A semiconductor device for amplifying radiation comprises an n-type substrate (1), a p-type layer (3) between which is located an active layer (2). Portions of the p-type layer (3) are removed by an etching process to create a central contact portion (11). Typically, the p-type layer portions are removed by an etching process in combination with a photoresist mask deposited on the top surface of the p-type layer (3). By using an etching process the contact portion can be shaped so that the photoresist layer then overhangs the contact portion (11). N-type ions are then implanted into the active layer (2) where the p-type layer (3) has been removed and the overhanging photoresist layer (4) creates non-implanted regions (10) in the active layer (20) which separates the implanted regions (8, 9) from the contact portion (11). The implanted regions (8, 9) have the effect of creating a potential barrier between the non-implanted regions of the first layer and the implanted regions and this prevents current injected into the contact portion (11) from spreading horizontally in the active layer (2).