The annealing temperature dependence of FWHM of Xray rocking curves of pGaN layers grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD) were studied. The results show that the quality of pGaN became worse at annealing temperature higher than 1150℃. The implantation conditions were simulated by TRIM. The planar GaN pn detectors were fabricated by Si implantation into pGaN. The currentvoltage (IV) curve at room temperature shows that the dark current density is 4.7nA/cm2 at zero voltage bias. The peak responsivity is 0.065 A/W and 0.039A/W at 368 nm at room temperature and 80 K, respectively. It decreases obviously with the decrease of temperature as a result of the changes in bandgap, series resistance, and buildin potential with temperature.