GaN High Electron Mobility Transistors Devices Research Articles

Study on irradiation effect and damage mechanism in cascode GaN HEMT irradiated by 10 MeV electron

This study investigated the irradiation effect of cascode-structure GaN HEMT (High Electron Mobility Transistor) devices, employing high-energy electrons with an energy of 10 MeV and irradiation doses ranging from 5 to 80 Mrad(Si). The variation of electrical properties of the device under annealing condition was analyzed. Geant4 and TCAD simulations were used to analyze the irradiation effect and damage mechanisms. The results show that the threshold voltage has obvious negative drift and the drain current increases after irradiation. The threshold voltage deviation amplitude of the device increases with the increase of irradiation dose, and the maximum deviation is 1.41V. Annealing at high temperatures (80 °C, 120 °C and 145 °C) partially restores the electrical properties, with a 0.49 V restoration in threshold voltage at 145 °C. Geant4 simulations reveal that enhanced Si MOSFET is more susceptible to total dose effects. TCAD simulations of enhanced Si MOSFET devices demonstrate an increase in electric field intensity, trapped electron concentration, and hole concentration in Si and SiO2 layers with the increase of irradiation dose. These findings can provide support for the space application and irradiation hardening of cascode GaN HEMT devices.

Temperature-induced degradation of GaN HEMT: An in situ heating study

High-power electronics, such as GaN high electron mobility transistors (HEMTs), are expected to perform reliably in high-temperature conditions. This study aims to gain an understanding of the microscopic origin of both material and device vulnerabilities to high temperatures by real-time monitoring of the onset of structural degradation under varying temperature conditions. This is achieved by operating GaN HEMT devices in situ inside a transmission electron microscope (TEM). Electron-transparent specimens are prepared from a bulk device and heated up to 800 °C. High-resolution TEM (HRTEM), scanning TEM (STEM), energy-dispersive x-ray spectroscopy (EDS), and geometric phase analysis (GPA) are performed to evaluate crystal quality, material diffusion, and strain propagation in the sample before and after heating. Gate contact area reduction is visible from 470 °C accompanied by Ni/Au intermixing near the gate/AlGaN interface. Elevated temperatures induce significant out-of-plane lattice expansion at the SiNx/GaN/AlGaN interface, as revealed by geometry-phase GPA strain maps, while in-plane strains remain relatively consistent. Exposure to temperatures exceeding 500 °C leads to almost two orders of magnitude increase in leakage current in bulk devices in this study, which complements the results from our TEM experiment. The findings of this study offer real-time visual insights into identifying the initial location of degradation and highlight the impact of temperature on the bulk device’s structure, electrical properties, and material degradation.

Investigation of RF performance of Ku-band GaN HEMT device and an in-depth analysis of short channel effects

In this paper, we have characterized an AlGaN/GaN High Electron Mobility Transistor (HEMT) with a short gate length (Lg ≈ 0.15 μm). We have studied the effect of short gate length on the small signal parameters, linearity parameters and gm-gd ratio in GaN HEMT devices. To understand how scaling results in the variation of the above-mentioned parameters a comparative study with higher gate length devices on similar heterostructure is also presented here. We have scaled down the gate length but the barrier thickness(tbar) remained same which affects the aspect ratio (Lg/tbar) of the device and its inseparable consequences are the prominent short channel effects (SCEs) barring the optimum output performance of the device. These interesting phenomena were studied in detail and explored over a temperature range of −40 °C to 80 °C. To the best of our knowledge this paper explores temperature dependence of SCEs of GaN HEMT for the first time. With an approach to reduce the impact of SCEs a simulation study in Silvaco TCAD was carried out and it is observed that a recessed gate structure on conventional heterostructure successfully reduces SCEs and improves RF performance of the device. This work gives an overall view of gate length scaling on conventional AlGaN/GaN HEMTs.

