58b join to form a closed member for enclosing an insulated island such as island 52 when viewed in plan view such as shown in fig2 e the device shown in fig3 employs the island configuration shown in fig2 d as its starting element more specifically the passivation layer mentioned with reference to fig2 d is formed on the surface of the device shown in fig2 d having a plurality of openings for exposing at least the upper surface 52a and 54a of the islands 52 and 54 respectively as an alternate procedure for forming devices according to the teaching of the present invention the p type diffusion step is made over the entire surface of the composite structure comprising the plurality of islands 52 and 54 the insulating layers 56 and the polycrystalline member 58 without the formation of a diffusion mask thereon the insulating layers 56 and the use of a polycrystalline member 58 for backfilling the grooves 42 44 and 46 makes this possible in this manner the diffusion is made into the entire upper surface of such composite structure where a uniform diffusion region 62 of p type conductivity is achieved in each island 52 and 54 and in the surrounding polycrystalline silicon portion 58a 58b and 58c as represented by a line 63 and an additional line 64 respectively a p type diffusion step is made through the aforementioned openings forming an opposite conductivity type region 62 in the islands 52 and 54 a line 63 shows the junction between the first conductivity type region 32 and the opposite conductivity type region 62 of the island 52 an additional masking layer 65 is formed over.