Advances in power electronics have been one of the primary enablers of the unprecedented growth of renewable energy sources on the electrical power grid over the past several decades along with the current evolution toward ever-higher levels of electrification. As a result, power electronic circuits will play an important role in all decarbonization efforts for mitigating climate change and future sustainability. For commercial importance, GaN is one of the most promising semiconductors for the next-generation power switch used in power electronics. Vertical architecture GaN devices have the potential to go beyond the performance and capabilities of Si and SiC solutions to enable a decarbonized economy. This chapter introduces vertical p–n diodes and vertical junction field effect transistors and other transistor structures fabricated on pseudo-bulk low defect density (104–106 cm−2) bulk GaN substrates. The basic building block of most semiconductor devices, the p-n junction, is discussed in detail. Topics included in the chapter are device design, edge termination strategies, epitaxial growth, and incoming substrate specifications, back-side processing and metallization schemes, packaging, temperature studies down to the cryogenic regime, impact ionization-based avalanche breakdown, and reliability studies.