Abstract: A Metal-Semiconductor-Metal (MSM) photodetector comprises a semiconductor substrate, a semiconductor barrier layer on the substrate, a thin semiconductor active layer on the barrier layer, and at least two electrical contacts to the active layer. The barrier layer prevents carriers generated deep in the substrate from reaching the contacts. As it is the delayed detection of these carriers which limits the useful operating speed or bandwidth of conventional MSM photodetectors, the MSM photodetector according to the invention is capable of higher speed operation than conventional MSM photodetectors.