This book covers two broad domains: state-of-the-art research in GaN HEMT and Ga2O3 HEMT. Each technology covers materials system, band engineering, modeling and simulations, fabrication techniques, and emerging applications. The book presents basic operation principles of HEMT, types of HEMT structures, and semiconductor device physics to understand the device behavior. The book presents numerical modeling of the device and TCAD simulations for high-frequency and high-power applications. The chapters include device characteristics of HEMT including 2DEG density, Id-Vgs, Id-Vds, transconductance, linearity, and C-V. The book emphasizes the state-of-the-art fabrication techniques of HEMT and circuit design for various applications in low noise amplifier, oscillator, power electronics, and biosensor applications. The book focuses on HEMT applications to meet the ever-increasing demands of the industry, innovation in terms of materials, design, modeling, simulation, processes, and circuits. The book will be primarily helpful to undergraduate/postgraduate, researchers, and practitioners in their research.