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Series of experiments were performed to simulate the invasion of formation sand into and the plugging process of gravel-pack at different viscosities and flowing rates of fluid. Two types of formation sands with the medium size of 0.10 mm and 0.16 mm and the quartz sand and ceramsite of 0.6—1.2 mm were used in the experiments. A new viscosity-velocity index (the product of fluid viscosity and velocity) was put forward to characterize the influencing mechanism and law of physical property and flow condition of formation fluid on gravel-pack plugging, and a new method to optimize the production rate of wells controlling sand production with gravel-packing was proposed. The results show that the permeability of formation sand invaded zone and final permeability of plugged gravel-pack have negative correlations with viscosity and flow velocity of fluid, the higher the flow velocity and viscosity, the lower the permeability of formation sand invaded zone and final permeability of plugged gravel-pack will be. The flow velocity and viscosity of fluid are key factors affecting plugging degree of the gravel zone. The viscosity-velocity index (v-v index) can reflect the flow characteristics of fluid very well and make it easier to analyze the plugging mechanism of gravel zone. For different combinations of fluid viscosity and flow velocity, if the v-v index is the same or close, their impact on the final gravel permeability would be the same or close. With the increase of the v-v index, the permeability of plugged gravel zone decreases first, then the reduction rate slows down till the permeability stabilizes. By optimizing production and increasing production step by step, the optimal working scheme for sand-control well can reduce the damage to gravel-pack zone permeability caused by sand-carrying fluid effectively, and increase well productivity and extend the sand control life. Key words: gravel-pack, sand retention experiment, plugging law, viscosity-velocity index, working scheme, optimization method, plugging simulation

This paper presents a novel nonlinear control approach for a medium density fiberboard continuous hot pressing hydraulic system. Uncertainties, disturbances, and input saturation are explicitly taken into account. The proposed controller incorporates a smooth function by using a hyperbolic tangent function to substitute for the saturation nonlinearity in the system. Moreover, a novel backstepping-like slab thickness tracking controller is developed for a third-order cascade system within two steps. Taking advantages of radial basis function neural network (RBFNN) technique, a RBFNN-based reconstruction law is introduced to approximate the composite term consisting unknown function, disturbances, and saturation error. Lyapunov stability analysis shows that the designed control algorithm guarantees the asymptotic stability of the system with great robustness. Numerical simulation results are also exhibited to authenticate and validate the benefits of the proposed control scheme.

This paper presents a novel nonlinear control approach for medium-density fiberboard continuous hot pressing electro-hydraulic servo system (EHSS). The EHSS is subject to input saturation and unknown external disturbance. The approach is developed in the framework of the prescribed performance control design. The transient and steady behaviors are both considered by virtue of an appropriate performance function. A new robust adaptive neural control algorithm is then developed by introducing a dynamic surface control scheme. Moreover, the compensation control is designed for the nonlinear term arising from the input saturation. It is shown with the Lyapunov stability analysis that all the signals in the closed-loop system are semi-globally uniformly ultimately bounded. In addition, the system tracking performance regarding transient and steady-state behaviors is guaranteed effectively with input saturation via the designed control approach. Finally, numerical simulation results are presented to authenticate and validate the effectiveness of the proposed control scheme.

Hybrid white light emitting diodes (LEDs) have been developed utilizing micro-LEDs radiatively pumping down-conversion phosphor materials. The cyan InGaN/GaN multiple quantum wells LEDs with 95 μm in square are fabricated into 4 × 4 pixels as one unit in 4 in wafer by photolithography and inductively coupled plasma etching techniques. The relatively low turn on voltage and low leakage of cyan micro-LEDs indicate good electrical performance. A significant reduction in the efficiency droop characteristics of cyan micro-LED has been observed in comparison with that of standard green LED. Finally, based on a systemic optimization for white emission indexes, high-quality hybrid white light emission has been demonstrated by the combination of blue and cyan micro-LEDs with two types of phosphors, which have a high color rendering index up to 87 at the correlated color temperatures of 6500 K. It also demonstrates a way to adjust the color component by modifying the output power of cyan micro-LED in hybrid LED, showing a promising candidate in a large range of applications including micro-displays, bioinstrumentation, photolithography, and lab-on-chip systems in the future.

