In recent decades, gallium nitride (GaN) high-electron-mobility transistor (HEMT) has become increasingly popular for microwave applications due to its wide bandgap and high saturated electron velocity. However, self-heating inhibits the improvement of its electrical characteristics and reduces device reliability. In this study, a thermoelectric analysis based on embedded microfluidic cooling is performed. By embedding the microchannel into the silicon carbide (SiC) substrate, the coolant can be introduced directly to realize active near junction cooling. According to the thermal resistance model, embedded cooling shortens the thermal path and enhances the capacity of heat convection, and the improvement in the HEMT output characteristics is also verified experimentally. The saturation current increases by 19.5%, and the heat flux of the gates reaches 6349.2 W/mm2 at 70 mL/min, which is 1172% higher than that of traditional remote cooling. The maximum temperature is only 67.4 °C. Therefore, the embedded microfluidic cooling scheme can markedly suppress the self-heating effect and further explore the electrical potential of GaN-based devices.

Self-heating inhibits the electrical characteristics improvement of GaN devices. The local hotspots generated in the gate region significantly affect the output performance of GaN devices. This study proposes a near-junction cooling technique for the thermal regulation of a GaN-on-SiC monolithic microwave integrated circuit power amplifier (MMIC PA). An embedded microchannel structure is integrated into the GaN-on-SiC high-electron-mobility transistor (HEMT) substrate. The dc characteristic curve of the actual HEMT reveals that the designed embedded cooling microchannel induces a 22.32% increase in saturation current and an 11.74% enhancement in device transfer characteristics and effectively mitigates the impact of device heat on dc output properties. The microchannel design of a single HEMT model is applied to design the characteristic parameters of the MMIC PA. Embedded cooling can effectively eliminate the thermal coupling between two HEMTs on MMIC PA at a certain distance. Consequently, the implementation of embedded cooling in the thermal management of high-power MMIC PAs markedly augments the output performance. This is an effective and innovative method for the thermal management of high-power amplifiers.

This study introduces an enhanced thermal management strategy for efficient heat dissipation from GaN power amplifiers with high power densities. The advantages of applying an advanced liquid-looped silicon-based micro-pin fin heat sink (MPFHS) as the mounting plate for GaN devices are illustrated using both experimental and 3D finite element model thermal simulation methods, then compared against traditional mounting materials. An IR thermography system was equipped to obtain the temperature distribution of GaN mounted on three different plates. The influence of mass flow rate on a MPFHS was also investigated in the experiments. Simulation results showed that GaN device performance could be improved by increasing the thermal conductivity of mounting plates’ materials. The dissipated power density of the GaN power amplifier increased 17.5 times when the mounting plate was changed from LTCC (Low Temperature Co-fired Ceramics) (k = 2 Wm⁻¹ K⁻¹) to HTCC (High-Temperature Co-fired Ceramics) (k = 180 Wm⁻¹ K⁻¹). Experiment results indicate that the GaN device performance was significantly improved by applying liquid-looped MPFHS, with the maximum dissipated power density reaching 7250 W/cm². A thermal resistance model for the whole system, replacing traditional plates (PCB (Printed Circuit Board), silicon wafer and LTCC/HTCC) with an MPFHS plate, could significantly reduce θ_js (thermal resistance of junction to sink) to its theoretical limitation value.

This paper reports a dual-mode switching of highly sensitive thermal flow sensors based on calorimetric mode that is more suitable for microflow measurements. Upstream heating and downstream heating modes (which are both calorimetric) are adopted in this paper. The sensor consists of two vertically stacked suspended membrane MEMS chips. A gas micro channel is fabricated inside the chip and the chip is mounted perpendicular to the flow direction. Due to the Laval effect, the gas flowing through the internal channels of the chip increases the gas flow velocity and improves the heat exchange efficiency between the chip and the gas. Moreover, the dual-chip stacked design increases the thermal resistance of solid heat conduction and the heat exchange area of the gas, thus improving its sensitivity. Simulation results show that the gas flow velocity and heat exchange efficiency increased seven times and three times, respectively. Experimental results show that the sensor achieves high sensitivity measurements of 0.77 mV/(mL/min) for 0-18 mL/min and 0.16 mV/(mL/min) for 18-100 mL/min over the entire range by dual-mode switching. The response time is 25 ms. To the best of the authors’ knowledge, this is the first report of a thermal flow sensor with dual-mode switching of upstream and downstream heating in calorimetric mode. In addition, the sensor can be widely used in semiconductors, hydrogen energy, scientific instruments and other fields.

An improved physical model was proposed for accurate electrothermal modeling Pirani vacuum gauges (PVG) by accounting for the micro-nano scale effects of heat transfer in sensitive elements and the non-negligible corner heat dissipation from multiple heat sinks. This model is verified by published experimental data and can reveal the influences of geometrical, physical and process parameters, such as device dimensional size, temperature and doping concentrations, on the micro-nano scale heat transfer process, which is a great consideration for microelectromechanical systems (MEMS) PVG designing. Although the model is initially envisaged for MEMS PVG, it could be straightforwardly adopted for the thermal sensors with silicon heaters. Finally, the characteristics of PVG were comprehensively investigated based on the presented model validated by the experimental data. The results show that expanding the area ratio of heat sinks to heater and the thermal resistance ratio of heater to gas is an effective design idea to optimise the maximum sensitivity and central pressure of device.