The photonic mixer comprises a couple of an injecting contact region(3,4) for injecting the majority carrier current into the semiconductor substrate (1) and a detector region (7,8) for collecting the photocurrent. The injecting contact region (3,4) is doped with a dopant of the first conductivity type (p + ) at a higher dopant concentration than the semiconductor substrate (1). The detector region (7,8) is doped with a dopant of a second conductivity type (n + ) opposite the first conductivity type and has a junction (11,12) with the semiconductor substrate (1), a zone of the semiconductor substrate (1) around said junction (11,12) being a depleted substrate zone (101, 102). The couple further comprises a field shaping zone (13, 14) of the first conductivity type (p - ) defining a lateral edge of the couple and having a dopant concentration higher than the dopant concentration of the semiconductor substrate (1), for example between the dopant concentrations of the semiconductor substrate (1) and the injecting contact region (3,4), which field shaping zone (13, 14) is designed to limit said depleted substrate zone (101, 102) laterally.