To improve the RadCom system of performing both data transmission and radar-sensing functions, here a waveform design method for orthogonal frequency division multiplexing (OFDM) signal is proposed to solve the problem of high range sidelobe level in radar detection process while maintaining a reasonable channel capacity. The method only transmits data in the sub-channels with preferable channel quality. Then, calculate the weight of the chosen subcarriers through the convex optimisation method to traditionally minimise the peak-to-sidelobe (PSL) ratio. Simulation results prove that, compared with traditional range sidelobe suppression method, this algorithm is more suitable for RadCom system.

Junction temperature (Tj) and current have important effects on light-emitting diode (LED) properties. Therefore, the electroluminescence (EL) spectra of blue and green LEDs were investigated in a Tj range of 120−373 K and in a current range of 80−240 mA based on accurate real-time measurements of Tj using an LED with a built-in sensor unit. Two maxima of the emission peak energy with changing Tj were observed for the green LED, while the blue LED showed one maximum. This was explained by the transition between the donor-bound excitons (DX) and free excitons A (FXA) in the green LED. At low temperatures, the emission peak energy, full width at half maximum (FWHM), and radiation power of the green LED increase rapidly with increasing current, while those of the blue LED increase slightly. This is because when the strong spatial potential fluctuation and low exciton mobility in the green LED is exhibited, with the current increasing, more bonded excitons are found in different potential valleys. With a shallower potential valley and higher exciton mobility, excitons are mostly bound around the potential minima. The higher threshold voltage of the LEDs at low temperatures may be due to the combined effects of the band gap, dynamic resistance, piezoelectric polarization, and electron-blocking layer (EBL).

The dc and radio-frequency (rf) characteristics of [Al.sub.0.3][Ga.sub.0.7]N/GaN/[In.sub.0.1][Ga.sub.0.9]N/GaN double-heterojunction HEMT (DH-HEMT) structure are analyzed. The E-mode devices with good dc, RF small signal, and power performances are fabricated on the InGaN-notch DH-HEMT structure by using C[F.sub.4] plasma treatment before the deposition of the gate electrodes.

A fabrication method with an accurate control of threshold voltages ([V.sub.th] of AlGaN/GaN high-electron mobility transistors (HEMTs) using a fluoride-based plasma treatment is presented, whereby the [V.sub.th] of AlGaN/GaN HEMTs can be continuously shifted from -4V in a conventional depletion-mode HEMT to 0.9V in an enhancement-mode HEMT. A physical model of the threshold voltage is provided to analyze the effects of the fluoride-based plasma treatment on AlGaN/GaN HEMTs.

Authentic travel documentaries provide language learners with a deep insight info the target language and culture. A travel documentary not only takes people to travel hut also serves as a miniature encyclopedia encompassing such aspects of the target country as history, politics, culture, geology, music, military, society, economy, etc. which is a rich resource for language learners to probe into. The combination of travel documentaries and iBooks offers an immersion-like learning experience where people feel like they are personally on the scene. It is hoped that this can offer a unique and innovative perspective for language teaching and learning. [ABSTRACT FROM AUTHOR]

