Waveguide photodetector in germanium on silicon

It is described a photodetector structure (1,1′,1"), comprising a silicon-based waveguide (2) in which optical signals to be detected travel in a given direction (X) and are confined therein; a germanium-based layer (4) disposed on a portion of the silicon-based waveguide (2), the germanium layer (4) including a first mesa (10) defining an active region having a length (L) along the signal propagating direction (X) and a width (W) in a direction (Z) substantially perpendicular to the propagating direction (X) so that an evanescent tail of the propagating optical signal in the waveguide (2) is coupled into the active region, and a second mesa (30; 40) separated by a distance d from the first mesa (10) in said direction (Z) substantially perpendicular to the signal propagating direction. The photodetector further includes a first (7) and a second metallic contact (9a,9b) having opposite polarities, the first metallic contact (7) being located on the active region of the first mesa (10) and the second metallic contact (9a, 9b) being located on said second mesa (30), said first and second contact being used to collect electrons generated by light absorption to obtain an output electric signal.