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We studied how irradiation with fast (14 MeV) and thermal (<0.4 eV) neutrons affected the properties of GaN PIN diodes, measuring their I-V characteristics before and after irradiation. Irradiation with fast neutrons caused the carrier removal effect when the reverse bias was low. Irradiation with thermal neutrons significantly increased the reverse-bias current, possibly because of damage to the passivation layer, defects induced in the GaN, and defects near the metal/GaN interface acting as tunneling sites. However, the forward current decreased both after fast neutron radiation and after thermal neutron radiation. Irradiation with thermal neutrons destroyed the SiO2 passivation layer. The crystal quality and optical properties of the GaN PIN diodes were characterized by examining changes in the photoluminescence (PL) and micro-Raman scattering spectra. As the fluence of neutrons increased, the yellow luminescence (YL) intensity increased because of the increase in \text V\mathrm {{Ga}} -related defects. Irradiation with fast and thermal neutrons destroyed the lattice integrity and changed the strain state of the devices, which was the main reason for the shift in the \text E\mathrm {{2}}^{\mathrm {{high}}} phonon mode. [ABSTRACT FROM PUBLISHER]