Wide bandgap III-Nitride semiconductors are a promising material system for the development of ultraviolet avalanche photodiodes (APDs) that could be a viable alternative to photomultiplier tubes. In this paper, we report the epitaxial growth and physical properties of device quality GaN layers on high quality AlN templates for the first back-illuminated GaN p-i-n APD structures on transparent sapphire substrates. The 25 μm × 25 μm device characteristics were measured, and compared with the same devices grown on GaN templates, under low bias and linear mode avalanche operation where they exhibited gains near 1500 after undergoing avalanche breakdown. The breakdown electric field in GaN was determined to be 2.73 MV/cm. The hole impact ionization coefficients were shown to be greater than those of electrons. These APDs were also successfully operated under Geiger mode.