Abstract: A planar, p-n junction device has electrical contact made to the active region, the capping layer (4) having been selectively removed through a mask (5, 6). The selective removal of the capping layer may be performed prior to a diffusion step in which the junction is formed, or the dopant may be diffused through the capping layer (4) and the selective removal performed subsequently. In the latter instance the capping layer (4) is preferably etched through a second mask (8,9) having a smaller window (9) in register with the window (6) in the mask layer (5) which is used for the diffusion stage. The device finds particular application as a photodiode detector in optical communications.