A photodetector structure includes a silicon-based waveguide in which optical signals to be detected travel in a given direction and are confined therein and a germanium layer disposed in contact with a portion of the silicon-based waveguide so that an evanescent tail of the propagating optical signal in the waveguide is coupled into the germanium layer. In addition, the germanium layer includes a mesa having a length along the signal propagating direction and a width in a direction substantially perpendicular to the propagating direction, in which the width of said mesa is smaller than its length. The photodetector also comprises a first and a second metal contacts, the first metallic contact being located on the germanium layer, the said second metallic contact being located on the silicon-based waveguide, the first and second contacts being used to collect electrons generated by light absorption to obtain an output electric signal.