Thermal management is one of the main challenges for the future of electronics [1]-[5]. With the ever increasing rate of data generation and communication, as well as the constant push to reduce volume and costs of industrial converter systems, the power density of electronics rises [6]. Consequently, cooling has an increasingly large environmental impact [7], [8], and new technologies are needed to efficiently handle the heat in a sustainable and cost-effective way [9]. Embedding liquid cooling directly inside the chip is a promising approach for a more efficient thermal management [5], [10], [11]. However, even in state-of-the-art approaches, the electronics and cooling are treated separately, leaving the full energy-saving potential of embedded cooling untapped. Here we demonstrate that co-designing microfluidics and electronics into the same semiconductor substrate, to produce a monolithically-integrated manifold microchannel (mMMC) cooling structure, provides efficiency beyond the state-of-theart. Our results show that heat fluxes exceeding 1.7 kW/cm 2 can be cooled down using only 0.57 W/cm 2 of pumping power. We observed an unprecedented coefficient of performance (>10 4 ) for single-phase water-cooling of heat fluxes exceeding 1 kW/cm 2 , corresponding to a 50-fold increase compared to straight microchannels, as well as a remarkably high average Nusselt number of 16. The proposed cooling technology enables further miniaturization of electronics, potentially extending Moore's law and greatly reducing energy consumption worldwide. Furthermore, by removing the need for large external heat sinks, we demonstrate how this approach enables ultra-compact

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 20 mbar, enabling the cooling of a total of 300 W of losses in the converter using several mW of pumping power, which can be realized with small micropumps. Experimental results show a 10fold 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.

In this paper, a high step-up magnetic-less DC/DC nX converter is designed and experimentally evaluated. GaN transistors are applied in a nX converter topology, yielding ultrahigh power density and high conversion efficiency. The absence of magnetic materials results in a constant efficiency throughout the power range; the power capability of the system is only limited by the ratings of the semiconductor devices. To effectively extract the dissipated power, a novel micro-fluidic heat sink is designed, based on microchannels fabricated on Silicon substrate and a laser-cut acrylic manifold. The developed liquid cooling heat sink yields a much smaller volume and higher cooling capability compared to conventional heat sinks. A 10X converter prototype with the integrated micro-fluidic heat sink is experimentally evaluated at various operating conditions and different flow rates for the cooling system. At a transferred power of 1.2 kW the converter exhibits an overall efficiency of 96%, while occupying 260 mL of volume, resulting in 4.62 W/cm 3 , a notable power density for such a high step-up DC/DC converter.