Investigation of the Gate Degradation Induced by Forward Gate Voltage Stress in p-GaN Gate High Electron Mobility Transistors.

In this work, we investigated the degradation of the p-GaN gate stack induced by the forward gate voltage stress in normally off AlGaN/GaN high electron mobility transistors (HEMTs) with Schottky-type p-GaN gate. The gate stack degradations of p-GaN gate HEMTs were investigated by performing the gate step voltage stress and the gate constant voltage stress measurements. In the gate step voltage stress test, the positive and negative shifts of threshold voltage (VTH) depended on the range of the gate stress voltage (VG.stress) at room temperature. However, the positive shift of VTH in the small gate stress voltage was not observed at 75 and 100 °C and the negative shift of VTH was started from a lower gate voltage at a high temperature compared to room temperature. In the gate constant voltage stress test, the gate leakage current increased with three steps in the off-state current characteristics as the degradation progressed. To investigate the detailed breakdown mechanism, we measured the two terminal currents (IGD and IGS) before and after the stress test. The difference between the gate-source current and the gate-drain current in the reverse gate bias indicated that the increase of the leakage current was attributed to the degradation between the gate and the source while the drain side was not affected.

High Selectivity Hydrogen Gas Sensor Based on WO3/Pd-AlGaN/GaN HEMTs.

We investigated the hydrogen gas sensors based on AlGaN/GaN high electron mobility transistors (HEMTs) for high temperature sensing operation. The gate area of the sensor was functionalized using a 10 nm Pd catalyst layer for hydrogen gas sensing. A thin WO3 layer was deposited on top of the Pd layer to enhance the sensor selectivity toward hydrogen gas. At 200 °C, the sensor exhibited high sensitivity of 658% toward 4%-H2, while exhibiting only a little interaction with NO2, CH4, CO2, NH3, and H2S. From 150 °C to 250 °C, the 10 ppm hydrogen response of the sensor was at least eight times larger than other target gases. These results showed that this sensor is suitable for H2 detection in a complex gas environment at a high temperature.

Temperature- and Frequency-Dependent Ferroelectric Characteristics of Metal-Ferroelectric-Metal Capacitors with Atomic-Layer-Deposited Undoped HfO2 Films.

In this study, we evaluated the temperature- and frequency-dependent ferroelectric characteristics of TiN/undoped HfO2/TiN metal-ferroelectric-metal (MFM) capacitors in which an undoped HfO2 film was deposited through atomic layer deposition (ALD). Successful ferroelectric characteristics were achieved after postdeposition annealing at 650 °C, which exhibited a remanent polarization of 8 µC/cm2 and a coercive electric field of 1.6 MV/cm at 25 °C (room temperature). The ferroelectric property was maintained at 200 °C and decreased as the temperature increased. The ferroelectric property was completely lost above 320 °C and fully recovered after cooling. The frequency dependency was evaluated by bias-dependent capacitance-voltage and s-parameter measurements, which indicated that the ferroelectric property was maintained up to several hundred MHz. This study reveals the ultimate limitations of the application of an undoped HfO2 MFM capacitor.

Response Enhancement of Pt-AlGaN/GaN HEMT Gas Sensors by Thin AlGaN Barrier with the Source-Connected Gate Configuration at High Temperature.

AlGaN/GaN HEMT hydrogen gas sensors were optimized by AlGaN barrier thickness in the gate-source connected configuration demonstrated high response and robust stability up to 500 °C. First, we found that the hydrogen sensing performance of a conventional normally-on HEMT-based sensor was enhanced when zero voltage was applied on the gate in comparison with a floating-gate condition due to a reduced level of the base current. In the next step, to take advantage of the response increase by VGS = 0 V, a new type of sensor with a source-connected gate (SCG) was fabricated to utilize the normally-on operation of the GaN HEMT sensor as a two-terminal device. AlGaN barrier thickness was thinned by the dry-etching process to gain higher transconductance at a zero-gate bias with the reduction of the distance from the 2DEG channel to the AlGaN surface, thereby significantly improve the hydrogen response. The SCG GaN sensor with an ultra-thin AlGaN barrier (9 nm) exhibited responses of 85% and 20% at 200 and 500 °C, respectively, onto 4%-hydrogen gas, which demonstrates a promising ability for harsh environment applications.

Performance Optimization of Nitrogen Dioxide Gas Sensor Based on Pd-AlGaN/GaN HEMTs by Gate Bias Modulation.

