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Ultrashallow p+-n--n+ silicon photodiodes, fabricated by a pure boron deposition technology, show excellent performance for detection of Deep Ultra Violet (DUV) radiation due to the nanometer deep pn-junctions. The dark current of photodiode is degraded by the damage of the silicon/oxide interface at the diode perimeter region caused by DUV radiation. Reducing the depletion region width across the p+ n- junction at the silicon/oxide interface will also invariably increase the electric field, reducing the breakdown voltage and increasing the perimeter component of the junction capacitance. In this paper, the trade-off between the depletion region width, breakdown voltage and junction capacitance is examined for ultrashallow p+-n--n+ photodiodes where an additional ultrashallow doped p-region is introduced as an extension to the p-type guard rings. An optimal doping profile is proposed for the added p-region to obtain minimal degradation of electric characteristics for peak doping of 1018 cm-3, 5 · 1018 cm-3 and 5 · 1019 cm-3 at junction depths of 50 nm, 10 nm and 2 nm, respectively, and a distance of 0.5 μm between the added p-region and the surrounding n+ channel stop.