Al0.2Ga0.8N/GaN/Al0.2Ga0.8N multilayer structures and GaN monolayer structures with AIN as the buffer layers were grown on Si substrates by metal-organic chemical vapour deposition. The photocurrent responses of these structures were measured and analysed. The multilayer structures showed a high response in a narrow range of wavelengths. The peak wavelength is located at 365 nm at which the responsivity is as high as 24 A/W under 5.5 V bias; this is much higher than the GaN monolayer structure. This high responsivity results mainly from the high polarization electric field in the GaN layer of the Al0.2Ga0.8N/GaN/Al0.2Ga0.8N heterostructure.