1,720,988 research outputs found
2-mm-gate-periphery GaN high electron mobility transistors on SiC and Si substrates: A comparative analysis from a small-signal standpoint
No full textIn this paper, a comparative analysis has been conducted on GaN high electron mobility transistor (HEMT) technology on Si and SiC substrates. Small-signal characteristics of 2-mm GaN-on-Si and GaN-on-SiC devices have been investigated. Both devices have the same gate length of 0.5 μm. Special emphasis has been put on the temperature dependence of the buffer/substrate loading effects arising from the Si substrate. As a matter of fact, the “cold” pinch-off admittance (Y-) parameter measurement showed significant loading effect for the Si-based device with respect to the SiC-based one. This has been clearly supported by the analysis of the extracted parameters of the small-signal equivalent-circuit model. The model was also validated by simulating active scattering (S-) parameters, which showed a very good agreement with the corresponding measurements. The results of this paper highlight the impact of buffer/substrate leakage currents on small-signal characteristics and the importance of taking this into account during the modeling phase of the GaN-on-Si HEMT technology. The lower thermal conductivity of this substrate increases the internal temperature, thus stimulating more leakage and reduction of the device power efficiency
Design and investigation of a compact 3D MMIC patch antenna for mm wave applications
No full textThis work focused on the design, characterization and investigation of GaAs based multi-layered compact 3D MMIC based antenna. Different patch antennas were designed and characterized along with its S-parameters. The proposed models of multi-layered patch antennas use V-shaped feeder line or a spiral transmission feeder line. The newly propose planar antennas are more compact in area compared to using the normal planar feeder line; this design approach allows to have wider bandwidth, better input return loss and a slightly lower resonance frequency
The Degradation of ZnO Varistors Packed with Epoxy Insulation Layers
No full textZinc oxide (ZnO) varistor is a highly non-linear semiconducting device that is widely used as a transient voltage surge protector in electrical and electronic systems. One of the major challenges in the continuous development of varistors has been to reduce the long-term degradation. The degradation can take a number of forms when the varistor is subjected to a series of pulses or to a constant applied dc or ac voltage. There will be a distinct asymmetry in the I-V characteristics once the degradation has taken place, with the leakage current being greater in both directions of the applied field. Because varistors are usually operated under the voltage, any increase in leakage current is of major importance. This is especially so for the station-class varistors that are used in power distribution systems, because the power dissipated within the varistor represents a loss in distributable power. Under extreme conditions, particularly at elevated temperatures, the increase in leakage current can be sufficient for the associated Joule heating to cause thermal runaway of the varistor
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The degradation of epoxy resin-coated ZnO varistors at elevated temperatures and ambient humidity conditions
No full textThe degradation of the epoxy resin-coated commercial ZnO varistors at elevated temperatures and ambient humid conditions has been investigated experimentally. It has been observed that the leakage current of the ZnO varistors increases under the voltage stress at elevated temperatures with ambient humidity content. The change in the leakage current corresponding to a fixed electric field with respect to the initial current is taken as the dimensionless degradation index. This index is monitored at various experimental conditions in conjunction with the curing conditions of the epoxy resin powder. The results are carefully evaluated, and it has been found that the diffusion process of the moisture into the ZnO varistors plays a key role in the degradation process provided that these varistors had excellent property to begin with. The ionisation of the moisture at the interface between the ZnO block and the epoxy resin coating leads to the increase of the leakage current. Furthermore, the role of the ambient pressure corresponding to the elevated temperatures is considered as the variable to the degradation process. Those data are also monitored and analysed as a function of time
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Temperature-sensitivity of two microwave HEMT devices: AlGaAs/GaAs vs AlGaN/GaN heterostructures
No full textThe goal of this paper is to provide a comparative analysis of the thermal impact on the microwave performance of high electron-mobility transistors (HEMTs) based on GaAs and GaN technologies. To accomplish this challenging goal, the relative sensitivity of the microwave performance to changes in the ambient temperature is determined by using scattering parameter measurements and the corresponding equivalent-circuit models. The studied devices are two HEMTs with the same gate width of 200 μm but fabricated using different semiconductor materials: GaAs and GaN technologies. The investigation is performed under both cooled and heated conditions, by varying the temperature from −40◦ C to 150◦ C. Although the impact of the temperature strongly depends on the selected operating condition, the bias point is chosen in order to enable, as much as possible, a fair comparison between the two different technologies. As will be shown, quite similar trends are observed for the two different technologies, but the impact of the temperature is more pronounced in the GaN device
The origin of varistor property of SrTiO3-based ceramics
No full textThe role of oxygen in the heat treatment process of SrTiO3 varistor ceramics has been investigated in this paper. The varistor voltage of SrTiO3 ceramics has been found independent of the sample thickness and it increases with the heat treatment temperature. It has been further revealed that the dielectric property is mainly governed by a highly resistive surface layer. The XPS results of Mn 2p and O 1s suggest that the surface layer is formed by oxygen diffusion and surface chemisorption at grain boundaries during the heat treatment in air. The chemisorption of oxygen in grain boundaries, which leads to the highly resistive surface layer, is the origin of the varistor property of SrTiO3 ceramics
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