In this work, we describe a new approach for compact and energy-efficient cooling of converters where multiple miniaturized microfluidic cold-plates are attached to transistors providing local heat extraction. The high pressure drop associated with microchannels was minimized by connecting these cold-plates in parallel using a compact 3D-printed flow distribution manifold. We present the modeling, design, fabrication and experimental evaluation of this microfluidic cooling system and provide a design strategy for achieving energy-efficient cooling with minimized pumping power. An integrated cooling system is experimentally demonstrated on a 2.5 kW switched capacitor DC-DC converter, cooling down 20 GaN transistors. A thermal resistance of 0.2 K/W was measured at a flow rate of 1.2 ml/s and a pressure drop of 600 mbar, enabling the cooling of a total of 300 W of losses in the converter using only 75 mW of pumping power, which can be realized with small micropumps. Experimental results show a 10-fold increase in power density compared to conventional cooling, potentially up to 30 kW/l. This proposed cooling approach offers a new way of co-engineering the cooling and the electronics together to achieve more compact and efficient power converters.

GaN transistors are being employed in an increasing number of applications thanks to their excellent performance and competitive price. Yet, GaN diodes are not commercially available and little is known about their performance and potential impact on power circuit design. In this work, we demonstrate scaled-up GaN-on-Si Tri-Anode Schottky Barrier Diodes (SBDs), whose excellent DC and switching performance are compared to commercial Si fast recovery diodes and SiC SBDs. Moreover, the advantageous lateral GaN-on-Si architecture enables to integrate several devices on the same chip, paving the way to power integrated circuit. This is demonstrated by realizing a diode-multiplier Integrated Circuit (IC), which includes up to 8 monolithically-integrated SBDs on the same chip. The IC was integrated on a DC-DC magnetic-less boost converter able to operate at a frequency of 1 MHz. The IC performance and footprint are compared to the same circuit realized with discrete Si and SiC vertical devices, showing the potential of GaN power ICs for efficient and compact power converters.

Many high power (opto-) electronic devices such as transistors, diodes, and lasers suffer from significant hot spot temperature rises due to the high heat fluxes generated in their active area, which limits their performance, reliability, and lifetime. Employing high thermal conductivity materials near the heat source, known as near-junction heat spreaders, offers a low-cost and effective thermal management approach. Here, we present analytical heat spreader models and a methodology to evaluate their performance. Experimental demonstration of near-junction diamond heat spreaders on vertical GaN PiN diodes revealed significantly reduced spreading resistances, along with very low temperature gradients across the device. The findings in this work provide design guidelines and demonstrate excellent prospects, especially for the devices on low thermal conductivity substrates. The theoretical analysis of optimized diamond heat spreaders shows an 86% reduction of spreading resistance for GaN devices and 98% for Ga₂O₃ devices. In addition, our results show that a 3 μm-thick layer of high-quality CVD-deposited diamond heat spreaders on GaN-on-Si devices can provide better heat spreading than GaN-on-SiC devices and perform similar to GaN-on-diamond devices, highlighting the significant potential of heat spreaders as an effective and low-cost thermal management approach.

A detailed analysis of the Sawyer-Tower method used in the measurement of large-signal output capacitance ( C _o ) of power transistors is presented, followed by important design recommendations to obtain accurate results. Key factors affecting the proper implementation of the technique, such as power amplifier characteristics, load slew rate, reference capacitor ( C _ref ), and reverse conduction of the device, are addressed, with accompanying simulation and experimental results for Si, SiC, and GaN devices. A thorough investigation of the selection of C _ref is presented, with a new equation to correctly determine its value for a given voltage swing and output capacitance range of the device under test (DUT). We report that the Sawyer-Tower circuit imposes the DUT to enter steady-state reverse conduction under certain conditions, thus leading to charge-voltage (QV) hysteresis patterns unrelated to C _o . Our analysis reveals that the origin of this phenomenon is related to DUT's leakage current and it could be minimized by the proper selection of excitation frequency. This article intends to provide an effective guide on designing and using the Sawyer-Tower circuit, and to induce further scientific insight on characterizing C _o .

Wide-Band-Gap semiconductors have enabled considerable miniaturization of power devices, which requires, however, new thermal management solutions to handle the resulting high heat fluxes. Recently, embedded liquid cooling in GaN-on-Si devices was demonstrated as a promising solution by flowing a coolant through microchannels etched in the silicon substrate. However, its impact on power devices’ electrical characteristics, especially at high voltage, is yet to be investigated, which is crucial to assess the viability of the technology. Besides, previous demonstrations were limited to relatively low-power devices, while embedded liquid cooling for high-current and high-voltage (650 V) commercial GaN transistors would show the full potential of the technology. Here, we integrate embedded liquid cooling on 650 V, 50 $\text{m}\Omega $ GaN-on-Si commercial power devices. We demonstrate no negative impact on the device dc or switching performance due to the embedded liquid cooling, which proves the robustness and validity of the technology. Besides, liquid-cooled devices show more than $4\times $ higher current capability and much-improved ${R}_{\mathrm{ON}} \times \,\,{E}_{{\text {oss}}}$ figure-of-merit in a large output current range compared to forced-convection air-cooling, highlighting their potential for high-current applications. Finally, deionized water and a dielectric fluid (3M Novec 7200) are compared as coolants, revealing a trade-off between thermal performance and reliability during high-voltage operation.