A low phase-noise X-band monolithic-microwave integrated-circuit voltage-controlled oscillator (VCO) based on a novel high-linearity and low-noise composite-channel A10.3 Ga0.7N/A10.05 Ga0.95 N/GaN high electron mobility transistor (HEMT) is presented. The HEMT has a 1 μm × 100 μm gate. A planar inter-digitated metal-semiconductor-metal varactor is used to tune the VCO's frequency. The polyimide dielectric layer is inserted between a metal and GaN buffer to improve the Q factor of spiral inductors. The VCO exhibits a frequency tuning range from 9.11 to 9.55 GHz with the varactor's voltage from 4 to 6 V, an average output power of 3.3 dBm, and an average efficiency of 7% at a gate bias of -3 V and a drain bias of 5 V. The measured phase noise is -82 dBc/Hz and -110 dBc/Hz at offsets of 100 kHz and 1 MHz at a varactor's voltage (Vtune) = 5 V. The phase noise is the lowest reported thus far in VCOs made of GaN-based HEMTs. In addition, the VCO also exhibits the minimum second harmonic suppression of 47 dBc. The chip size is 1.2 × 1.05 mm2. [ABSTRACT FROM AUTHOR]

We present the design, fabrication, and characterization of AIGaN-GaN double-channel HEMTs. Two carrier channels are formed in an AIGaN-GaN-AIGaN--GaN multilayer structure grown on a sapphire substrate. Polarization field in the lower AIGaN layer fosters formation of a second carrier channel at the lower AIGaN-GaN interface, without creating any parasitic conduction path in the AIGaN barrier layer. Unambiguous double-channel behaviors are observed at both dc and RF. Bias dependent RF small-signal characterization and parameter extraction were performed. Gain compression at a high current level was attributed to electron velocity degradation induced by interface scattering. Dynamic IV measurement was carried out to analyze large-signal behaviors of the double-channel high-electron mobility transistors. It was found that current collapse mainly occurs in the channel closer to device surface, while the lower channel suffers minimal current collapse, suggesting that trapping/detrapping of surface states is mainly responsible for current collapse. This argument is supported by RF large-signal measurement results. [ABSTRACT FROM AUTHOR]

The correlation between the resistivity of an undoped GaN/Al2O3 interfacial layer and in-situ reflectance spectrum in metalorganic chemical vapor deposition and the mechanism of this correlation were investigated. The first minimum reflectance during the initial high-temperature GaN growth was found to be a good indicator of the resistivity of the GaN buffer. The background electron concentration and mobility were both higher in the samples with higher indicative reflectance at that point. The resistivity of the GaN buffer layer was predominantly determined by an ∼0.25-µm-thick layer near the GaN/Al2O3 interface. Atomic force microscope (AFM) and high-resolution x-ray diffraction (HRXRD) results showed that the samples with higher indicative reflectance had smaller sized but higher density nuclei before the high-temperature GaN growth and lower screw threading dislocation (TD) density in the initially grown GaN. The difference in the background electron concentration and mobility of the interfacial layer was related to the relatively higher concentration of the O and Al diffused from Al2O3, which is also dependent on the size and density of the nuclei. These differences were found not to affect the structural and electrical properties or the surface morphology of AlGaN/GaN high electron-mobility transistors (HEMTs, except for the buffer conduction) when the GaN buffer is thick enough (e.g., ∼2.5 µm). [ABSTRACT FROM AUTHOR]

An InGaN/GaN superlattice (SL) with Mg-doped barriers was designed and inserted into the InGaN-based blue light-emitting diodes (LEDs) as a hole gathering layer (HGL) to promote hole injection into the active region. The fabricated LEDs with the SL HGL show 36.4% increase in light output power at an injection current of 200 mA. Meanwhile, the efficiency droop is also mitigated effectively, as compared to the traditional LEDs. The improved performance is attributed to increased hole injection efficiency and decreased electron leakage into the p-type region. [ABSTRACT FROM PUBLISHER]

