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The shape of the tube in submerged floating tunnels (SFTs) plays a critical role in determining their performance and safety in marine environments. In this study, a gradient optimization procedure based on the discrete adjoint method is performed to minimize the drag force under uniform currents action. The free form deformation approach is employed to parameterize the design variables. The physical mechanisms of the optimization process are revealed via unsteady numerical simulations and experimental tests. Moreover, the hydrodynamic performance of the tube with the optimal shapes subjected to a wave–current combination is also evaluated. The results show that the drag coefficient is significantly reduced through optimization by reducing the pressure amplitude in the negative-pressure area. Additionally, the lift oscillation is also suppressed to delay structural fatigue, owing to the strength reduction and increased distance of the wake vortex. The experimental analysis indicates the advantage of the optimal shape in reducing the loads under wave–current actions, accompanied by changes in the frequency distribution of the force and vortex structure. The elliptical shape of the SFT's tube provides significant advantages in drag reduction at high Reynold number. Suggestions on the shape design of the section of SFT are given according to different types of constraints. [ABSTRACT FROM AUTHOR]

This study performed a two-dimensional experiment to reveal the load characteristics of a submerged horizontal cylinder with vortex patterns in the wake region under wave–current flows. Wave–current conditions were tested with both following and opposing currents. Instantaneous representations of the velocity field, streamline topology, and vorticity patterns, obtained using particle image velocimetry, provided insight into the origin of the unsteady loading of the cylinder. The spatiotemporal characteristics of the pressure distribution on the cylinder were analyzed using Fourier series expansion across the circumference of the cylinder. The results demonstrate that, unlike the action of current only, the presence of waves modifies the frequency and position of vortex shedding, resulting in three distinct modes in the frequency domain of the vertical force on the cylinder, where each mode exhibits a distinct mode of vortex shedding. The variation in frequency domain of Fourier coefficients of pressure around the cylinder is consistent with the force corresponding to different modes. Moreover, velocity rate β and the frequency relationship between the wave and current-induced vortex shedding play important roles in the frequency characteristics of the vertical force. [ABSTRACT FROM AUTHOR]

In this Letter, trapping effects of a Schottky lightly Mg-doped p-GaN gate stack for low-power applications have been investigated, and further analysis focusing on AlGaN/GaN interface traps under γ-ray irradiation has been carried out. A negligible shift in the flatband voltage with γ-ray irradiation dose up to 800 krad indicates the superior radiation tolerance of the p-GaN gate structure. The difference between capacitance dispersion at the measurement frequency below and above 500 kHz is observed, which is attributed to trapping effects in different locations with varying gate voltage. Moreover, the frequency-dependent conductance method is put forward to assess the effects of different doses of γ-ray irradiation on the AlGaN/GaN interface traps. Based on that method, aside from the shallow trap states [the trap activation energy (ET) is about 0.334–0.338 eV] previously found in the traditional normally on high electron mobility transistor (HEMT), another type of deeper trap states at the AlGaN/GaN interface (ET is about 0.467–0.485 eV) is detected. It is observed that the ET of shallow trap states distributes at a deeper and broader range as the irradiation dose increases. Additionally, the trap density decreased after 600 krad doses irradiation but increased after 800 krad doses irradiation for both deep and shallow ET. Transmission electron microscopy and atomic force microscopy are used to demonstrate the smooth AlGaN/GaN interface morphology, which will not be greatly damaged after 800 krad doses of γ-ray irradiation. This work can provide a further understanding of radiation tolerance and trapping effects of p-GaN gate HEMTs for low-voltage applications. [ABSTRACT FROM AUTHOR]

In this Letter, we report on the enhanced radio frequency (RF) performance in sub-micrometer scaled β-Ga2O3 tri-gate FinFETs. With a 200-nm-thick β-Ga2O3 bulk channel and a 0.35 μm gate length, the FinFETs exhibit an improved current-gain cutoff frequency of 5.4 GHz and a maximum oscillation frequency of 11.4 GHz, which are 20% and 58% improved with respect to the planar counterpart, respectively. The improved RF performance results from the enhanced gate control capability and the suppressed short-channel effects, as evidenced by the improved pinch-off characteristics, the improved transconductance, and the suppressed output conductance. It suggests that the tri-gate multi-fin architecture is a promising strategy to break the scaling limitation of the gate-channel aspect ratio toward high-performance β-Ga2O3 RF MOSFETs. [ABSTRACT FROM AUTHOR]

Properties of minority carrier (electron) traps in Schottky type p-GaN gate high electron mobility transistors were explicitly investigated by optical deep level transient spectroscopy (ODLTS). By temperature-scanning ODLTS, three electron traps, namely, E1, E2, and E3, were revealed, together with activation energy, capture cross section, and trap concentration. A thermally accelerated electron-releasing process of traps was quantitatively studied by Laplace ODLTS with individual emission time constant disclosed. At 300 K, the emission time constant was determined to be 0.21 and 1.40 s for E2 and E3, respectively, which adjacently existed in the bandgap and held activation energies of over 0.6 eV. As varying the optical injection pulse duration, a three-dimensional mapping of capacitance transient was obtained for each trap, attesting to the electron capture capability of each trap. By varying the reverse bias, the analysis of the ODLTS signal amplitude indicates that all three electron traps are located inside the p-GaN layer rather than the surface defect related. [ABSTRACT FROM AUTHOR]

