AlGaN-based Schottky barrier photodiodes, aimed to monitor the solar UV radiation, have been fabricated and characterized. AlGaN semiconductor layers were grown by metal-organic vapor phase epitaxy on sapphire substrates, and the Al mole fraction was varied between zero and 0.35. The evolution of the characteristics of these UV detectors with the Al content is presented. Schottky barrier AlxGa1-xN photodetectors show a very fast response that is independent of the optical power, and their visible rejection ratio is higher than 103. Al0.2Ga0.8N Schottky barriers are very good candidates for monitoring the UV-B band. It is also reported that by using the proper Al mole fraction, the erythema-weighted action of the solar UV radiation can be directly determined by AlGaN photodiodes, without any filter, for the first time. ¿ 2000 American Geophysical Union