in the example gaas is chosen for the active layer 13 whereas layers 12 and 14 are made of algaas these three layers are grown in a mbe process with an amphoteric dopant such as si arranged on top of the upper cladding layer 14 are a cathode 22 and at each side thereof anodes 20 and 21 these electrodes are as indicated connected to appropriate negative or positive voltage sources the surface of the substrate 11 is patterned to exhibit a protruding ridge having a horizontal surface and sloped edges that lead to lower horizontal planes of neighboring recesses or grooves the horizontal surface sections are of a first crystallographic orientation in the example 100 whereas the edges exhibit a different higher crystallographic index orientation in the example 111a due to the above mentioned effect that the conductivity of epitaxially grown gaas if doped with an amphoteric dopant depends on the crystallographic orientation of the underlying gaas substrate the sections of layers 12 13 and 14 above the horizontal surfaces are n type forming n columns whereas the sections of these layers above the sloped edges are p type forming p columns this results in the formation of p n junctions 15 through 18 at the intersections of the n.