GaN-based electronic devices have great potential for future power applications, thanks to their wide band-gap, high breakdown electric field, and high electron mobility. In addition, these devices can be integrated on large-size Si substrates and enable novel monolithic power integrated circuits (ICs), providing an exceptional cost-effective GaN-on-Si platform to revolutionize current power conversion systems with much higher power density and greater energy efficiency. Despite all these remarkable advantages, the performance of current GaN-on-Si power devices is still far away from the prospect promised by this material, and further enhancement requires a signif-icant reduction in the ON-resistance of a unit area (RON·A) and an increase in the breakdown voltage (VBR) of the device. Moreover, the family of GaN-on-Si power devices is not yet complete, as high-voltage power GaN-on-Si Schottky barrier diodes (SBDs) are still missing on the market in spite of the great demand, restricting the full functionality of GaN-on-Si power solutions. This thesis proposes tri-gate technologies to overcome these challenges. The common drawback of increased RON in tri-gate GaN high electron mobility transistors (HEMTs) is resolved, and the excep-tional merit of the tri-gate for high VBR is discovered. A novel slanted tri-gate structure is invented to improve the VBR for GaN-on-Si metal-oxide-semiconductor HEMTs (MOSHEMTs) at a fixed A, resulting in a much reduced RON·A product and a record high-power figure-or-merit among GaN-on-Si power transistors. High-voltage power GaN-on-Si SBDs are also achieved based on a judicious design of the tri-gated anode region, demonstrating unprecedented reverse-blocking performance that is dramatically improved from existing technologies, along with excellent integratability with GaN transistors, which is demonstrated in reverse-blocking GaN MOSHEMTs with record voltage-blocking capabilities. Furthermore, a novel multi-channel tri-gate structure is developed in this the-sis, deploying multiple 2DEG channels to dramatically reduce the RON·A value while maintaining a high VBR thanks to the tri-gate, resulting in novel multi-channel tri-gate power GaN-on-Si normally-ON/OFF MOSHEMTs and SBDs with state-of the-art performance. The results in this thesis reveal the extraordinary value of the tri-gate in enhancing the VBR for high-voltage GaN power devices, demonstrate high-performance power GaN-on-Si SBDs that can be rated for 650 V, and unleash the enormous potential of the multi-channel tri-gate approach to dra-matically improve the performance of power GaN devices, offering a complete and effective plat-form towards the full capabilities of GaN for future efficient power conversion.