Gallium Nitride (GaN) has enabled groundbreaking developments in the field of optoelectronics and radio frequency communication. More recently, GaN devices for power conversion applications have demonstrated excellent potential. Thanks to Gallium Nitride wide band-gap, large breakdown electric field, and high electron mobility, GaN power devices lead to higher efficiency and smaller footprint compared to silicon devices. Besides, their higher operating frequency results in considerable miniaturization of the overall converter. For these reasons, GaN power devices are gaining a considerable market share for applications such as fast chargers, electric vehicles, and data centers. Despite its fast development, several aspects of the GaN power technology need to be improved for its widespread adoption. From a device point of view, the performance of current GaN power transistors is far from the material limit, requiring a significant reduction in the on-resistance and increase in the breakdown voltage. Besides, GaN Schottky barrier diodes (SBDs) still suffer from several shortcomings, which limit the versatility of the GaN power technology. In addition, GaN power integrated circuits (ICs) are yet to be developed, which would enable further size and cost reduction, and higher switching frequency. Finally, the drastic footprint reduction of GaN devices requires new thermal management techniques to effectively extract the resulting high heat fluxes. This thesis aims at addressing the aforementioned challenges. To significantly improve the performance of power devices, a multi-channel tri-gate technology employing several 2DEG channels and a three-dimensional gate architecture is demonstrated, leading to a considerable reduction of the device resistance while achieving key features such as Enhancement–mode operation and reduced current collapse. In addition, intrinsic polarization super junctions (i-PSJs) are proposed to further enhance the device performance by improving its off-state electric field distribution. A robust platform enabling excellent charge matching for single- and multi-channel structures is presented resulting in optimal field profile in the whole drift region. The potential of i-PSJs compared to conventional HEMTs is unveiled by an analytical model showing a significant improvement in both RON,SP vs VBR and RON x Eoss figures-of-merit. Furthermore, high-performance GaN SBDs are demonstrated by designing the anode region with the use of a p-GaN cap layer or a tri-gate architecture. This results in excellent dc and switching characteristics for the GaN SBDs, which also present outstanding performance with respect to commercially available devices. The potential of GaN-on-Si power ICs is demonstrated by monolithically integrating several high-performance GaN SBDs to realize a magnetic less DC-DC boost converter, which presents high-frequency operation and largely increased power density compared to Si and SiC solutions. Lastly, to manage the unprecedented heat fluxes deriving from the drastic device miniaturization, embedded liquid cooling in the device substrate is investigated showing its great potential to greatly improve the device thermal performance without impacting its electrical characteristics. The results here presented address several key challenges of GaN power devices and demonstrate the extraordinary potential of the GaN technology for high-efficiency, cost-effective, and ultra-compact future power conversion.