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Strain plays an important role in the performance and reliability of AlGaN/GaN high electron mobility transistors (HEMTs). However, the impact of strain on the performance of proton irradiated GaN HEMTs is yet unknown. In this study, we investigated the effects of strain relaxation on the properties of proton irradiated AlGaN/GaN HEMTs. Controlled strain relief is achieved locally using the substrate micro-trench technique. The strain relieved devices experienced a relatively smaller increase of strain after 5 MeV proton irradiation at a fluence of 5 × 1014 cm−2 compared to the non-strain relieved devices, i.e., the pristine devices. After proton irradiation, both pristine and strain relieved devices demonstrate a reduction of drain saturation current (Ids,sat), maximum transconductance (Gm), carrier density (ns), and mobility (μn). Depending on the bias conditions the pristine devices exhibit up to 32% reduction of Ids,sat, 38% reduction of Gm, 15% reduction of ns, and 48% reduction of μn values. In contrast, the strain relieved devices show only up to 13% reduction of Ids,sat, 11% reduction of Gm, 9% reduction of ns, and 30% reduction of μn values. In addition, the locally strain relieved devices show smaller positive shift of threshold voltage compared to the pristine devices after proton irradiation. The less detrimental impact of proton irradiation on the transport properties of strain relieved devices could be attributed to reduced point defect density producing lower trap center densities, and evolution of lower operation related stresses due to lower initial residual strain. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The addition of CHF3 to Cl2/Ar inductively coupled plasmas operating at low dc self-biases (<100 V, corresponding to incident ion energies <125 eV) leads to etch selectivity for Ga2O3 over (Al0.18Ga0.82)2O3 of >30, with a maximum value of 55. By sharp contrast, without CHF3, the etching is nonselective over a large range of source and rf chuck powers. We focused on low ion energy conditions that would be required for device fabrication. This result has a direct application to selective removal of Ga2O3 contact layers to expose underlying (Al0.18Ga0.82)2O3 donor layers in high-electron-mobility transistor structures. It is expected that formation of nonvolatile AlF3 species helps produce this selectivity. X-ray photoelectron spectroscopy does detect F residues on the etched surface for the Cl2/Ar/ CHF3 plasma chemistry. [ABSTRACT FROM AUTHOR]

Radiation damage in electronic devices is known to be influenced by physics, design, and materials system. Here, we report the effects of biasing state (such as ON and OFF) and pre-existing damage in GaN high electron mobility transistors exposed to γ radiation. Controlled and accelerated DC biasing was used to prestress the devices, which showed significant degradation in device characteristics compared to pristine devices under ON and OFF states after γ irradiation. The experiment is performed in situ for the ON-state to investigate transient effects during irradiation until the total dose reaches 10 Mrad. It shows that threshold voltage, maximum transconductance, and leakage current initially decrease with dosage but slowly converge to a steady value at higher doses. After 10 Mrad irradiation, the OFF-state device demonstrates larger RON and one order of magnitude increased leakage current compared to the ON-state irradiated device. The micro-Raman study also confirms that the ON-state operation shows more radiation hardness than OFF and prestressed devices. Prestressed devices generate the highest threshold voltage shift from −2.85 to −2.49 V and two orders of magnitude higher leakage current with decreased saturation current after irradiation. These findings indicate that high electric fields during stressing can generate defects by modifying strain distribution, and higher defect density can not only create more charges during irradiation but also accelerate the diffusion process from the ionizing track to the nearest collector and consequently degrade device performances. [ABSTRACT FROM AUTHOR]

High Aluminum content channel (Al0.85Ga0.15N/ Al0.7Ga0.3N) High Electron Mobility Transistors (HEMTs) were operated from room temperature to 500°C in ambient. The devices exhibited only moderate reduction, 58%, in on-state forward current. Gate lag measurements at 100 kHz and 10% duty only showed a slight reduction in pulsed current from DC at 500°C and high gate voltages. Interfacial trap densities were $2 \times 10^{11}$ over the range 25–300°C and $3 \times 10^{12}$ cm $^{-2}$ from 300–500°C from the subthreshold swing. These low interfacial trap densities and the near ideal gate lag measurement indicate high-quality epi layers. The insulating properties of the barrier layer led to low gate induced drain leakage current of $\sim 10^{-12}$ A/mm and $\sim 10^{-8}$ A/mm at 25 and 500°C, respectively. Low leakage current was enabled by the high Schottky barrier of the Ni/Au gate, 1.1 eV and 3.3 eV at 25 and 500°C, respectively. These properties of the AlGaN channel HEMTs demonstrate their potential for high power and high temperature operation. [ABSTRACT FROM AUTHOR]

