In this work, we demonstrate a new thermal management approach for direct cooling of GaN-on-Si power integrated circuits (ICs), in which the Si substrate functions as a microfluidic heat sink, turning Si into a cost-effective, high thermal performance substrate. Flowing coolant through microchannels etched in the backside of the substrate enables a much denser integration of GaN power devices in a single chip. As a proof of concept, an integrated full-wave bridge rectifier (FWBR) was realized based on high-performance tri-anode GaN Schottky barrier diodes (SBDs), together with a novel hybrid printed circuit board (PCB) that provides fluidic and electric connections to the liquid-cooled power IC. A device-level heat flux of 417 W/cm 2 was cooled using only 60 mW of pumping power. Compared to natural-convection air-cooling, the temperature rise was reduced by 98% and the converter output power was increased by 30 times, up to 120 W, by eliminating self-heating degradation. The high cooling efficiency, large heat extraction capabilities and low-cost fabrication process of embedded microchannels on GaN-on-Si, in combination with new PCB-based coolant delivery, can be an enabling technology for the next generation of ultra-high power-density ICs.