Solar blind metal-semiconductor-metal detectors have been fabricated based on AlGaN grown on Si by molecular-beam epitaxy. Submicron finger spacings were obtained by electron-beam lithography, and allowed us to demonstrate a significant improvement of the responsivity and the spectral selectivity. These results were explained by numerical two-dimensional calculations of the electric-field distribution. The simulation also explained the dependence of the response on applied bias. (C) 2002 American Institute of Physics.