Wide-bandgap GaN-based avalanche photodetectors (APDs) are important for photodetectors operating in UV spectral region. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates such as sapphire and SiC, a high density of defects is introduced, thereby causing device failure by premature microplasma breakdown before the electric field reaches the level of the bulk avalanche breakdown field, which has hampered the development of III-nitride based APDs. In this study, we investigate the growth and characterization of GaN and AlGaN-based APDs on bulk GaN and AlN substrates. Epitaxial layers of GaN and AlxGa1-xN p-i-n ultraviolet avalanche photodiodes were grown by metalorganic chemical vapor deposition (MOCVD). Improved crystalline and structural quality of epitaxial layers was achieved by employing optimum growth parameters on low-dislocation-density bulk substrates in order to minimize the defect density in epitaxially grown materials. GaN and AlGaN APDs were fabricated into 30μm- and 50μm-diameter circular mesas and the electrical and optoelectronic characteristics were measured. APD epitaxial structure and device design, material growth optimization, material characterizations, device fabrication, and device performance characteristics are reported.