AlGaN based ultraviolet solar blind photodiodes, operating at a peak wavelength of 274 nm with external quantum efficiencies of up to 46% at zero bias, were grown by low pressure MOCVD. The photodiodes operate via backside illumination, allowing the use of Mg doped GaN instead of a high Al mole fraction p-AlGaN layer. The effects of: the individual layer thicknesses, the Al composition difference between the n-type layer and intrinsic layer, the use of an AlGaN composition grade at the i-n interface, and the inclusion of an electron blocking layer at the p-i interface on the device performance were investigated. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)