Threshold voltage analysis can help reveal the reliability of semiconductor transistors and its underlying mechanism. Herein, negative constant voltage stress (NCVS)-induced threshold voltage instability is studied in hydrogen-terminated diamond metal-oxide-semiconductor field-effect transistors (HD MOSFETs) with an Al2O3 dielectric layer deposited via atomic layer deposition at 90 °C. An unusual bidirectional shift in threshold voltage (Vth) can be observed with time. When a weak gate NCVS is applied, Vth gradually decreases during the first 500 s but increases in the next 500 s. A similar but opposite phenomenon is observed when the HD MOSFETs are in a recovery stage upon removing the NCVS, i.e., Vth increases in the first 500 s but decreases in the next 4500 s. A kinetic hydrogen motion model shows that this phenomenon can be attributed to the larger characteristic time constant of the unactuated oxygen-dangling bonds (UODBs) compared to that of the traps in the gate dielectric. Consequently, the trapping effect dominates and decreases Vth at the onset of NCVS. After 500 s, the UODB effects can be observed, increasing Vth. In the recovery stage, Vth is larger than the initial value. Further, modified hydrogen kinetic equations accounting for the dynamic effects of UODBs and traps are provided to quantitatively analyze the results. [ABSTRACT FROM AUTHOR]

RF power characteristics of hydrogen-terminated diamond metal-insulator-semiconductor field-effect transistors operating at 10 GHz have been reported, and an output power density of 182 mW/mm has been achieved. The diamond devices were fabricated by using a self-aligned process combined with 0.1 μm gate-length T-shaped gates. A high quality atomic layer deposition Al2O3 film with low leakage current was deposited as a gate dielectric by using H2O as the oxidant at a low temperature of 90 °C. The direct current output characteristics of the devices were measured by applying gate voltage in opposite directions, and a maximum drain current density of 741 mA/mm has been obtained. By small-signal measurements, the diamond device demonstrates a high extrinsic cutoff frequency fT of 66 GHz and a maximum frequency of oscillation fmax of 55 GHz. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of microscopic features on the tooth surface of a geared rotor-bearing system on the system's dynamic behavior. First, by upgrading the Weierstrass–Mandelbrot function to redefine the gear backlash equation and combining this with a nonlinear dynamic model of a geared system, an improved nonlinear dynamic model is established. Second, the effects of four crucial model parameters on the gear dynamics are examined through a series of numerical simulations, allowing us to explore the corresponding dynamic response and the model's feasibility. The results show that surface roughness plays a significant role in the dynamic behavior of the gears, with lower surface roughness corresponding to better dynamic performance. The fractal dimension is found to be a non-negligible factor with respect to gear dynamics, and decreasing the fractal dimension proves advantageous to the dynamic response. As the stiffness coefficient decreases and the gear meshing damping increases, the stability of the system improves. Finally, the model is validated through a comparison of our theory with experimental data and another certified model. Our theory constructs a mathematical relationship between the gear dynamics and tooth surface micro-characteristics, forming a basis for future tooth surface topographies as well as a reference for the dynamic analysis of other highly paired systems. [ABSTRACT FROM AUTHOR]

Asymmetrical shift behaviors of capacitance-voltage (C-V) curve with opposite direction are observed in two AlGaN/GaN metal-ferroelectric-semiconductor (MFS) heterostructures with Pb(Zr,Ti)O3 and LiNbO3 gate dielectrics. By incorporating the switchable polar nature of the ferroelectric into a self-consistent calculation, the coupling effect between the ferroelectric and the interface charges is disclosed. The opposite initial orientation of ferroelectric dipoles determined by the interface charges is essentially responsible for the different C-V characteristics. A critical fixed charge density of -1.27 × 1013 cm-2 is obtained, which plays a key role in the dependence of the C-V characteristic on the ferroelectric polarization. The results pave the way for design of memory devices based on MFS structure with heteropolar interface. [ABSTRACT FROM AUTHOR]

Cubic In2O3 microtubes were prepared by thermally oxidizing InN counterparts under controlled conditions. Similar to the starting InN sample, the converted In2O3 microtubes are hexagonal in cross section and exhibit helical motifs of both right-handed and left-handed architectures with helical angles of 0°–30°. InN microtubes can be retrieved by renitridation of the oxide microtubes. This work demonstrates that it is feasible to fabricate specific oxide nano-/microstructures from its corresponding nitride counterparts via controlled oxidation manner and vice versa. Temperature-dependent Raman scattering in both InN and In2O3 microtubes was measured and obvious downshifts of Raman modes were observed with increasing temperature. [ABSTRACT FROM AUTHOR]

Monolithic integrated enhancement/depletion (E/D)-mode AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated on an AlGaN/GaN heterostructure with an engineered triple-cap-layer. The energy band of the cap layer is greatly tailored by the polarizations within it, which improves the controllability of D-to-E mode conversion with gate recess. The uniformity of the threshold voltage (Vth) across a 3″ wafer is assessed and the standard deviations of Vth are 0.1 V and 0.14 V for E-mode and D-mode devices, respectively. Direct-coupled field-effect transistor logic E/D HEMT inverter and 17-stage ring oscillator are demonstrated, and the latter shows a oscillation frequency of 201 MHz at a supply voltage of 1 V, corresponding to a propagation delay of 146 ps/stage and a power-delay product of 1.96 pJ/stage. [ABSTRACT FROM AUTHOR]