In-situ S/TEM DC biasing of p-GaN/AlGaN/GaN heterostructure for E-mode GaN HEMT devices

This work describes an in-situ electrical DC bias study of the E-mode GaN high electron mobility transistor (HEMT) device. A single transistor structure is biased and studied in real-time. The sample was made from an E-mode GaN HEMT device using Focused Ion Beam (FIB) milling and upright lift-off. The device lamella is subjected to forward gate bias to understand the device operation and physical changes under the bias. Active device area and micron level changes due to biasing were studied and identified as crucial factors affecting device reliability during continuous operation. Electric bias-induced physical changes are observed at the p-GaN layer and AlGaN interface on the p-GaN and GaN sides. Localized damage and defect formation, along with elemental diffusion, is observed. The formation of new defects over existing growth defects was seen in the p-GaN/AlGaN/GaN heterostructure. The study helped us identify the exact location of the failure, the region affected under bias, and the occurrence of physical changes due to the electrical bias on the in-situ device. Based on the study, gate breakdown failure and its location at the metal/p-GaN interface are understood to result from physical changes activated by electrical bias.

Research on the Reliability of Threshold Voltage Based on GaN High-Electron-Mobility Transistors.

With the development of high-voltage and high-frequency switching circuits, GaN high-electron-mobility transistor (HEMT) devices with high bandwidth, high electron mobility, and high breakdown voltage have become an important research topic in this field. It has been found that GaN HEMT devices have a drift in threshold voltage under the conditions of temperature and gate stress changes. Under high-temperature conditions, the difference in gate contact also causes the threshold voltage to shift. The variation in the threshold voltage affects the stability of the device as well as the overall circuit performance. Therefore, in this paper, a review of previous work is presented. Temperature variation, gate stress variation, and gate contact variation are investigated to analyze the physical mechanisms that generate the threshold voltage (VTH) drift phenomenon in GaN HEMT devices. Finally, improvement methods suitable for GaN HEMT devices under high-temperature and high-voltage conditions are summarized.

The effect of the barrier thickness on DC and RF performances of AlGaN/GaN HEMTs on silicon

In this study, the effect of barrier thickness on the DC and RF performances of AlGaN/GaN high-electron mobility transistors (HEMTs) on silicon for millimeter wave applications is experimentally investigated. GaN HEMT devices with different barrier thicknesses were fabricated and characterized. While the device with the thinnest barrier exhibited the highest extrinsic transconductance (gm) resulting from the shortest gate-to-channel distance, such configuration suffered from the lowest unit current-gain cut-off frequency (fT) due to the increase of the total gate capacitance. Moreover, degradation in the output power was observed for devices with thinner barriers. Such degradation was related to the severe knee walkout as evidenced from drain-lag characterization.

Impact of the Low Temperature Ohmic Contact Process on DC and Forward Gate Bias Stress Operation of GaN HEMT Devices

In AlGaN/GaN high electron mobility transistors (HEMTs), high temperature processes (such as ohmic annealing with >800°C value) could deform the crystal structure and induce trap states within the bulk and surface. Expanded defect densities cause crucial problems, such as threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\text {th}}$ </tex-math></inline-formula> ) instability, current collapse, and high leakages. In this work, a low temperature ohmic contact process (630°C, 10 minutes) is adopted with recess etch, and contact resistances <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&lt; 0.1\Omega ~\cdot $ </tex-math></inline-formula> mm with low sheet resistances are achieved. The positive impact of this low thermal budget process on surface morphology, DC operation, long-term stability, and forward gate bias stress of the device is studied.

