This paper proposes an efficient parameter extraction algorithm for GaN high electron mobility transistors smallsignal equivalent circuit model. The algorithm combines parameter scanning and iteration methods to solve the problem of error accumulation in conventional methods and is implemented in MATLAB programming. By using the iteration method, the algorithm each time uses more accurate element values thus makes the results converge to the optimal value faster. A 20-element small-signal equivalent circuit model of GaN high electron mobility transistors is used to validate the proposed algorithm, and the results show that the calculated S-parameters agree well with the measured S-parameters within the frequency range of 0.1 to 40 GHz.

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Non-ideal thermodynamics of solid solutions can greatly impact materials degradation behavior. We have investigated an actinide silicate solid solution system (USiO4–ThSiO4), demonstrating that thermodynamic non-ideality follows a distinctive, atomic-scale disordering process, which is usually considered as a random distribution. Neutron total scattering implemented by pair distribution function analysis confirmed a random distribution model for U and Th in first three coordination shells; however, a machine-learning algorithm suggested heterogeneous U and Th clusters at nanoscale (~2 nm). The local disorder and nanosized heterogeneous is an example of the non-ideality of mixing that has an electronic origin. Partial covalency from the U/Th 5f–O 2p hybridization promotes electron transfer during mixing and leads to local polyhedral distortions. The electronic origin accounts for the strong non-ideality in thermodynamic parameters that extends the stability field of the actinide silicates in nature and under typical nuclear waste repository conditions.

In this paper, the performance difference of AlGaN/GaN high-electron mobility transistors (HEMTs) with same epitaxial structure fabricated silicon carbide (SiC) and transferred to diamond substrate is examined based on the surface-potential (SP) model. The thermal resistances of these devices are extracted through finite element method (FEM) thermal analysis. Results show that GaN-ondiamond device has a lower thermal resistance than conventional GaN-on-SiC device, which demonstrates the thermal performance improvement of GaN-on-Diamond technology. By embedding thermal characteristic into carrier mobility in the conventional SP model, the effectiveness of model is validated through good agreement between simulation and measurements of DC and RF performance. Additionally, large-signal performance (output power P out , power added efficiency P AE and Gain) on these two similar devices are compared under identical bias and temperature conditions based on the improved SP model, making this work be effective for improving the process of GaN-on-Diamond HEMTs. The excellent performance of GaN materials including wide bandgap, high thermal conductivity, high electron saturation drift velocity makes it very suitable for the development of highfrequency, high-power microwave and millimeter-wave device and circuit applications.1,2 SiC material is presently a major choice to implement high-performance GaN HEMT due to its high thermal conductivity that is an order of magnitude greater than that of other materials such as sapphire. However, the heat dissipation issue appears more and more prominent with the development to direction of smaller size, greater output power and higher frequency. Therefore, to solve the degradation of device performance and reliability caused by heat dissipation, it is of utmost importance to reduce thermal resistance to get good thermal management. [3][4][5] Recently, GaN HEMT devices fabricated on diamond substrate (GaN-on-diamond) have been developed 6,7 due to its high thermal conductivity (2000 W/m · K) that is three to four times that of SiC. The application of diamond substrate can significantly reduce the temperature rise of device, which is expected to solve the performance degeneration under conditions of high bias and power drive. [8][9][10] Accurate device model, especially physical model, is essential for predicting device performance and guiding process development. This paper discusses the comparison result of performance between GaN HEMTs fabricated on SiC and diamond substrates based on the SP model. The paper is organized as follows. In Device structure and fabrication section, the structure and fabrication process of GaN-onDiamond HEMTs are described in detail. Then the 3D FEM model and SP model are described briefly in Thermal analysis setup and results section and SP model description for following performance verification and discussion in RF Performance verification section. Finally, Conclusion section is the conclusion. Device Structure and FabricationThe cross-section view o...