This report details the use of a Si Metal Oxide Semiconductor Field Effect Transistor (MOSFET) with the gate connected to a disposable antibody-functionalized electrode for the detection of β-2-transferrin (β2T) as a marker of cerebral spinal fluid (CSF). Through the design of a Si based PCB the need for GaN High Electron Mobility Transistors has been mitigated and brings the cost of production for commercial application closer to realization. Through the use of a double pulse method to the test electrode and the drain of the Si MOSFET, ionicity of the test solution is no longer a concern as a double layer is produced and perturbed on the test electrode giving rise to current changes related to the concentration of the biomarker. Additionally, the PCB provides connections for an oscilloscope readout or an onboard display entirely removing the need for any expensive semiconductor parameter analyzers. Clinically, the cutoff for β2T as indicator of CSF leakage has traditionally been limited to ~2 μg/mL and takes several hours; with this newly designed board, CSF was detected at levels from 0.1 ng/mL to 100 μg/mL within 5 minutes, greatly improving the detection limits and feedback time for medical professionals. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation energy on the electrical properties of SiNx/AlGaN/GaN metal-insulator semiconductor high electron mobility transistors (MISHEMTs) using in situ grown silicon nitride as the gate dielectric were studied. The SiNx/AlGaN/GaN MISHEMT devices were irradiated with protons at energies of 5, 10, or 15 MeV at a fixed fluence of 2.5 × 1014 cm−2. The largest amount of device degradation was shown in the samples irradiated with the lowest irradiation energy of 5 MeV. The DC saturation current was reduced by 10.4%, 3.2%, and 0.5% for MISHEMTs irradiated with proton energies of 5, 10, and 15 MeV, respectively. Device performance degradation was more pronounced in the irradiated samples under high-frequency operation. At a frequency of 100 kHz, the percent saturation drain current reduction at a gate voltage of 3 V was 40%, 19%, and 17% after proton irradiation at 5, 10, and 15 MeV, respectively. The carrier removal rates for the MISHEMT devices were in the range of 21–144 cm−1 for the proton irradiation energies studied. The measured DC degradation and carrier removal rates are lower than the values reported for AlGaN/GaN metal-gate high electron mobility transistor devices irradiated at similar conditions, which can be attributed to the SiNx insulating layer reducing the total damage on the AlGaN surface. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on sapphire substrates, using ZrTi buffer layers to provide in-plane lattice-matching to hexagonal GaN. X-ray diffraction (XRD) as well as cross-section transmission electron microscopy (TEM) were used to assess the quality of the HEMT structure. The XRD 2h scans showed full-width-at-half-maximum values of 0.16, 0.07°, and 0.08° for ZrTi alloy, GaN buffer layer, and the entire HEMT structure, respectively. TEM studies of the GaN buffer layer and the AlN/ZrTi/AlN stack showed the importance of growing thin AlN buffer layers on the ZrTi layer prior to growth of the GaN buffer layer. The density of threading dislocations in the GaN channel layer of the HEMT structure was estimated to be in the 108 cm2 range. The HEMT device exhibited a saturation drain current density of 820mA/mm, and the channel of the fabricated HEMTs could be well modulated. A cutoff frequency (fT) of 8.9GHz and a maximum frequency of oscillation (fmax) of 17.3GHz were achieved for HEMTs with gate dimensions of 1 x 200 μm. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation on the dc performance of InAlN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) with Al2O3 as the gate oxide were investigated. The InAlN/GaN MOSHEMTs were irradiated with doses ranging from 1 x 1013 to 1 x 1015cm-2 at a fixed energy of 5MeV. There was minimal damage induced in the two dimensional electron gas at the lowest irradiation dose with no measurable increase in sheet resistance, whereas a 9.7% increase of the sheet resistance was observed at the highest irradiation dose. By sharp contrast, all irradiation doses created more severe degradation in the Ohmic metal contacts, with increases of specific contact resistance from 54% to 114% over the range of doses investigated. These resulted in source-drain current- voltage decreases ranging from 96 to 242mA/mm over this dose range. The trap density determined from temperature dependent drain current subthreshold swing measurements increased from 1.6 x 1013cm-2V-1 for the reference MOSHEMTs to 6.7 x 1013cm-2V-1 for devices irradiated with the highest dose. The carrier removal rate was 1287 ± 64 cm-1, higher than the authors previously observed in AlGaN/GaN MOSHEMTs for the same proton energy and consistent with the lower average bond energy of the InAlN. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation doses on dc characteristics of reference, electrically stressed under off-state conditions and stressed/annealed AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. The HEMTs were irradiated with protons at a fixed energy of 5MeV and doses ranging from 1013 to 1015cm-2. As expected, in all three types of HEMTs, more degradation of the device dc characteristics was observed for higher doses due to the larger displacement damage in two-dimensional electron gas channel of the HEMTs. The electrically stressed HEMTs after proton irradiation showed more degradation compared with reference and stressed/annealed HEMTs. After proton irradiation at a dose of 1015cm-2, the drain saturation current and maximum transconductance of stressed HEMTs were decreased by 28.5% and 15%, respectively, compared to 24% and 11.5%-12%, respectively, for reference and stressed/annealed devices. The dc characteristics of stressed/annealed HEMTs after proton irradiation showed similar degradation trends compared with those of reference HEMTs, confirming that annealing is effective in removing defects created by the off-state stressing. In some cases, the authors also annealed stressed/irradiated HEMTs after the proton irradiation step. The drain current and transconductance of stressed/irradiated HEMTs were slightly increased after subsequent thermal annealing at 450 °C for 10 min, while reverse gate leakage current after annealing was decreased more than an order of magnitude. The interface trap density of stressed HEMTs after proton irradiation at a dose of 1015cm-2 increased from 3.05×1012 to 1.37×1013/cm²V and were reduced to 6.01×1012/cm²V following thermal annealing. Our results are consistent with the notion that off-state stressing creates defects that have a common origin with those created by proton irradiation, and thus, irradiation of off-state stressed devices leads to more defects than in unstressed devices that are subsequently irradiated. Annealing the stressed devices prior to irradiation makes them behave the same as unstressed HEMTs when both are irradiated with protons. [ABSTRACT FROM AUTHOR]

