as the drain bias was increased, the critical voltage decreased. However, the cumulative bias between the gate and drain remained constant, further indicating that the electric field is the main mechanism for degradation.     

Reliability investigation of AlGaN/GaN high electron mobility transistors under reverse-bias stress

Impact of reverse-bias stress on the reliability of AlGaN/GaN high electron mobility transistors was investigated in this paper. We found that inverse piezoelectric effect could induce noisy characteristics of stress current, and the “critical voltage” increased with the drain–source bias in the step-stress experiments. Although the degradation of the gate leakage current and drain-to-source leakage current are non-recoverable, the maximum output current can recover almost completely through electron de-trapping procedure after stress. The de-trapping activation energy was estimated to be 0.30 eV by the dynamic conductance technique. The surface morphology of the electrically stressed devices was investigated after removing the gate metallization by chemical etching, and no pits or cracks under the gate contact were observed.     

A model for hydrogen-induced piezoelectric effect in InP HEMTs and GaAs PHEMTs

We have developed a model for the impact of the hydrogen-induced piezoelectric effect on the threshold voltage of InP HEMTs and GaAs PHEMTs. We have used two-dimensional (2-D) finite element simulations to calculate the mechanical stress caused by a Ti-containing metal gate that has expanded due to hydrogen absorption. This has allowed us to map the 2-D piezoelectric charge distribution in the semiconductor heterostructure. We then used a simple electrostatics model to calculate the impact of this piezoelectric polarization charge on the threshold voltage. The model explains experimental observations of hydrogen-induced threshold voltage shifts, both in InP HEMTS and in GaAs PHEMTs. It also suggests ways to mitigate the hydrogen sensitivity of these devices.     

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.     

GaN基微腔传感器悬空隔膜的力电转换的ANSYS研究

为了深入研究氮化镓薄膜的压电效应和机械特性,基于气体直接吸收红外辐射的原理,将基于氮化镓悬空隔膜的充气微腔红外传感器项目作为背景,以氮化镓/铝镓氮薄膜作为敏感单元,在材料力学以及压电效应方面,采用有限元分析软件ANSYS 14.0进行了理论分析和验证,取得了薄膜形状、厚度和面积等尺寸与压电薄膜输出电压以及薄膜灵敏度的逻辑关系数据,验证了薄膜力-电信号转换机制可行性。结果表明,GaN薄膜材料具有良好的压电特性以及线性度,有助于对探测器输出信号进行准确的预测,并进行温度补偿,突出GaN材料在应用中的优势。此研究对设计性能良好、灵敏度高的微腔红外传感器是有帮助的。     

GaN基HEMT器件抗逆压电性能研究

可靠性问题是GaN基HEMT器件走向实用化的关键,逆压电效应导致器件退化是近年来比较引人瞩目的理论之一。对于GaN基HEMT器件,当其承受外加电场时,由于逆压电效应,电场最终转化成弹性势能。当电场足够大,突破势垒层材料所能承受的临界弹性势能,势垒层材料就会发生松弛。根据逆压电效应导致器件退化机理,从弹性势能的角度出发,对低Al组分AlGaN势垒层、InAlN势垒层和AlGaN背势垒等三种结构进行理论分析。分析表明,三种结构均能较大程度地改善GaN基HEMT器件的抗逆压电能力,从而提高器件可靠性。     

Hot electrons induced degradation in lattice-matched InAlN/GaN high electron mobility transistors

In this paper, lattice-matched Pt/Au-In_0.17Al_0.83N/GaN high electron mobility transistors (HEMTs) were fabricated, and the degradation characteristics of the gate leakage current were investigated by drain-to-source voltage (V_DS) step-stress measurements under the ON, semi-ON, and OFF stress conditions and at different temperatures, respectively. It is found that, (1) there exists a critical value of V_DS, beyond which the gate leakage current begins to increase significantly; and (2) the degradation of gate leakage current has a positive temperature coefficient, indicating that high temperature can accelerate the degradation. A hot electron model is used to explain the experimental results, emphasizing that the hot electrons from the channel can induce additional negatively charged defects at the InAlN/GaN interface, which can increase the local electrical field and introduce a thinner surface barrier and finally enhance the vertical leakage current component, leading to the current degradation.     

Impact of gate placement on RF power degradation in GaN high electron mobility transistors

We have investigated the RF power degradation of GaN high electron mobility transistors (HEMTs) with different gate placement in the source–drain gap. We found that devices with a centered gate show different degradation behavior from those with the gate placed closer to the source. In particular, centered gate devices degraded through a mechanism that has a similar signature as that responsible for high-voltage DC degradation in the OFF state and is likely driven by electric field. In contrast, offset gate devices under RF power stress showed a large increase in source resistance, which is not regularly observed in DC stress experiments. High-power pulsed stress tests suggest that the combination of high voltage and high current stress maybe the cause of RF power degradation in these offset-gate devices.     

