Premature breakdown is a key obstacle in developing AlGaN-based avalanche photo diodes (APD) for ultraviolet (UV) light detection. Novel materials growth techniques, such as Hydride Vapor Phase Epitaxy (HVPE) allows for reduction of dislocation density only to approximately 10^7 cm-2. In order to reduce the number of growth defects and dislocation density in AlGaN-based APDs we will use single crystal bulk AlN substrates. We recently demonstrated that the use of a bulk AlN substrate allows us to reduce the dislocation density in the epitaxial AlN layers by more than four orders of magnitude down to 10^4-10^5 cm-2. The reduction of dislocation density below 10^4 cm-2 would enable us to fabricate up to 100 ?m diameter devices in dislocation-free areas of the wafers. Hence, we expect that AlGaN-based APDs on bulk AlN substrates will exhibit major improvements in the breakdown voltages required for stable performance of the devices.