A photoresistor comprises a silicon-on-insulator substrate (101) comprising a device layer (4). In an example embodiment and mode at least two non-contiguous first highly conductive regions (2, 3) of semiconductor material are formed on a surface of the device layer, and at least one active region (1) of a high resistivity semiconductor material of a same conductivity type as the first highly conductive regions are formed to propagate through a whole thickness of the device layer and to electrically contact the at least two non-contiguous first highly conductive regions.