We report metal-insulator-semiconductor photodetectors fabricated on a silicon-on-insulator substrate, exhibiting an enhanced responsivity in the near-infrared due to the incorporation of Au-nanoantennas at the optical input port. The optimal antenna design and the photoresponse enhancements have been calculated using detailed finite-difference time-domain simulations. These nanoantennas modify the propagation of the input optical signals. In this regime, strong plasmonic local field enhancements are observed. The maximum responsivity enhancement, 40%, was obtained at 780 nm; while at wavelengths shorter than 720 nm, the responsivity is reduced due to absorption losses in the Au-nanoantennas. © 2002-2012 IEEE.