Impact of strain relaxation of AlmGa1−mN layer on 2-DEG sheet charge density and current voltage characteristics of lattice mismatched AlmGa1−mN/GaN HEMTs

An accurate charge control model to investigate the effect of aluminum composition, strain relaxation, thickness and doping of the Al_mGa_1−mN barrier layer on the piezoelectric and spontaneous polarization induced 2-DEG sheet charge density, threshold voltage and output characteristics of partially relaxed Al_mGa_1−mN/GaN HEMTs is proposed. The strain relaxation of the barrier imposes an upper limit on the maximum 2-DEG density achievable in high Al content structures and is critical in determining the performance of lattice mismatched Al_mGa_1−mN/GaN HEMTs. The model incorporates the effects of field dependent mobility, parasitic source/drain resistance and velocity saturation to evaluate the output characteristics of Al_mGa_1−mN/GaN HEMTs. Close proximity with published results confirms the validity of the proposed model.     

Changes of Electrical Characteristics for AlGaN/GaN HEMTs Under Uniaxial Tensile Strain

This letter investigates the characteristics of unpassivated AlGaN/GaN high-electron mobility transistors (HEMTs) under uniaxial tensile strain. Mechanical stress can produce additional charges that change the HEMT channel current. This phenomenon is dependent upon gate orientation and may be the result of the piezoelectric effect and changes in electron mobility due to the applied uniaxial stress. In addition, results show that tensile strain reduces the transient current, which is likely due to the additional donorlike surface states created through the piezoelectric effect.      

Implications of gate-edge electric field in AlGaN/GaN high electron mobility transistors during OFF-state degradation

Gate degradation in high electron mobility transistors (HEMTs) under OFF-state stress results from the high electric field near the gate edge. We investigate the evolution of this field over time in AlGaN/GaN HEMTs upon OFF-state stress using a combination of electroluminescence (EL) microscopy and spectroscopy. EL analysis suggests that the electric field at the sites of generated surface defects is lowered after the stress, with greater lowering at higher stress temperature. The ON-state EL spectrum remains unchanged after the stress, suggesting that the regions without generated defects are not affected during the degradation. A finite element model is employed to further demonstrate the effect of surface defects on the local electric field. A correlation is observed for the spatial distribution of the EL intensity before and after the generation of leakage sites, which provides a prescreening method to predict possible early failures on a device.     

Characterization of stress degradation effects and thermal properties of AlGaN/GaN HEMTs with photon emission spectral signatures

The influence of stress degradation and device temperature variation on the device properties has been investigated with electrical and photon emission (PE) measurements. To degrade the devices the type of short-time stress tests, namely DC-Step-Stress-Tests of GaN HEMTs have been performed on wafers with and without GaN cap to additionally check the behaviour of various technological processes. It has been found that wafers with GaN cap show much higher critical voltages as compared to non-capped epitaxial designs and have different PE spectral signatures. Thermo-electrical topics like high power dissipation and self-heating of GaN based HEMTs were also investigated with electrical characterization and electroluminescence in various operating conditions.     

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.     

Role of stress voltage on structural degradation of GaN high-electron-mobility transistors

In GaN high-electron-mobility transistors, electrical degradation due to high-voltage stress is characterized by a critical voltage at which irreversible degradation starts to take place. Separately, cross-sectional TEM analysis has revealed significant crystallographic damage for severely degraded devices. Furthermore, a close correlation between the degree of drain current degradation and material degradation has been reported. However, the role of the critical voltage in physical degradation has not been explored. In this work, we investigate the connection between electrical degradation that occurs around and beyond the critical voltage and the formation of crystallographic defects through detailed electrical and TEM analysis, respectively. We find that a groove in the GaN cap starts to be generated around the critical voltage. At higher voltages, a pit develops that penetrates into the AlGaN barrier. The size of the pit increases with stress voltage. We also observe a good correlation between electrical and material degradation.     

Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems

Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8 × 60 μm wide and 0.5 μm long AlGaN/GaN HEMTs at a drain voltage of V_d = 50 V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance-voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers.     

质子辐照对场板AlGaN／GaN HEMT器件电特性的影响

分别采用3MeV和10MeV的质子对GaN基HEMT（High Electron Mobility Transistor）器件进行辐照。实验发现：低注量辐照引起了体材料载流子浓度增加,高注量辐照引起了HEMT器件漏电流下降,跨导减小,阈值电压显著退化的结果。通过分析发现辐射感生受主缺陷引起的2DEG浓度降低是上述器件退化的主要原因。此外基于实验结果,采用辐射感生受主缺陷退化模型仿真并计算了HEMT器件主要参数随受主浓度的退化规律,仿真结果与实验结果有较好的一致性。本文实验结果也表明场板结构和SiN钝化层有效地阻止了电子陷落在表面态中,屏蔽了绝大部分的辐照损伤,是很有效的辐射加固手段。