Low-frequency noise in gallium nitride epitaxial layers with different degrees of order of mosaic structure

The correlation between the noise level 1/f and the degree of mosaic-structure order in gallium nitride epitaxial layers was studied for the first time. Samples with a doping level of N _d ?N _a ≈8×10¹⁶ cm^?3 and a relatively high degree of order were characterized by the Hooge parameter α≈1.5×10^?3. This value is unprecedently low for thin GaN epitaxial films. The Hooge parameter was significantly higher for samples with N _d ?N _a ≈1.1×10¹⁸ cm^?3 and a low degree of order despite the fact that α generally decreases with increasing doping level at the same degree of order. Thus, the degree of mosaic-structure order affects not only the optical and electrical characteristics but also the fluctuation parameters of GaN epitaxial layers.     

Detection, mixing, and frequency multiplication of terahertzradiation by two-dimensional electronic fluid

We show that a short channel High Electron Mobility Transistor (HEMT) has a resonance response to electromagnetic radiation at the plasma oscillation frequencies of the two dimensional electrons in the device. This response can be used for new types of detectors, mixers, and multipliers. These devices should operate at much higher frequencies than conventional, transit-time limited devices, since the plasma waves propagate much faster than electrons. The responsivities of such devices may greatly exceed the responsivities of Schottky diodes currently used as detectors and mixers in the terahertz range. A long channel HEMT has a nonresonant response to electromagnetic radiation and can be used as a broadband detector for frequencies up to several tens of terahertz     

Enhanced CAD model for gate leakage current in heterostructurefield effect transistors

A simple and accurate circuit model for Heterostructure Field Effect Transistors (HFETs) is proposed to simulate both the gate and the drain current characteristics accounting for hot-electron effects on gate current and the effect of the gate current on the channel current. An analytical equation that describes the effective electron temperature is developed in a simple form. This equation is suitable for implementation in circuit simulators. The model describes both the drain and gate currents at high gate bias voltages. It has been implemented in our circuit simulator AIM-Spice, and good agreement between simulated and measured results is achieved for enhancement-mode HFETs fabricated in different laboratories. The proposed equivalent circuit and model equations are applicable to other compound semiconductor FETs, i.e., GaAs MESFETs     

Enhanced Plasma Wave Detection of Terahertz Radiation Using Multiple High Electron-Mobility Transistors Connected in Series

We report on enhanced room-temperature detection of terahertz radiation by several connected field-effect transistors. For this enhanced nonresonant detection, we have designed, fabricated, and tested plasmonic structures consisting of multiple InGaAs/GaAs pseudomorphic high electron-mobility transistors connected in series. Results show a 1.63-THz response that is directly proportional to the number of detecting transistors biased by a direct drain current at the same gate-to-source bias voltages. The responsivity in the saturation regime was found to be 170 V/W with the noise equivalent power in the range of ${hbox{10}}^{-7}~{hbox{W/Hz}}^{0.5}$ . The experimental data are in agreement with the detection mechanism based on the rectification of overdamped plasma waves excited by terahertz radiation in the transistor channel.      

Simultaneously achieving high energy storage performance and low electrostrictive strain in BT-based ceramics

Dielectric ceramics with high recoverable energy storage density (W_rec) and high energy storage efficiency (η) are urgently needed due to their potential application in pulse capacitor devices. However, the low  η and breakdown strength (BDS) have produced a bottleneck for achieving high W_rec at high electric field. Here, we introduce Bi(Mg_0.5Ti_0.5)O₃ (BMT) into Ba(Ti_0.92Sn_0.08)O₃ (BTS) matrix to enhance the relaxor character of BTS–xBMT and reduce the electrostrictive strain generated during electric field loading. The enhanced relaxor character is beneficial for increasing the efficiency, whereas the reduced electrostrictive strain is profitable to increase the BDS. Furthermore, the BDS is significantly improved by the polymer viscous rolling process. Finally, the electrostrictive effect was considerably lowered in an optimized BTS–0.1BMT composition. More crucially, a high W_rec of 4.34 J/cm³ was attained accompanied by excellent temperature stability (variation ≤±5% between 30 and 120°C). The current results show that the developed dielectric ceramics can be used in pulse capacitor devices for energy storage.     

Gas permeation of polymer blends. I. PVC/ethylene–vinyl acetate copolymer (EVA)

The transport behavior of O₂ and N₂ were studied for series of physical blends of PVC with EVA having different vinyl acetate (VAc) contents in the EVA (45 and 65 wt-%) and using different milling temperatures (160° and 185°C). The polymer blends were further characterized by dynamic mechanical measurements, density measurements, and x-ray diffraction. At higher VAc content in EVA and with higher milling temperature, the rate of permeation (P) and the rate of diffusion (D) decrease, and the activation energy of D (from Arrhenius plots) increases. Furthermore, the experimental density values of PVC/EVA-45 blends agree well with calculated values, assuming volume additivity of the two components, while those of PVC/EVA-65 blends are higher than the calculated densities. These results are interpreted as due to denser packing of polymer molecules and increased PVC-EVA interaction at higher VAc content and with higher milling temperature, indicating better compatibility between the blend components. The x-ray diffraction data give no evidence of crystallinity. Sharp increases in P and D values at about 7.5% EVA (by weight) are found for PVC/EVA-45 blends (in agreement with our previous work) but not for PVC/EVA-65 blends. This is interpreted as due to a phase inversion at increasing EVA content in the former blends but not in the latter blends. The dynamic mechanical measurements show that the PVC/EVA-65 blends milled at 160°C behave largely as semicompatible systems with maximum interaction between the two polymers at compositions of about 50/50 by weight.     

Enhanced electrostrains in PMN–xPZN solid solutions driven by a rather small electric field

Pb(Mg_1/3Nb_2/3)O₃ (PMN) relaxors have gained a lot of interest due to their unusual dielectric relaxation and high electrostrictive electrostrain. However, the T_m (temperature associated with maximum permittivity) of PMN is lower than room temperature, which limits their future development of electrostrain and practical applications. In this study, we increased the T_m by incorporating a relaxor ferroelectric (FE) end member Pb(Zn_1/3Nb_2/3)O₃ (PZN) rather than a conventional high Curie temperature FE end member to create (1−x)PMN–xPZN solid solutions with x = 0.2–0.5. Their dielectric, FE, and electrostrain properties were systematically investigated. In x = 0.4 composition, we get a maximum electrostrain of 0.134% and an equivalent piezoelectric coefficient $d_{33}^{\ast }$ of 936 pm/V under a rather small driving field of 5 kV/cm. Furthermore, the electrostrain of the x = 0.5 is greater than 0.1% between 20 and 80°C, indicating its possible applicability in precision displacement actuators. Our findings not only clarify the electrostrain and electrostrictive properties of (1 − x)PMN–xPZN system but also show an innovative way to improve electrostrain properties by constructing relaxor–relaxor type solid solutions that can be applied to other FE systems.