Planar Al/GaN/Ni Schottky diodes were realized on GaN films deposited on sapphire substrates and characterized in the dark and under illumination. The optoelectronic characteristics of GaNphotodetectors appear largely influenced by structural defects and impurities, which are clearly detected in photocurrent yield measurements. In particular, an exponential increase of the photocurrent is observed and explained in terms of a barrier lowering photoeffect, hence a light-induced shrinking of the space charge region, related to carrier trapping at defects and impurities. Trapping events are also responsible for a dispersive behavior of the AC responsivity with the light chopping frequency. Such effects point out the importance of a proper selection of bias voltage and working frequency for GaNphotodetector operations.