We investigated the sensing characteristics of NO2 gas sensors based on Pd-AlGaN/GaN high electron mobility transistors (HEMTs) at high temperatures. In this paper, we demonstrated the optimization of the sensing performance by the gate bias, which exhibited the advantage of the FET-type sensors compared to the diode-type ones. When the sensor was biased near the threshold voltage, the electron density in the channel showed a relatively larger change with a response to the gas exposure and demonstrated a significant improvement in the sensitivity. At 300 °C under 100 ppm concentration, the sensor's sensitivities were 26.7% and 91.6%, while the response times were 32 and 9 s at VG = 0 V and VG = -1 V, respectively. The sensor demonstrated the stable repeatability regardless of the gate voltage at a high temperature.

High-Sensitivity Hydrogen Sensor Based on AlGaN/GaN Heterojunction Field-Effect Transistor.

We have developed a Pd-functionalized hydrogen gas sensor based on a recessed AlGaN/GaN heterostructure field-effect transistor. The AlGaN barrier layer under the Pd catalyst was partially etched to enhance its sensitivity. Both low-power consumption and high sensitivity were achieved by employing a recessed structure. Sensor characterization was carried out at the temperature range from room temperature to 250 °C, among which the best sensing characteristics were observed at 200 °C. A sensitivity of 380% with a response time of 0.25 s was achieved at a bias voltage of 0.3 V at 200 °C under a hydrogen exposure concentration of 4%. The standby power consumption was only 2 µW for the sensing area of 100×28 µm² due to the low standby current, which was caused by the recessed AlGaN barrier layer.

Extracorporeal membrane oxygenation for takotsubo cardiomyopathy that developed after mitral valve replacement.

Takotsubo cardiomyopathy is a transient systolic and diastolic left ventricular dysfunction that presents several wall-motion abnormalities, while the coronary artery shows normal findings. Because patients with Takotsubo cardiomyopathy present with symptoms similar to acute coronary syndrome, the initial diagnosis and treatment are often difficult. The condition is often precipitated by acute emotional or physical stress and frequently occurs in postmenopausal women. Takotsubo cardiomyopathy may also occur in the perioperative period after cardiac and noncardiac surgery; surgery-associated Takotsubo cardiomyopathy reportedly accounts for 3%-23% of all cases. Of these perioperative cases, cardiothoracic surgery accounted for 16%. However, few cases have been reported in patients undergoing cardiac surgery and managed with extracorporeal membrane oxygenation (ECMO). We report a case of Takotsubo cardiomyopathy managed with ECMO in a patient in the intensive care unit after mitral valve replacement.

Influence of Oxygen-Plasma Treatment on In-Situ SiN/AlGaN/GaN MOSHEMT with PECVD SiO2 Gate Insulator.

The influence of oxygen-plasma treatment on in situ SiN/AlGaN/GaN MOS high electron mobility transistor with SiO2 gate insulator was investigated. Oxygen-plasma treatment was performed on in situ SiN, before SiO2 gate insulator was deposited by plasma-enhanced chemical vapor deposition (PECVD). DC I-V characteristics were not changed by oxygen plasma treatment. However, pulsed I-V characteristics were improved, showing less dispersion compared to non-treated devices. During short-term gate bias stress, the threshold voltage shift was also smaller in a treated device than in an untreated one. X-ray photoemission spectroscopy also revealed that SiO2 on in situ SiN with oxygen-plasma treatment has an O/Si ratio close to the theoretical value. This suggests that the oxygen plasma treatment-modified surface condition of the SiN layer is favorable to SiO2 formation by PECVD.

Proton Irradiation Effects on the Time-Dependent Dielectric Breakdown Characteristics of Normally-Off AlGaN/GaN Gate-Recessed Metal-Insulator-Semiconductor Heterostructure Field Effect Transistors.

In this work, we investigated the time-dependent dielectric breakdown (TDDB) characteristics of normally-off AlGaN/GaN gate-recessed metal-insulator-semiconductor (MIS) heterostructure field effect transistors (HFETs) submitted to proton irradiation. TDDB characteristics of normally-off AlGaN/GaN gate-recessed MISHFETs exhibited a gate voltage (VGS) dependence as expected and showed negligible degradation even after proton irradiation. However, a capture emission time (CET) map and cathodoluminescence (CL) measurements revealed that the MIS structure was degraded with increasing trap states. A technology computer aided design (TCAD) simulation indicated the decrease of the vertical field beneath the gate due to the increase of the trap concentration. Negligible degradation of TDDB can be attributed to this mitigation of the vertical field by proton irradiation.

Investigation of Stability and Power Consumption of an AlGaN/GaN Heterostructure Hydrogen Gas Sensor Using Different Bias Conditions.