Improved X-Band Performance and Reliability of a GaN HEMT With Sunken Source Connected Field Plate Design

The world class performance and reliability of a high-power density AlGaN/GaN high electron mobility transistor (HEMT) with an innovative sunken source connected field plate (SCFP) is reported. The optimized HEMT structure implements a novel sunken SCFP design that has significant advantages over the conventional field plated GaN HEMT. The new design reduced parasitic capacitances (C_gs and C_gd) whilst suppressing the peak electric fields in the drift region to improve breakdown voltage and reliability. This sunken SCFP demonstrates the improved RF performance and reliability with respect to leading GaN HEMT devices for x-band applications. The fabricated device produces 10 GHz performance with saturated output power &#x003E;10 W/mm, linear gain &#x003E;18 dB and PAE &#x003E; 60&#x0025; &#x0040; 50 V. The improved gain is a direct result of the reduction in Cgs and Cgd, by 15&#x0025; and 60&#x0025;, respectively. The increased power is achieved from lower trapping and increased drain voltage rating of 25&#x0025;. Accelerated RF-driven reliability testing is shown to determine the expected lifetime of the device was &#x003E;6E7 hours at 225&#x00B0;C junction temperature. This is a more than 2 orders of magnitude increase over the conventional field plate design.

Lifetime prediction and analysis of AlGaN/GaN HEMT devices under temperature stress

AlGaN/GaN HEMT (High Electron Mobility Transistor) devices are widely used in power electronic equipment due to their excellent properties such as low on-resistance and high breakdown voltage. Power electronic equipment usually faces high frequency, high temperature and other extreme environments when they are in use. Excessive environmental stress will accelerate the aging of devices. When the aging reaches a certain level, it will cause devices to fail, and even cause the entire circuit to fail. Therefore, the lifetime of AlGaN/GaN HEMT devices has always been a research hotspot. Based on the three temperature DC tests results of GaN HEMT devices with different gate widths, the lifetime of GaN HEMT devices at normal operating temperature is predicted by using multiple linear regression method and graph estimation method. The channel temperature of devices is 150 °C. The prediction results show that when GaN HEMT devices are in normal use, the median lifetime is greater than 107 h. The lifetime of the 1.25 mm gate width devices is significantly longer than that of the 3.6 mm gate width devices. It can be inferred that the smaller the gate width of devices, the longer the service lifetime. The acceleration factor of 3.6 mm gate width devices is 5.9 × 104, the acceleration factor of 1.25 mm gate width devices is 2 × 104. The larger the gate width of the device, the larger the acceleration factor and the easier the device will fail.

High Power Normally-OFF GaN/AlGaN HEMT with Regrown p Type GaN

In this paper is presented a Normally-OFF GaN HEMT (High Electron Mobility Transistor) device using p-doped GaN barrier layer regrown by CBE (Chemical Beam Epitaxy). The impact of the p doping on the device performance is investigated using TCAD simulator (Silvaco/Atlas). With 4E17 cm−3 p doping, a Vth of 1.5 V is achieved. Four terminal breakdowns of the fabricated device are investigated, and the origin of the device failure is identified.

Innovative remote plasma source for atomic layer deposition for GaN devices

High-quality dielectric films could enable GaN normally off high-electron-mobility transistors (HEMTs). Plasma atomic layer deposition (ALD) is known to allow for controlled high-quality thin-film deposition, and in order to not exceed energy and flux levels leading to device damage, the plasma used should preferably be remote for many applications. This article outlines ion energy flux distribution functions and flux levels for a new remote plasma ALD system, Oxford Instruments Atomfab™, which includes an innovative, RF-driven, remote plasma source. The source design is optimized for ALD for GaN HEMTs for substrates up to 200 mm in diameter and allows for Al2O3 ALD cycles of less than 1 s. Modest ion energies of &amp;lt;50 eV and very low ion flux levels of &amp;lt;1013 cm−2 s−1 were found at low-damage conditions. The ion flux can be increased to the high 1014 cm−2 s−1 range if desired for other applications. Using low-damage conditions, fast ALD saturation behavior and good uniformity were demonstrated for Al2O3. For films of 20 nm thickness, a breakdown voltage value of 8.9 MV/cm was obtained and the Al2O3 films were demonstrated to be suitable for GaN HEMT devices where the combination with plasma pretreatment and postdeposition anneals resulted in the best device parameters.

