In this work ultraviolet metal-semiconductor-metal and metal-insulator-semiconductor photodetectors based on GaN epitaxial layers were fabricated. N-polar GaN epitaxial layers were synthesized by plasma-assisted molecular beam epitaxy on nitrided sapphire substrates. To form Schottky barrier contacts a Ni/Au metallization was chosen. SiO2 layers were deposited by plasma enhanced chemical vapor deposition. I–V characteristics of fabricated photodetectors were studied. It was found that the dark current of the photodetectors decreased by 49 times after introducing a 20 nm thick SiO2 dielectric layer, and the photocurrent to dark current ratio increased by a maximum of 35 times.