In this work, the prototypes of visible-blind ultraviolet metal-semiconductor-metal photodetectors based on GaN epitaxial layers were implemented. For this purpose, ultrathin GaN epitaxial layers were synthesized by plasma assisted molecular beam epitaxy on sapphire substrates. The morphology and electrical properties of the obtained samples were studied. To form electric contacts with the Schottky barrier, an interdigitated electrode design with Ni/Au metallization was chosen and standard lift-off laser lithography procedure was used. It has been established that the formed photodetectors have the highest sensitivity to radiation with a wavelength of 350–360 nm. It was found that rapid thermal annealing of photodetector structures at a temperature of 500 °C made possible to reduce the dark current by a maximum of 30 times. In addition, it was shown that high temperature annealing led to the increase in Schottky barrier height and decrease in the ideality factor. Thus, it was confirmed that use rapid thermal annealing method can improve the characteristics of metal-semiconductor-metal visible-blind ultraviolet photodetectors based on GaN.