A scalable large‐signal model with self‐heating effect based on a hybrid‐scaling rule for GaN high‐electron‐mobility transistors

AbstractIn this paper, a novel hybrid‐scaling rule for GaN high‐electron‐mobility transistors (HEMTs) scalable large‐signal model with self‐heating effect is proposed. Due to the complex parasitic characteristics of large gate‐width GaN HEMT devices, nonlinear scaling rules are used to consider the dependence of the extrinsic inductances, extrinsic resistances, and thermal resistance on the gate width. In addition, a drain‐source current model and gate capacitance models are proposed to improve the accuracy of traditional Angelov model. A one stage resistor‐capacitor network is attached in the current model to characterize self‐heating effect. Therefore, a scalable large‐signal model based on a hybrid‐scaling rule is presented for the first time. Compared with traditional linear scalable model, the proposed model has a good prediction for multi‐bias S‐parameters and direct current (DC) I‐V curves of different size devices, and can also significantly improve the fitting degree of S22 parameter at high frequency. Moreover, the proposed scalable model shows a good prediction of large‐signal characteristics for large size GaN HEMT devices.

Effect of x-ray irradiation on threshold voltage of AlGaN/GaN HEMTs with p-GaN and MIS gates

Commercially available AlGaN/GaN high-electron-mobility transistors (HEMTs) are beginning to enter the public scene from a range of suppliers. Based on previous studies, commercial GaN-based electronics are expected to be tolerant to different types of irradiation in space. To test this assumption, we compared the characteristic electrical curves obtained at different X-ray irradiation doses for GaN HEMT devices manufactured by Infineon and Transphorm. The p-GaN-based device was found to be more robust with a stable threshold voltage, whereas the threshold voltage of the device with a metal-insulator-semiconductor gate was found to shift first in the negative and then the positive direction. This dynamic phenomenon is caused by the releasing and trapping effects of radiation-induced charges in the dielectric layer and at the interface of irradiated devices. As such, the p-GaN-gate-based GaN HEMT provides a promising solution for use as an electric source in space.

Real time detection of Hg2+ ions using MoS2 functionalized AlGaN/GaN high electron mobility transistor for water quality monitoring

A sensor for highly sensitive, selective, and rapid determination of the trace amount of toxic Hg2+ ions is developed for the first-time using molybdenum disulfide (MoS2) functionalized AlGaN/GaN high electron mobility transistor (HEMT). The vertically aligned, flower-like MoS2 structures are synthesized through a simple hydrothermal route and applied on the gate region of AlGaN/GaN HEMT. The scanning electron microscopy, Raman spectroscopy, and X-ray diffraction are performed for structural characterization of MoS2. Further, the sensing of Hg2+ ions is performed by electrical characterizations of MoS2 functionalized AlGaN/GaN HEMT. The sensor showed an excellent sensitivity of 0.64 μA/ppb and detection limit of 0.01152 ppb with the rapid response time of 1.8 s. The sensor exhibits the linear range of detection from 0.1 ppb to 100 ppb and highly selective behavior towards Hg2+ ions. The results demonstrated that the MoS2 possess excellent Hg2+ ions capture property, that could be attributed to the complexation of Hg2+ ions with sulfur and the electrostatic interaction between MoS2 and Hg2+ ions alters the drain to source current (IDS) of the HEMT at a constant drain to source voltage (VDS). Therefore, the MoS2 based AlGaN/ GaN HEMT devices have a huge potential for next-generation ion sensing applications.

Investigations on electrical parameters degradation and recovery of E-mode GaN high-electron mobility transistors under repetitive unclamped inductive switching stresses based on low-frequency noise

