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We present the characteristics of an AlGaN/GaN Schottky barrier ultraviolet (UV) photodetectors (PDs) with and without a GaN sandwich layer. The effect of inserting this secondary GaN buffer layer on the growth mode and crystal properties of the whole epitaxial GaN layer was reported. It was found that we could reduce defect density and thus improve crystal quality of the AlGaN/GaN Schottky barrier UV PDs by using a GaN sandwich layer. It was also found that we could use the GaN sandwich structure to suppress photoconductive gain (PCG), enhance UV-to-visible rejection ratio (UTV-RR), reduce noise level, and enhance the detectivity of the fabricated PDs.