The lack of a selective-area doping technique in GaN has hindered the development of high-performance vertical power devices, such as current-aperture vertical electron transistors (CAVETs) and junction-barrier Schottky (JBS) diodes. To overcome this obstacle, we took the approach of selective-area etching to create trenches in n—GaN, followed by filling them with p-GaN. In this process, producing a damage-free etch-and-regrowth interface has been the most critical and challenging step. In this chapter, we will discuss the use of a novel precursor, tertiarybutylchloride (TBCl), to enable in situ (selective-area) etching of GaN in the MOCVD system, thereby replacing the high-damage dry etching process. Through both material and electrical characterizations, TBCl etching was demonstrated to be an intrinsically clean and low-damage etching technique compatible with high-power applications. In addition, nonplanar selective-area growth of GaN was also investigated toward the formation of lateral PN junction devices. The growth evolution was studied and explained by the kinetic Wulff diagram. The origin of nonuniform p-type doping profile in the selectively grown p-GaN region was found to be related with local growth rate by employing the atom probe tomography (APT). Understanding the growth evolution and the cause of nonuniform Mg distribution enables controlling the 3D doping profile.