A semiconductor structure comprising: a substrate; a seed layer supported by the substrate; an elemental semiconductor layer disposed over a first portion of the seed layer; and a compound semiconductor layer disposed on a second portion of the seed layer. The first portion of the seed layer is electrically insulated from the second portion of the seed layer. A first semiconductor device is formed in the elemental semiconductor layer. A second semiconductor device is formed in the compound semiconductor layer. The second semiconductor device includes: a first electrode in contact with a first region of the compound semiconductor layer; a second electrode in contact with a second region of the compound semiconductor layer; and a third electrode. The third electrode controls carriers passing in a third region of the compound semiconductor layer disposed between the first region and the second region. A fourth electrode is in electrical contact with the second portion of the seed layer.