During no or minimal avalanche capability, the surge ruggedness of the E-mode AlGaN/GaN high-electron mobility transistors (HEMTs) with p-GaN gate has not been fully understood. This work unveils the comprehensive results from repetitive unclamped inductive switching (UIS) measurements on commercial p-gate GaN HEMTs. The Electrical parameters degradation and recovery characteristics have been investigated. Firstly, a careful study has been made about the degradation of output, transfer, gate leakage, and gate capacitance parameters of GaN HEMT devices before and after repetitive UIS pulses, which acting on the device channel. With the help of the low-frequency noises (LFNs), the test results show that the device characteristics have gradual degradation with an increase in UIS pules, including increased threshold voltage (V th), and decreased gate leakage current (I gss). Subsequently, the LFN testing is carried out with the increase of UIS pulses. Finally, the recovery characteristics of the devices are studied and the physics associated are discussed after putting the device at room temperature in the air for 1–15 d, which includes DC characteristics and trap density. These results physically confirm that the mechanism of the degradation and recovery of device characteristics, that is the charge trapping effects of electrons. After repetitive UIS stresses, the electrons are trapping and de-trapping on interfaces and in the bulk layer of devices, which can change electric field distribution below the grids. These have a strong impact on the electrical performance of devices.

(Invited) Coupled Experimental and Numerical Investigation of High-Voltage GaN Hemts

The wide band gap compound semiconductor Gallium Nitride and its ternary alloys have found commercial acceptance in the fabrication of high electron-mobility transistor (HEMT) rf devices. In addition, high power applications have been very attractive due to their high breakdown voltage and thus high output power. Higher power limit improvements can be reached by several means, including the integration of field-plates above the conduction channel, which tend to improve the uniformity of the electric-field distribution and reduce its peaks. These field-plates can be connected to the source or to the gate. The reduction of electrical and thermal “hotspots,” where severe degradation is thought to initiate, increases the allowable power levels for HEMT devices by as many as five folds. In this study, GaN HEMT devices with source-connected field-plates are characterized thermally to quantify their performance. Deep submicron resolution thermoreflectance (TR) temperature imaging is used to measure the temperature profile of the exposed surface features of the device, which are mainly the source and drain metal contacts as well as the GaN channel region. The TR thermography method is based on detecting changes in surface reflectivity (ΔR) that result from changes in local surface temperature (ΔT). The relationship between the changes in reflectivity and temperature is linear to first-order effects, and is typically referred to as the coefficient of thermoreflectance (CTR). As expected, this coefficient depends on the material and the wavelength of light, but also on any transparent layers (e.g., passivation) in the optical path between the first reflective surface and the sensor where the light is collected. Therefore, the success of a TR measurement requires that the surface of interest exhibit both a high reflective index and be thermally-reflective, i.e., have a good value of CTR. By using a near ultra-violet source to illuminate the device under test (DUT), the GaN is rendered opaque, thus ensuring that the obtained temperature fields are those of the GaN surface and not an integrated average over multiple semi-transparent layers. However, the presence of the field-plate obstructs imaging access to the gate where the peak temperatures are expected. Therefore, this investigation couples the measurements to 3D computations in order to produce temperature fields that would otherwise be inaccessible to direct measurement. Both quasi-steady-state and transient temperature fields of the GaN HEMTs are obtained at different activation power levels and an experimental assessment is made with regard to the electrical activation limits and the thermal self-heating. In addition to the measurements, thermal computations are performed using a self-adaptive, multi-scale, numerical simulation engine. The 3D transient thermal model is then driven by the experimentally-obtained temperature maps to provide a more detailed and device-specific interpretation of the coupled electro-thermal effect of the field-plates on reducing the temperature and increasing the break-down voltage. Moreover, the full 3D thermal model also helps provide physically-accurate temperature fields of regions of critical channel regions, where the heat sources are located, and where localized hotspots that are hidden underneath field-plates are inaccessible to optical measurement approaches. The attached figure contains two temperature fields, numerical on the left and experimental on the right. These results are representative of the type of useful data that can be obtained from the computational and measurement techniques. The presentation will additionally include the results of the coupling of the two approaches and the insights that can be gained, especially for those features that are optically inaccessible. Figure 1

