A semiconductor structure , comprising: a Group III-N semiconductor layer ; a source electrode structure (18 1 -18 2 ) and a drain electrode (22 1 -22 3 ) structure in Ohmic contact with the Group III-N semiconductor layer; a gate electrode structure (14 1 -14 4 ), disposed between the source electrode structure and the drain electrode structure, in contact with the Group III-N semiconductor layer; and wherein the source electrode structure, drain electrode structure and gate electrode structure comprising: an electrical contact structure (42 1 -42 7 , 14 GC ) and an electrode contact (54 1 -54 11 ) on the electrical contact structure, each electrode contact being of the same material, wherein each one of the electrode contacts (54 1 -54 11 ) is a gold-free electrically conductive material, and wherein each of the electrode contacts (54 1 -54 11 ) includes an upper layer of copper having the bottom and sides lined with an adhesion and copper diffusion barrier layer ; wherein electrical contact structures of the source electrode structure and drain electrode structure each include: (A) a gold-free Ohmic contact structure , having a bottom layer of titanium or tantalum; a layer of aluminum or Si doped aluminum, Al 1-x Si x , on the bottom layer , where the Si doping, x, is ≤ 0.05; and a top metal layer ; and (B) a gold-free electrically conductive etch stop layer (42 ES ) disposed on the Ohmic contact structure , wherein the etch stop layer (42 ES ) is nickel or molybdenum or platinum. a Group III-N semiconductor layer ; a source electrode structure (18 1 -18 2 ) and a drain electrode (22 1 -22 3 ) structure in Ohmic contact with the Group III-N semiconductor layer; a gate electrode structure (14 1 -14 4 ), disposed between the source electrode structure and the drain electrode structure, in contact with the Group III-N semiconductor layer; and wherein the source electrode structure, drain electrode structure and gate electrode structure comprising: an electrical contact structure (42 1 -42 7 , 14 GC ) and an electrode contact (54 1 -54 11 ) on the electrical contact structure, each electrode contact being of the same material, wherein each one of the electrode contacts (54 1 -54 11 ) is a gold-free electrically conductive material, and wherein each of the electrode contacts (54 1 -54 11 ) includes an upper layer of copper having the bottom and sides lined with an adhesion and copper diffusion barrier layer ; wherein electrical contact structures of the source electrode structure and drain electrode structure each include: (A) a gold-free Ohmic contact structure , having a bottom layer of titanium or tantalum; a layer of aluminum or Si doped aluminum, Al 1-x Si x , on the bottom layer , where the Si doping, x, is ≤ 0.05; and a top metal layer ; and (B) a gold-free electrically conductive etch stop layer (42 ES ) disposed on the Ohmic contact structure , wherein the etch stop layer (42 ES ) is nickel or molybdenum or platinum. (A) a gold-free Ohmic contact structure , having a bottom layer of titanium or tantalum; a layer of aluminum or Si doped aluminum, Al 1-x Si x , on the bottom layer , where the Si doping, x, is ≤ 0.05; and a top metal layer ; and (B) a gold-free electrically conductive etch stop layer (42 ES ) disposed on the Ohmic contact structure , wherein the etch stop layer (42 ES ) is nickel or molybdenum or platinum. <EOS>