Analysis of the Survivability of GaN Low-Noise Amplifiers

This paper presents a detailed analysis of the stressing mechanisms for highly rugged low-noise GaN monolithic-microwave integrated-circuit amplifiers operated at extremely high input powers. As an example, a low-noise amplifier (LNA) operating in the 3-7-GHz frequency band is used. A noise figure (NF) below 2.3 dB is measured from 3.5 to 7 GHz with NF<1.8 dB between 5-7 GHz. This device survived 33 dBm of available RF input power for 16 h without any change in low-noise performance. The stress mechanisms at high input powers are identified by systematic measurements of an LNA and a single high electron-mobility transistor in the frequency and time domains. It is shown that the gate dc current, which occurs due to self-biasing, is the most critical factor regarding survivability. A series resistance in the gate dc feed can reduce this gate current by feedback, and may be used to improve LNA ruggedness     

Recrystallization of Cold Spray-Fabricated CP Titanium Structures

Cold gas dynamic spray (cold spray) is a promising rapid deposition technology in which particles deposit at supersonic velocities. The effect of isothermal annealing on recrystallization and mechanical properties of commercial purity (CP) titanium structures that were directly fabricated through cold spray deposition is studied. The optimized cold spray parameters led to a dense cold spray structure. Results show that annealing improves ductility of the cold-sprayed CP titanium structure. The mechanism for softening is the nucleation and growth of equiaxed grains, which include an ultrafine grain structure. A physical-based model for recrystallization of the cold spray CP titanium is proposed. The results show that recrystallization does not eliminate preferred orientation inherited from the cold spray material.     

A modification on ECAP process by incorporating torsional deformation

In the present study, integration of equal channel angular pressing (ECAP), as a well known severe plastic deformation (SPD) technique, and torsion deformation, is studied by using three dimensional finite element analysis. This process is to be named as torsional-equal channel angular pressing (T-ECAP). In this modification a part of the exit channel in the ECAP die is rotating around its axis, to impose extra shear strains to the samples. To study deformation behavior in the T-ECAP process, three-dimensional finite element analysis (FEA) was carried out by using the elasto-plastic finite element analysis ABAQUS/Explicit Simulation. To investigate the validity of the simulation results, experimental studies were furthermore performed on commercially pure aluminum (AA 1050). Vickers hardness test was used to determine the distribution of hardness on both normal and longitudinal sections of the deformed samples with respect to the exit channel of the die. The hardness test results showed more uniform distribution of hardness in both sections of the T-ECAP processed samples regarding the ones produced by ECAP process. The load requirement comparison for performing both processes showed lower value for the T-ECAP with respect to the ECAP process. The simulation results for the strain values showed higher magnitude and more uniform distribution for the T-ECAP with respect to the ECAP process.     

Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems

Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8 × 60 μm wide and 0.5 μm long AlGaN/GaN HEMTs at a drain voltage of V_d = 50 V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance-voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers.     

Estimating equilibration times and heating/cooling rates in heat treatment of workpieces with arbitrary geometry

Estimation of both equilibration times and heating/cooling rates is very important in heat treatment practices. An improved, practical virtual sphere method recently developed by two of the authors, Gao and Reid, based on the theory of heat conduction in solids, is further examined and evaluated in this article. The method is capable of estimating equilibration times for any workpiece with arbitrary geometry. The requirement of input data for this method is kept to a minimum. The method is simple and cost effective to use and offers much better accuracy than those methods based on rules of thumb. The engineering approach of applying the virtual sphere method for determining both equilibration times and heating/cooling rates at the center is provided in this article. It can be easily implemented in a computer program, and hand calculations are also possible. The test cases presented in this article for evaluating the method include a long cylinder, an infinite plate, and a short cylinder with exact analytical solutions and a H13 die with experimentally measured temperature history for a heat treatment cycle. The method is recommended for engineering applications.     

PHEV powertrain co-design with vehicle performance considerations using MDSDO

Structural and Multidisciplinary Optimization - The complexity of plug-in hybrid-electric vehicles (PHEVs) motivates the simultaneous integration of component design and supervisory control...     

Approximation of integrable functions based on \phi -transform

Let \(E\) be a bounded subset of real line which contains its infimum and supremum. In this paper, we have defined the \(\phi -\) transform and its inverse, where \(\phi \) is a function from \(E\) into \((0,1]\) . We will have shown that real-valued integrable functions on \([a, b]\) and real-valued continuous functions on \(E\) can be approximated by this transformation with an arbitrary precision.     

First-principles study of impurity segregation in zirconia,hafnia, and yttria-stabilized-zirconia grain boundaries

The impurity segregation behavior in symmetric tilt Σ5 (310)/[001] grain boundaries (GBs) of cubic ZrO₂, HfO₂, and yttria-stabilized ZrO₂ (YSZ) were studied using first-principles calculations. The substitutional impurities, including divalent (Mg, Ca), trivalent (Al, Y), and tetravalent (Si, Ti) elements, were considered. It is found that divalent and trivalent impurities tend to segregate to a narrow region within 4 Å of the GB plane, while the tetravalent impurities have a much more scattered segregation profile extending 8–15 Å from the GB plane. For ZrO₂ GB, the calculated grain boundary impurity enrichment factor for yttrium dopant is about 1.7, which is in good agreement with the experimental value of 1.9. For YSZ GB, there exist a strong GB segregation tendency for Mg, Ca, and Si, and a very weak segregation tendency for Ti in the YSZ GB, which is consistent with experimental findings.