We report the degree to which the resonances associated with metal island films can be used to enhance the sensitivity of very thin semiconductor photodetectors. The island films can couple incident light into the waveguide modes of the detector, resulting in increased absorption. To characterize the coupling, silver-, gold-, and copper-island layers were formed on the surface of a thin-film photodetector fabricated in the 0.16 μm thick silicon layer of a silicon-on-insulator (SOI) wafer. The copper islands gave the best result, producing more than an order of magnitude enhancement in the photocurrent for light of wavelength 800 nm. The enhancements appear to be due primarily to coupling between the metal island resonances and the waveguide modes supported by the SOI structure.