A Pd-functionalized hydrogen gas sensor was fabricated on an AlGaN/GaN-on-Si heterostructure platform. The AlGaN layer under the Pd catalyst area was partially recessed by plasma etching, which resulted in a low standby current level enhancing the sensor response. Sensor stability and power consumption depending on operation conditions were carefully investigated using two different bias modes: constant voltage bias mode and constant current bias mode. From the stability point of view, high voltage operation is better than low voltage operation for the constant voltage mode of operation, whereas low current operation is preferred over high current operation for the constant current mode of operation. That is, stable operation with lower standby power consumption can be achieved with the constant current bias operation. The fabricated AlGaN/GaN-on-Si hydrogen sensor exhibited excellent sensing characteristics; a response of 120% with a response time of < 0.4 s at a bias current density of 1 mA/mm at 200 °C. The standby power consumption was only 0.54 W/cm2 for a sensing catalyst area of 100 × 24 µm2.

Replacement of dexmedetomidine loading with midazolam for sedation in elderly patients with spinal anesthesia.

BACKGROUND: Dexmedetomidine is an effective sedative during spinal anesthesia. However, it requires a loading dose, which can result in transient hypertension, hypotension, bradycardia, and/or sinus arrest. In addition, the time required to reach an appropriate depth of sedation may cause anxiety to the patients. Therefore, we examined whether an intravenous bolus of midazolam could replace the loading dose of dexmedetomidine for sedation during surgery in elderly patients who received spinal anesthesia. METHODS: Patients aged over 60 years who scheduled to undergo total knee arthroplasty under spinal anesthesia were enrolled in this study. The patients were randomized into 2 groups. Patients in dexmedetomidine group (group D) (n = 20) were administered a loading dose of dexmedetomidine (1.0 µg/kg over 10 min) intravenously followed by dexmedetomidine maintenance (0.5 µg/kg/h). Patients in group MD (n = 20) were administered an intravenous midazolam (0.05 mg/kg) followed by dexmedetomidine maintenance (0.5 µg/kg/h) intravenously. Heart rate (HR), mean arterial blood pressure (MBP), peripheral oxygen saturation (SpO2), and patient state index (PSI) were recorded. Ramsay sedation scale (RSS) scores were evaluated at 10 minutes after drug administration and the end of surgery. RESULTS: A total of 40 subjects were enrolled in the present study. At baseline, there was no between-group difference in HR. Ten minutes after drug administration, group D had lower HR than group MD (62.1 ±â€Š9.4 versus 69.6 ±â€Š13.4, P = .047). PSI was significantly lower in group MD at 10 minutes after drug administration (82.8 ±â€Š13.0 versus 72.0 ±â€Š16.0, P = .024); there was no between-group difference at 30 and 60 minutes, and lower values in group D at the end of surgery (70.2 ±â€Š22.6 versus 79.7 ±â€Š10.9, P = .011). The RSS score showed statistically significantly deeper sedation in group MD 10 minutes after drug administration, but no difference at the end of surgery. CONCLUSIONS: An intravenous bolus of midazolam is a viable alternative to dexmedetomidine loading for sedation during surgery in elderly patients who received spinal anesthesia. This is especially effective for patients who are at high risk for bradycardia or who want a faster sedation.

Enhanced UV absorption of GaN photodiodes with a ZnO quantum dot coating layer.

Light absorption at the surface of a photodiode can be enhanced by employing nanostructures smaller than the wavelength of interest. In this study, a ZnO quantum dot (QD) coating layer was investigated for improving the light absorption of gallium nitride (GaN) ultraviolet (UV) photodiodes. A GaN surface coated with a ZnO QD solution exhibited significantly lower surface reflection than an uncoated GaN surface, which, in turn, improved the responsivity of the GaN photodiode. In comparison with other nanostructure or multilayer thin film processes, the proposed ZnO QD coating process is simple and effective in enhancing UV light absorption.

Simple large-scale synthesis of hydroxyapatite nanoparticles: in situ observation of crystallization process.

The noble synthesis method for hydroxyapatite (HAp) nanoparticles was exploited using a fairly simple reaction of Ca(OH)(2) and H(3)PO(4), which does not generate residual harmful anions and consequently does not need an additional washing process. HAp nanoparticles were found to yield from dicalcium phosphate dehydrate (DCPD) as the only intermediate phase, which was monitored by in situ observation study using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), (1)H and (31)P magic-angle spinning (MAS) NMR. Furthermore, we found that the phase evolution of HAp was preceded by heteronucleation of HAp onto the DCPD surface. The combination of scanning electron microscopy (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-ES) analysis gave more information on the HAp crystallization process, which was found to be retarded by the residual Ca(OH)(2) and slow diffusion process of Ca ions into the interface between HAp and DCPD. These results demonstrate that the synthesis of pure HAp nanoparticles with high throughput can be achieved by controlling the residual Ca(OH)(2) and diffusion process of Ca ions.