Ka band High RF performance AlGaN/GaN HEMTs

To realize high RF performance AlGaN/GaN high electron mobility transistor (HEMT), two SiNx passivation layers were respectively grown on the surface of AlGaN/GaN epi wafer by inductively coupled-plasma chemical vapor deposition (ICP-CVD) and plasma enhanced chemical vapor deposition (PECVD). The two-stage passivation can effectively suppress the current collapse and decrease the parasitic parameters. To verify the performance improvement, a 6 × 50 μm GaN HEMT device was fabricated and measured. The device shows a maximum drain current of about 1067 mA/mm and a maximum transconductance of about 433 mS/mm, the cut-off frequency (fT) of 45.4 GHz at a gate bias of -1.5 V and a drain bias of 20 V. The device shows a power density of 3.7 W/mm, a power-added efficiency (PAE) of 46.5% and power gain of 8.5 dB by load-pull measurement. In addition, a 34-36 GHz GaN monolithic microwave integrated circuit (MMIC) was fabricated, which exhibits an output power of about 41.5 dBm, a PAE of 20% and a gain of 15.5 dB.

Study on Small-Signal Modeling of GaN HEMT Devices

Because of its advantages of high temperature, high frequency and high power, GaN high electron-mobility transistor (HEMT) devices have been widely used in microwave circuit design. Successful circuit design requires accurate device models. Therefore, small-signal modeling plays an important role in the design of microwave circuits. According to the theoretical analysis, this paper selects CGH40010F device for small-signal modeling, proposes 16-element small-signal circuit model, uses vector network analyzer (VNA) to measure S parameters under different biases at different frequency points, and uses the distributed extraction method to extract the parasitic parameters and intrinsic parameters of GaN HEMT device. The accuracy of the model was verified by comparing the test results with the simulation results.

Impact of Plasma‐Damaged‐Layer Removal on GaN HEMT Devices

Plasma‐damage during the dry‐etch process often induces degradation of the electric performance in GaN high electron mobility transistor (HEMT) devices. In this paper, the effects of wet‐etch processing on plasma‐damaged AlGaN films using various chemical solutions and propose the best chemical formulation are reported. Three chemical solutions are selected by a combinatorial method. A MIS‐capacitor TEG is fabricated on an AlGaN/GaN structure, and each surface is evaluated by XPS, SIMS, TEM, and AFM. From the results of C–V measurements of wet‐etched samples, the study reveals that the wet chemicals successfully remove charge traps at ≈−2.3 eV induced by the dry‐etch process. Moreover, these C–V results show excellent agreement with simulated trap models. The XPS results show that O/Ga ratios increase from 0.32 to 1.00 after the dry‐etch process, whereas after the wet‐etch process, the ratios return to less than 0.31. Moreover, the TEM observations reveal that an amorphous layer is only detected in the dry‐etched sample. Therefore, this amorphous layer should be an oxidized AlGaN layer. Thus, the selected chemical formulation can successfully remove the damaged‐layer induced by the dry‐etch process. The increase of surface roughness by the wet‐etch process is suppressed less than 0.1 nm.

Mathematical analysis of GaN high electron mobility transistor false turn‐on phenomenon

High-power density is required in power conversion systems used in various applications, especially in electric vehicles. Driving the power devices with high frequency is one of the successful solutions to downsize a circuit's magnetic components, and hence an overall high-power density of the circuit can be realised. Therefore, GaN high electron mobility transistor (HEMT) is widely used in many applications, owing to its high-speed switching characteristics, low power losses, and high heat and radiation resistances. However, the high-speed switching of GaN HEMT comes with a drawback. For instance, in a full-bridge inverter configuration, high-speed turn-on of one device may cause a false turn-on of the complementary device. If a power device has encountered a false turn-on, the GaN HEMT devices will suffer from overcurrent, excessive heat and is eventually destructed due to the short circuit. This phenomenon can be tackled by modifying the circuit conditions. In this Letter, the theoretical analysis of a three-phase full-bridge inverter is developed to figure out the behaviour of the GaN HEMT gate voltage, which is the key-factor parameter causing a false turn-on. On the basis of the proposed theoretical analysis, the circuit conditions which minimise the gate voltage spike are derived.