By varying the photon flux during sub-bandgap optical pumping, the locations of traps associated with sub-bandgap laser pumping in AlGaN/GaN high electron mobility transistors (HEMTs) were identified. Fixed photon flux of sub-bandgap optical pumping was previously employed to determine the activation energies of traps in AlGaN/GaN HEMT; traps with activation energies of 0.73 eV, both 0.73 and 1.91 eV or all three trap states detected for the 671, 532, or 477 nm laser illumination, respectively. However, by illuminating the HEMTs with lower photon fluxes from 532 or 447 nm lasers, the traps with 0.73 eV were absent, and only the traps associated with higher activation energies were activated by the laser light. This indicated that the traps related to the activation energies of 1.91 and 2.35 eV are likely located at the surface of the HEMTs. Photon flux dependent gate-lag measurements were also conducted. Laser illumination of 671 nm did not affect the drain current during the gate lag measurement, implying the traps linked to the activation energy of 0.73 eV are not present in the AlGaN layer of the HEMT structure. On the other hand, the 1.91 and 2.35 eV trap states were distributed in the AlGaN layer of the HEMTs. This approach to selectively exciting the defect states provides a nondestructive method of probing the ionization levels and spatial location of traps in wide bandgap heterostructures. [ABSTRACT FROM AUTHOR]

The effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron mobility (HEMTs) was studied. AlGaN/GaN HEMT structures with 2 and 5 µm GaN buffer layers on sapphire substrates from two different vendors with the same Al concentration of AlGaN were used. The defect densities of HEMT structures with 2 and 5 µm GaN buffer layer were 7×109 and 5×108cm–2, respectively, as measured by transmission electron microscopy. There was little difference in drain saturation current or in transfer characteristics in HEMTs on these two types of buffer. However, there was no dispersion observed on the nonpassivated HEMTs with 5 µm GaN buffer layer for gate-lag pulsed measurement at 100 kHz, which was in sharp contrast to the 71% drain current reduction for the HEMT with 2 µm GaN buffer layer. [ABSTRACT FROM AUTHOR]

The breakdown characteristics of AlGaN/GaN based metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs) using a 10nm thick AlN gate insulator and passivation layer deposited plasma enhanced atomic layer deposition. The AlN was effective in significantly reducing gate leakage current relative to Schottky gate devices and showed only small decreases in drain current during gate lag measurements. The devices exhibited a strong dependence of gate breakdown voltage on source-drain distance, reaching a value of 2000V for a source-drain distance of 40μm limited by the measurement instrument. The specific on-state resistance was 1.3 and 10.9 mΩ cm² for the devices with the gate-drain distance of 7.5 and 37.5μm, respectively. The saturation drain current was inversely dependent on source-drain distance and the on-off ratios were in excess of 108 due to the low gate leakage current in the MISHEMTs. [ABSTRACT FROM AUTHOR]

