MSM photodetectors and Schottky barrier photodetectors over spectral range 200—365 nm with characteristics close to those of the best foreign analogs, in particular, leakage current density of about 10^–8 A/cm^–2, have been obtained using domestic GaN grown by MOCVD on sapphire substrates of (0001) orientation. The characteristics of Al_0,1Ga_0,9N Schottky barrier photodetectors have been given. The effect of epilayer structural peculiarities on electro-physical properties of the epilayers and on parameters of the UV photodetectors has been investigated. Random distribution of charged centers, which is associated with domain boundaries of the mosaic structure being typical of III-nitrides, has been shown to result in a low Schottky barrier height, a high leakage current, and persistent photoconductivity in undoped GaN. The introduction of low Si concentration minimizes the effect as well as allows the Schottky barrier height to be obtained close to the difference between work functions of GaN and metal.