Gallium Nitride (GaN)- and Aluminum Gallium Nitride (AlGaN)-based devices have emerged as one of the best III-V material systems for power electronics applications. The purpose of this chapter is to provide insight into the GaN device and process technology and its potential as a major candidate for future Schottky barrier diodes in rectifying applications for power supplies. Specifically, this study investigates GaN-based Schottky diodes with the target turn-on voltage of Vf < 0.4V, and breakdown voltage of Vbr > 400V for applications in 230V AC-DC rectifiers. First, fundamental Schottky barrier theory with advanced models for transport of electrons by thermionic emission (TE), thermionic field emission (TFE), and field emission (FE) are described, taking into account the presence of defect states and their effects on Fermi-level pinning. Second, specific contact resistivity for ohmic contacts and the dependence on electron concentration for different Schottky barrier heights is presented. Third, the superior material properties of GaN for high voltage and high-frequency applications, as well as the role of various metal contacts and their Schottky barrier heights are reviewed. Finally, since traditional Schottky barriers diodes on GaN cannot achieve the target specified above, this chapter also explores the different device structures that can be engineered with field plate extension and recessed anodes for achieving low Vf while maintaining high Vbr. The review of research suggests that hetero-junction AlGaN/GaN device structures employing formation of two-dimension electron gas (2DEG) engineered with Schottky and ohmic contacts show promise for the aimed applications.