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We systematically investigated the wafer- bonded interfaces of p+GaAs/n+InGaAs and p+InGaAs/ n+InGaAs by using a circular transmission line method (CTLM) for the increased extraction accuracy. Based on the low-temperature bonding process at 50 °C, the bonded interfaces were successfully fabricated without degradation of the material quality. While the fabricated devices exhibited the linearly increased resistance as a function of channel distances, the p+InGaAs/n+InGaAs structure revealed the improved interfacial resistivity of $3.9\times 10^{-3} \,\, \Omega \cdot \text{cm}^{2}$ compared with $3.3\!\times \!10^{-2} \,\, \Omega \cdot \text{cm}^{2}$ of the p+GaAs/n+InGaAs. Since these values suggested good electrical properties in wafer-bonded structures, the developed wafer-bonded interfaces could be a good approach for integrating electronic and optoelectronic devices.

In this article, we have demonstrated 125-nm-gate InGaAs-on-insulator (InGaAs-OI) high-electron-mobility transistors (HEMTs) on Si substrates via wafer bonding. The highlights of this device were the i-In0.53Ga0.47As/i-InAs/i-In0.53Ga0.47As quantum-well channel for high RF performance by improving electron transport properties and the low processing temperature of 250 °C or less for monolithic 3-D integration. As a result, we obtained a current gain cutoff frequency ( ${f}_{\text {T}}$ ) of 227 GHz and a maximum oscillation frequency ( ${f}_{\text {MAX}}$ ) of 187 GHz. These results are the highest ever reported in monolithic 3-D RF transistors above ${L}_{\text {G}} = {100}$ nm. In addition, through the small-signal modeling, we have found that the impact of the back-gate (BG) on RF performances of top devices is a critical issue in M3D RF systems.

A recursive filtering technique is developed to improve the computational efficiency of explicit-filter large-eddy simulation in highly parallelized computational environments. In parallel computations on a partitioned domain, unlike in non-recursive explicit filtering in which the number of ghost cell layers for storing neighbor cell values to ensure communication with neighbor partitions is increased rapidly as the stencil of the filter becomes wider, the present recursive filtering technique requires only a single layer of ghost cells for storing immediate neighbor cell values regardless of the stencil width of the filter. A generalized algorithm for construction of recursive filters with desired filter widths is developed. The computational efficiency of the proposed recursive filtering technique over the conventional non-recursive broad-band filtering techniques in terms of memory usage and computational time is presented.

A new numerical method to solve three-dimensional wave equations in media with arbitrarily-shaped interfaces on a Cartesian grid is proposed. The present method aims to achieve two objectives to simulate wave propagation through realistic geometries: (1) handling wave interaction at the interface with high ratios of acoustic material properties and (2) treating complex geometries involving both smooth and non-smooth interfaces. To achieve the first objective, the present method extends the solution smoothly across the interface in the direction normal to the interface. A cell layer of ghost points on each side of the interface is used to enforce interface conditions, which support not only reflection but also transmission of incident waves. Ghost-point values are determined by applying a local coordinate-transform and a weighted least squares error method, which suppress numerical instabilities. To achieve the second objective, the interface geometry is approximated using an unstructured surface mesh, which does not require analytic information about the interface geometry. Finally, the accuracy and effectiveness of the present method are validated and demonstrated for wave propagation over or through several two-dimensional and three-dimensional obstacles.