This thesis investigates the fabrication and reliability of a high electron mobility transistor (HEMT) integrated circuit (IC) platform for power conversion, emphasizing the advantages of Gallium Nitride (GaN) materials in power electronics. Highlighting GaN's superior physical and electrical properties over traditional materials, this work addresses the challenges and advancements in GaN HEMT devices and GaN monolithic integrated circuits, culminating in developing an innovative IC platform that enhances device performance and reliability for power conversion applications. This work introduces GaN applications in power electronics, discussing the benefits of GaN materials and the critical characteristics of GaN HEMT devices. It reviews the progress in GaN monolithic integrated circuits, including developing p-GaN gate fabrication processes and a novel hydrogen plasma treatment method to improve electrical properties. Chapter 2 focuses on fabricating p-GaN gate devices, comparing hydrogen plasma treatment with conventional etching processes. The chapter details the fabrication of an etched and hydrogen-treated p-GaN gate device on the same wafer. It verifies the reliability improvements of hydrogen-treated devices through various characterization methods, such as pulse testing, with explanations grounded in physical mechanisms. In Chapter 3, the thesis builds on successful device fabrication by employing Technology Computer-Aided Design simulation to model hydrogen-treated devices. The impact of the treated p-GaN layer design on device electrical characteristics are investigated. Further modeling using the ASM model and parameter optimization via IC-CAP is conducted to simulate device dynamic characteristics accurately. Circuit simulations and validation of components like capacitors, resistors, and diodes are also performed on this platform. Chapter 4 presents the monolithic integrated circuit platform based on hydrogen treatment processes, detailing the complete fabrication process, device performance, circuit module performance, and high-temperature stability of the platform. Implementing a DC-DC buck converter demonstrates the platform’s 48 V power conversion feasibility in data center applications. Chapter 5 shifts focus to emerging GaN bidirectional switches. It overviews GaN bidirectional switches, modeling, simulation analysis, and experimental results. This thesis centers on the p-GaN gate HEMT innovation, which encompasses process development, device design, fabrication, modeling, and simulation, ultimately achieving the development of a monolithic integrated circuit platform and power conversion system. This work significantly advances the monolithic GaN power electronics by offering superior device characteristics, enhanced reliability, and improved performance through monolithic integration.