This paper presents a universal scalable thermal resistance model for AlGaN/GaN high electron-mobility transistors (HEMTs), which can be easily implemented into compact large-signal models and is very suitable for rapid device modeling. It contains all the key HEMT device geometric parameters such as gate length, gate width, gate pitch, and different layer thicknesses. The temperature-dependent conductivities of different materials are taken into consideration. The via-holes effects on the thermal resistance of a device are also included, which is considered for the first time in a compact large-signal model. Two uniform parameters are used to simplify the complicated analytical closed-form expression of thermal resistance, which is difficult to be implemented into the compact model. Another two uniform parameters are used to simplify the complicated transcendental function of the temperature rise with respect to the temperature-dependent thermal conductivity. Thus a simple, accurate, and scalable thermal resistance model can be obtained. The results are verified by comparing with experimental and finite-element numerical simulations. The dc I-V characteristics and large-signal performances of HEMTs devices with different gate lengths 0.15, 0.25, and 0.4 μm are used for further verifications. The proposed thermal resistance model can also be used to optimize the structural geometry of a device to obtain minimum thermal resistance.

In this paper, the buffer-related current collapse (CC) in AlGaN/GaN HEMTs is investigated by using pulsed I-V measurements and 2-D drift-diffusion simulations. The simulation results indicate that the negative potential (NP), appearing in the Fe-doped buffer under the gate, is found to account for the CC. The NP will induce threshold voltage shifts, in which an inflection point (IP) of the threshold voltage is found. It could be revealed in pulsed I-V characteristics which are significant to depict trapping effect when building a compact large-signal model. The formation mechanism of the IP is detailed. In the end, a simple threshold voltage model is proposed for verification.

In this paper, analytical gate capacitance models for a large-signal compact model of AlGaN/GaN high-electron mobility transistors are proposed. Different from the MOSFET devices, different depletion regions on either side of the gate are fully considered for high-voltage GaN devices. The depletion regions are bias-dependent to implement the capacitance models into the large-signal compact model. A transfer function is proposed to characterize the switching behavior of the capacitances between on- and off-states, which is essential to describe all the states of a device, for example, operating at Class-B. Different from previous works, the current saturation phenomenon is taken into account in determining the intrinsic capacitances which are induced by nonstatic channel charge. The capacitance models can be easily implemented into the virtual-source-based model to accurately predict the S-parameters and large-signal output characteristics.

The reconfigurable intelligent surface (RIS) is a promising route for the next generation of communication and has been widely investigated. The research on its nonlinear electromagnetic (EM) phenomenon has been reported extensively. However, few researchers focus on the quantitative analysis of the nonlinear distortion of the RIS due to the active components serving as the tuning devices in its unit cells. A method of assessing the third-order intermodulation of RIS is proposed in this article, which is implemented with a combination of the nonlinear active device modeling and full-wave simulation of the passive part of the RIS. Considering that the varactor diode is widely utilized, a nonlinear equivalent circuit model is proposed in this article. It contains an improved capacitance–voltage model with an m expression acquired with the nonlinear current calculation. The nonlinear diode model is used to form the meta-atoms in the RIS system to realize nonlinearity assessment. Several characteristics of the diode are measured, and the measurement data are employed to verify the accuracy of the nonlinear modeling work for the varactor diode. More importantly, consistent comparison results are also obtained from the nonlinear assessment of the RIS system, which indicates that the proposed method is precise. With the nonlinear modeling work of the RIS system, much analysis can be carried out. For example, we can study the nonlinear characteristics of each meta-atom to improve the linearity of the RIS system. Consequently, it demonstrates that the proposed assessment method is significant.

In this paper, a potentiometric method is used for monitoring the concentration of glutamine in the bioprocess by employing silicon nanowire biosensors. Just one hydrolyzation reaction was used, which is much more convenient compared with the two-stage reactions in the published papers. For the silicon nanowire biosensor, the Al₂O₃ sensing layer provides a highly sensitive to solution-pH, which has near-Nernstian sensitivity. The sensitive region to detect glutamine is from ≤40 μM to 20 mM. The Sigmoidal function was used to model the pH-signal variation versus the glutamine concentration. Compared with the amperometric methods, a consistent result from different devices could be directly obtained. It is a fast and direct method achieved with our real-time setup. Also, it is a label-free method because just the pH variation of the solution is monitored. The obtained results show the feasibility of the potentiometric method for monitoring the glutamine concentrations in fermentation processes. Our approach in this paper can be applied to various analytes.