The effects of postprocess annealing on the gate leakage current and breakdown voltage characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) was investigated. The fabricated AlGaN/GaN HEMTs were postannealed at 250, 300, 350, 400, or 450°C under a nitrogen (N2) atmosphere by using rapid thermal annealing, and both direct current (dc) and pulsed measurements were performed to characterize the changes in device performance. The reverse gate leakage current (IG) at VG = -10 V was reduced by one order of magnitude and the off-state drain breakdown voltage (Voff) increased by over three-fold after postprocess annealing at 450 °C. The reverse gate leakage current was found to be independent of gate-to-drain potential after annealing. The gate pulse measurements revealed the activation of deep traps during the postannealing at elevated temperatures. [ABSTRACT FROM AUTHOR]

InAlN/GaN high electron mobility transistors were irradiated from the front side with 340keV protons to a dose of 5 × 1013 cm-2. Raman thermography showed that the irradiated devices had higher channel temperatures than unirradiated control devices, but only by ~10% under typical biasing conditions. Accordingly, the irradiated devices have higher thermal resistance (400°C/W) compared to reference devices (350°C/W), based on the slope of the power versus channel temperature line. However, increases of 42% in off-state drain breakdown voltage (VBR) and of >92% in critical voltage (Vcri) were observed for the proton irradiated HEMT. This is ascribed to the reduction of the peak electric field at the gate edges by ~50% through the introduction of negative trap charges created from vacancies generated by the proton irradiation. [ABSTRACT FROM AUTHOR]

AlGaN/GaN based metal-insulator-semiconductor high-electron-mobility transistors (HEMTs) using a plasma enhanced atomic layer deposited 10 nm AlN as the gate insulator and passivation layer were demonstrated. A refractive index of 1.92 for the deposited AlN was measured using an ellipsometer, which was slightly lower than that of bulk AlN. The deviation of the refractive index from the ideal value was caused by AlN surface oxidation, and this was confirmed by X-ray photoelectron spectroscopy and Auger depth profiling analyses. The HEMT drain current was modulated with gate voltages ranging from -3 to +4 V. The HEMT exhibited an on-off ratio of 3.3 × 108 due to the low gate leakage current and a maximum saturation drain current of 600 mA/mm. Beside reducing the gate leakage current, the effectiveness of the HEMT passivation was confirmed by gate pulse measurements, which showed only a 7% decrease of the drain current. [ABSTRACT FROM AUTHOR]

We present a review of reliability issues in AlGaN/GaN and AlGaAs/GaAs high electron mobility transistors (HEMTs) as well as Heterojunction Bipolar Transistors (HBTs) in the AlGaAs/GaAs materials systems. Because of the complex nature and multi-faceted operation modes of these devices, reliability studies must go beyond the typical Arrhenius accelerated life tests. We review the electric field driven degradation in devices with different gate metallization, device dimensions, electric field mitigation techniques (such as source field plate), and the effect of device fabrication processes for both DC and RF stress conditions. We summarize the degradation mechanisms that limit the lifetime of these devices. A variety of contact and surface degradation mechanisms have been reported, but differ in the two device technologies: For HEMTs, the layers are thin and relatively lightly doped compared to HBT structures and there is a metal Schottky gate that is directly on the semiconductor. By contrast, the HBT relies on pn junctions for current modulation and has only Ohmic contacts. This leads to different degradation mechanisms for the two types of devices. [ABSTRACT FROM AUTHOR]

The effect of ambient temperature on the detection sensitivity of carbon monoxide (CO) using ZnO nanorod-gated AlGaN/GaN high electron mobility transistor (HEMT) sensors was studied over a range of temperatures from 25 to 400 °C. An increase of the HEMT drain current was observed for exposure to the CO-containing ambients, due to chemisorbed oxygen on the ZnO surface reacting with CO to form CO2 and releasing electrons to the oxide surface, increasing the counter charges in the two-dimensional electron gas channel of the HEMT. By increasing the detection temperature from 25 °C to 150 °C, the CO detection sensitivity, ΔI/I, and detection limit were significantly improved from 0.23% to 7.5% and from 100 ppm to ∼30 ppm, respectively. However, the sensitivity of the CO detection was degraded by the decrease of mobility and saturation drain current of HEMT at temperatures higher than 200 °C. [ABSTRACT FROM AUTHOR]

