The Silicon PIN photodiode (Si-PIN PD) with active area (10.0 x 10.0 mm 2 , 12.0 x12.0 mm 2 and 20.0 x 20.0 mm 2 ) was designed by using Silvaco ATLAS and ATHENA tools at Nuclear Radiation Detectors Applications and Research Center (NÜRDAM). To get Si-PIN PDs' specifications, capacitance-voltage (C-V) and dark current -voltage (I-V), spectral response measurements were accomplished with Bipolar and Shr model, and Newton method. The dark current and capacitance at -90 V of designed Si-PIN PD are (7.49 nA, 39 pF), (39 nA, 51 pF), (10 nA, 80 pF) for 10x10 mm 2 , 12x12 mm 2 , 20x20 mm 2 respectively. Si-PIN PDs have low dark current and capacitance at high reverse voltage and all photodiodes reach the full depletion mode at -20 V. Spectral response of each Si-PIN PD is 0.6 AW -1 . According to obtained results, designed Si-PIN PDs are likely to be used for medical application after fabrication and radiation test.