optoelectronic devices are formed on a first surface of a gallium arsenide substrate using selective ion implantation . signal processing devices may be formed on a second , opposite surface of the substrate using selective ion implantation and / or selective epitaxy ,. vertical interconnects are formed between the first and second surfaces . alternatively , a gallium arsenide buffer layer may be grown on the first surface of the substrate , and the signal processing devices formed on the buffer layer using selective ion implantation and / or selective epitaxy . dielectric and / or conductive metal layers may be formed on selected areas of the first surface , and the buffer layer grown from exposed areas of the first surface over the dielectric and / or metal layers using lateral epitaxial overgrowth organometallic chemical vapor deposition . the signal processing devices may be formed in areas of the buffer layer which overlie the exposed areas of the substrate , and / or over the dielectric and / or metal layers . the present method enables metal - semiconductor field effect transistors , high electron mobility transistors and heterojunction bipolar transistors to be monolithically integrated in a vertical fashion for close packing density .