Comprehensive Schottky Barrier Height Behavior and Reliability Instability with Ni/Au and Pt/Ti/Pt/Au on AlGaN/GaN High-Electron-Mobility Transistors

The reliability instability of inhomogeneous Schottky contact behaviors of Ni/Au and Pt/Ti/Pt/Au gate contacts on AlGaN/GaN high-electron-mobility transistors (HEMTs) was investigated via off-state stress and temperature. Under the off-state stress condition, Pt/Ti/Pt/Au HEMT showed abruptly reduced reverse leakage current, which improved the Schottky barrier height (SBH) from 0.46 to 0.69 eV by suppression of the interfacial donor state. As the temperature increased, the reverse leakage current of the Pt/Ti/Pt/Au AlGaN/GaN HEMT at 308 K showed more reduction under the same off-state stress condition while that of the Ni/Au AlGaN/GaN HEMT increased. However, with temperatures exceeding 308 K under the same off-state stress conditions, the reverse leakage current of the Pt/Ti/Pt/Au AlGaN/GaN HEMT increases, which can be intensified using the inverse piezoelectric effect. Based on this phenomenon, the present work reveals the necessity for analyzing the concurrent SBH and reliability instability due to the interfacial trap states of the MS contacts.

Investigating two−stage degradation of threshold voltage induced by off−state stress in AlGaN/GaN HEMTs

In this work, a two-step degradation phenomenon in D-mode Si3N4/AlGaN/GaN metal-insulator-semiconductor high−electron−transistors (MIS−HEMT) is discussed systematically. During off−state stress, threshold voltage shifts positively for a short duration, and is followed by a negative shift. In contrast, the off−state leakage continues to decrease throughout the entire stress. Results of varied measurement conditions indicate that carrier trapping at different regions dominates this phenomenon. It is interesting that under a large lateral electric field, electron−hole pairs are generated and will then be trapped at the gate dielectric layer. Furthermore, when increasing the stress temperature, impact ionization due to carriers from the gate electrode becomes more severe. Finally, devices with different gate insulator (GI) thicknesses are performed to verify the physical model of the degradation behavior.

Experimental and numerical investigation of Poole-Frenkel effect on dynamic R ON transients in C-doped p-GaN HEMTs

In this paper, we investigate the influence of Poole-Frenkel Effect (PFE) on the dynamic R ON transients in C-doped p-GaN HEMTs. To this aim, we perform a characterization of the dynamic R ON transients acquired during OFF-state stress (i.e., V GS,STR = 0 V < V T, V DS,STR = 25–125 V and we interpret the results with the aid of numerical simulations. We find that dynamic R ON transients at room temperature accelerate with V DS,STR 1/2, which is signature of PFE, as further confirmed by the simultaneous decrease of the activation energy (E A) extracted from the Arrhenius plot of the dynamic R ON transients at V DS,STR = 50 V and T = 30–110 °C. Results obtained by means of calibrated numerical simulations reproduce the exponential dependence of transients time constants (τ) on V DS,STR 1/2 and consequent E A reduction only when including PFE enhancement of hole emission from dominant acceptor traps in the buffer related to C doping. This result is consistent with the model that considers hole emission from acceptor traps (rather than electron capture) as the mechanism underlying dynamic R ON increase during OFF-state stress.

Suppressing Buffer-Induced Current Collapse in GaN HEMTs with a Source-Connected p-GaN (SCPG): A Simulation Study

Carbon doping in the buffer of AlGaN/GaN high-electron-mobility transistors (HEMTs) leads to the notorious current collapse phenomenon. In this paper, an HEMT structure with a source-connected p-GaN (SCPG) embedded in the carbon-doped semi-insulating buffer is proposed to suppress the buffer-induced current collapse effect. Two-dimensional transient simulation was carried out to show the successful suppression of buffer-induced current collapse in the SCPG-HEMTs compared with conventional HEMTs. The mechanism of suppressing dynamic on-resistance degradation by ejecting holes from the SCPG into the high resistive buffer layer after off-state stress is illustrated based on energy band diagrams. This paper contributes an innovative device structure to potentially solve the buffer-induced degradation of the dynamic on-resistance in GaN power devices.

On the Large Amplitude Forced Vibration Analysis of Composite Sectorial Plates

The nonlinear steady state large amplitude forced vibration response of a laminated composite annular sector plate is presented. The nonlinear governing equation of motion of the laminated composite annular sector plate has been obtained using kinematics of first-order shear deformation theory (FSDT) and employing Hamilton’s principle. The governing equations of motion have been solved in a time domain using a modified shooting method and arc-length/pseudo-arc length continuation technique. The influence of the boundary condition, sector angle, and annularity ratio on the linear as well as nonlinear steady state forced vibration response has been investigated. The strain/stress variation across the thickness of the annular sector plate is presented to explain the reason for a decrease/increase in hardening nonlinear behaviour. The periodic variation of the non-linear steady state stress has also been obtained to throw light into the factors influencing the unequal stress half cycles and multiple cyclic stress reversals, which is detrimental to the fatigue design of laminated composite annular sectorial plates. The frequency spectra of the steady state stress reveals large even and odd higher harmonic contributions for different cases due to changes in the restoring force dynamics. The modal interaction/exchange during a cycle is demonstrated using a deformed configuration of the laminated annular sector plate.

Assessing the Stress of Surgeons and Surgical Residents Using a Wearable Smart Device

Although surgeon is one of the most stressful professions, only few studies have attempted to evaluate surgeons’ stress using impractical methods. Meanwhile, many wearable devices have been introduced in the health-care market. This study aimed to assess surgeons’ stress using a wearable device. Data were collected from 13 participants from June to September 2019. We checked level of stress, heart rate (HR) using Vivosmart4 (Garmin, Schaffhausenm, Switzerland) at rest and perioperatively, and also checked their perioperative self-perceived stress using the short-form State-Trait Anxiety Inventory (STAI). The perioperative stress level and HR significantly increased compared with resting state (stress level: 28.6 ± 18.2 at rest vs. 49.6 ± 25.5 before surgery vs. 55.1 ± 25.5 after surgery, p < 0.001; HR: 81.1 ± 6.2 at rest vs. 85.0 ± 11.5 before surgery vs. 85.0 ± 12.2 after surgery, p = 0.001). Scores on the short-form STAI significantly decreased after surgery (12.6 ± 4.9 before surgery vs. 11.7 ± 3.6 after surgery, p = 0.001). Stress level at rest was significantly higher among fellows and residents compared with professors (fellows: 40.7 ± 15.3 vs. residents: 29.9 ± 12.0 vs. professors: 13.2 ± 7.3, p < 0.001). We assessed surgeons’ stress using a smart device and demonstrated that surgery significantly increased stress. The level of stress was higher among fellows and residents compared with professors.