We report an AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with high-mobility two-dimensional electron gas (2-DEG) and reduced buffer leakage. The device features a 3-nm thin lnxGa1-xN(x = 0.1) layer inserted into the conventional AlGaN/GaN HEMT structure. Assisted by the InGaN layers polarization field that is opposite to that in the AlGaN layer, an additional potential barrier is introduced between the 2-DEG channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. For a sample grown on sapphire substrate with MOCVD-grown GaN buffer, a 2-DEG mobility of around 1300 cm²/V·s and a sheet resistance of 420 Ω/sq were obtained on this new DH-HEMT structure at room temperature. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (fT) of 14.5 GHz, and a peak power gain cutoff frequency (fmax) of 45.4 GHz were achieved on a 1 ⊗ 100 μLm device. The off-state source-drain leakage current is as low ∼ 5 μ A/mm at VDS = 10 V. For the devices on sapphire substrate, maximum power density of 3.4 W/mm and PAE of 41% were obtained at 2 GHz. [ABSTRACT FROM AUTHOR]

We report broadband microwave noise characteristics of a high-linearity composite-channel HEMT (CC-HEMT). Owing to the novel composite-channel design, the CC-HEMT exhibits high gain and high linearity such as an output third-order intercept point (OIP3) of 33.2 dBm at 2 GHz. The CC-HEMT also exhibits excellent microwave noise performance. For 1-µm gate-length devices, a minimum noise figure (NFmin) of 0.7 dB and an associated gain (Ga) of 19 dB were observed at 1 GHz, and an (NFmi) of 3.3 dB and a Ga of 10.8 dB were observed at 10 GHz. The dependence of the noise characteristics on the physical design parameters, such as the gate-source and gate-drain spacing, is also presented. [ABSTRACT FROM AUTHOR]

We report a novel approach in fabricating high-performance enhancement mode (E-mode) AlGaN/GaN HEMTs. The fabrication technique is based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing with an annealing temperature lower than 500 °C. Starting with a conventional depletion-mode HEMT sample, we found that fluoride-based plasma treatment can effectively shift the threshold voltage from -4.0 to 0.9 V. Most importantly, a zero transconductance (gin) was obtained at Vgs = 0 V, demonstrating for the first time true E-mode operation in an AlGaN/GaN HEMT. At Vgs = 0 V, the off-state drain leakage current is 28 μA/mm at a drain-source bias of 6 V. The fabricated E-mode AlGaN/GaN HEMTs with 1 μm-long gate exhibit a maximum drain current density of 310 mA/mm, a peak gm, of 148 mS/mm, a current gain cutoff frequency fT of 10.1 GHz and a maximum oscillation frequency fmax of 34.3 GHz. [ABSTRACT FROM AUTHOR]

We report the characterization of quality (Q) -factor of RF metal-semiconductor-metal (MSM) planar interdigitated varactors fabricated by standard AlGaN/GaN HEMT process. The MSM varactors have wide tuning range and exhibit high quality-factor at both the maximum and minimum capacitance values. The fundamental limitation of the Q-factor in the medium capacitance range is also revealed. The elimination of ohmic contact resistance in the MSM varactor configuration pushed up the peak Q-factor to 92 at 0.5 GHz and 41 at 1.1 GHz. The operation of the MSM varactor is modeled by a physical equivalent circuit, with which the dependence of the Q-factor over the entire tuning voltage range can be explained. [ABSTRACT FROM AUTHOR]

We report an Al0.3Ga0.7N-Al0.05Ga0.95N-GaN composite-channel HEMT with enhanced linearity. By engineering the channel region, i.e., inserting a 6-nm-thick AlGaN layer with 5% Al composition in the channel region, a composite-channel HEMT was demonstrated. Transconductance and cutoff frequencies of a 1 × 100 µm HEMT are kept near their peak values throughout the low- and high-current operating levels, a desirable feature for linear power amplifiers. The composite-channel HEMT exhibits a peak transconductance of 150 mS/mm, a peak current gain cutoff frequency (fT) of 12 GHz and a peak power gain cutoff frequency (fmax) of 30 GHz. For devices grown on sapphire substrate, maximum power density of 3.38 W/mm, power-added efficiency of 45% are obtained at 2 GHz. The output third-order intercept point (OIP3) is 33.2 dBm from two-tone measurement at 2 GHz. [ABSTRACT FROM AUTHOR]