Various nanostructures and ultrathin layers attract increasing attention for the fabrication of different sensors and photodetectors. In this paper, near-UV photodetectors based on ultrathin gallium nitride layers grown by molecular  beam epitaxy with nitrogen plasma activation on sapphire substrates are presented. Two types of photodetectors are fabricated: metal−semiconductor−metal detectors and Schottky photodiodes, and their characteristics are studied.  It is shown that using a silicon dioxide layer in the detector design allows not only to reduce the dark current of such devices, but also to drastically increase the photocurrent of the detectors. Thus, gallium nitride surface passivation  with silicon dioxide allows to increase the photocurrent to the dark current ration of the detectors, and therefore has a positive effect on the performance of the photosensitive devices based on ultrathin gallium nitride layers.