The authors report the proton energy dependence of the degradation of AlGaN/GaN high electron mobility transistors (HEMTs) with GaN cap layers from 5 to 15 MeV at a fixed dose of 5 × 1015 cm-2. All the samples degraded after proton irradiation. However, higher damage in dc electrical properties was observed at lower proton energies. Saturation currents at VDS = 6 V and VGS = 0 V reduced by 47% after proton irradiation at 5 MeV energy, but the reduction was less by 25% and 9% at 10 and 15 MeV, respectively. Similar trends were observed in other electrical properties [transconductance (gm) and gate leakage currents]. This energy dependence from 5 to 15 MeV can be explained by the energy-dependent penetration depth of the proton. Protons with higher kinetic energy can penetrate deeper while creating less numbers of defects at shallow depths where the active layers of the HEMTs are located. These results are in good agreement with stopping and range of ions in matter results. The optimization of the AlGaN/GaN HEMT structure will be critical for space-borne applications where high fluxes of protons are encountered. [ABSTRACT FROM AUTHOR]

The effect of proton irradiation on the off-state drain breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates was studied by irradiating protons from the backside of the samples through via holes fabricated directly under the active area of the HEMTs. There was no degradation of drain current nor enhancement of off-state drain voltage breakdown voltage observed for HEMTs irradiated with 275 keV protons, for which the defects created by the proton irradiation were intentionally placed in the GaN buffer. HEMTs with defects positioned in the 2 dimensional electron gas channel region and AlGaN barrier using 330 keV protons not only showed degradation of both drain current and extrinsic transconductance but also exhibited an improvement of the off-state drain breakdown voltage. Finite-element simulations showed the enhancement of the latter were due to a reduction in electric field strength at the gate edges by introduction of charged defects. [ABSTRACT FROM AUTHOR]

The effects of proton irradiation energy on dc, small signal, and large signal rf characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. AlGaN/GaN HEMTs were irradiated with protons at fixed fluence of 5 × 1015/cm2 and energies of 5, 10, and 15 MeV. Both dc and rf characteristics revealed more degradation at lower irradiation energy, with reductions of maximum transconductance of 11%, 22%, and 38%, and decreases in drain saturation current of 10%, 24%, and 46% for HEMTs exposed to 15, 10, and 5 MeV protons, respectively. The increase in device degradation with decreasing proton energy is due to the increase in linear energy transfer and corresponding increase in nonionizing energy loss with decreasing proton energy in the active region of the HEMTs. After irradiation, both subthreshold drain leakage current and reverse gate current decreased more than 1 order of magnitude for all samples. The carrier removal rate was in the range 121-336 cm-1 over the range of proton energies employed in this study. [ABSTRACT FROM AUTHOR]

The dc characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were measured before and after irradiation with 2 MeV Ge+ ions at doses from 5 × 1010 to 5 × 1012 cm-2. The drain current, gate leakage current, and transconductance decreased monotonically with dose, while the drain-source resistance increased to a much greater extent than observed previously for proton irradiation of similar devices. The data are consistent with a strong decrease in electron concentration in the HEMT channel. During off-state electrical stressing of AlGaN/GaN HEMTs, the typical critical voltage for unirradiated devices was ∼13 V. By sharp contrast, no critical voltage was detected for proton irradiated HEMTs up to 35 V, indicating that the Ge irradiation had a strong influence on the electric field distribution near the gate electrode. [ABSTRACT FROM AUTHOR]

Abstract: In this study, we report on a demonstration of hydrogen sensing at low temperature using SnO2 functionalized AlGaN/GaN high electron mobility transistors (HEMT). The SnO2 dispersion was synthesized via a hydrothermal method and selectively deposited on the gate region of a HEMT device through a photolithography process. The high electron sheet carrier concentration of nitride HEMTs provides an increased sensitivity relative to simple Schottky diodes fabricated on GaN layers. The morphology and crystalline properties of the SnO2-gate, together with the texture of the multilayer films on the device were investigated by SEM, HRTEM, EDS and XRD. The effects of annealing treatment on the crystalline properties of the SnO2-gate, and gas sensing properties of SnO2-gated HEMT sensors were studied. The SnO2-gated HEMT sensor showed fast and reversible hydrogen gas sensing response at low temperature. [Copyright &y& Elsevier]