A semiconductor photodiode (18) is formed as a pn-junction between a region (2) of a first conductivity type and a region (6) of a second conductivity type. The region (6) of the second conductivity type is approximately hemispherical. A mini guard ring (8), i.e. a ring of the second conductivity type having a junction depth that is much smaller than the junction depth of the region (6) preferably surrounds the region (6) in order to prevent surface trapping. The photodiode (18) is operated with a high reverse bias so that light falling on the photodiode (18) produces the avalanche effect.