There is a need for semiconductor based UV photodetectors to support avalanche gain in order to realize better performance and more effectively compete with existing photomultiplier tubes. However, there are numerous technical issues associated with the realization of high-quality solar-blind avalanche photodiodes (APDs). In this paper, APDs operating at 280 nm, within the solar-blind region of the ultraviolet spectrum, are investigated. The devices consist of an Al0.38Ga0.62N active region grown atop a high quality AlN template layer designed to allow back illumination of the devices through the sapphire substrate. These devices perform well in the unbiased mode of operation. Under the application of large reverse bias these devices show a soft breakdown starting at relatively low electric fields. The devices achieve a maximum optical gain of ~1000 at a reverse bias of ~90 Volts, which corresponds to an electric field strength of 2.5 MV/cm. The origins of this gain are discussed in detail and modeling of the devices is used to investigate the electric field build up in the multiplication region.