We report on characterization and operation principle of a set of GaN/AlGaN multiple-quantum-well (MQW) photovoltaic detectors. The structures were grown by molecular beam epitaxy (MBE) on c-plane sapphire substrates and fabricated in the back-illuminated vertical Schottky geometry. Introduction of MQWs into the active region of devices is expected to enhance the quantum efficiency due to the high absorption coefficient. A nearly flat spectral responsivity between 325 and 350 run with 0.054 A/W peak responsivity was achieved from the single-side polished backside (rough) illuminated GaN/AlGaN MQW devices. The cutoff wavelength of the MQW photodetector can be tuned by adjusting the well width, well composition and barrier height. A model has been developed to gain insight into the operation principles of MQWs photodiodes. The peak responsivity increased with decreasing barrier thickness due to enhanced tunneling of photogenerated carriers. (C) 2003 Elsevier Science Ltd. All rights reserved.