A method, comprising: forming an opening in a front surface of a substrate comprising at least one Group III nitride-based transistor on the front surface; wherein the opening is a blind via with a base and an open end at the front surface, inserting conductive material into the opening from the front surface, wherein inserting the conductive material into the opening from the front surface comprises: forming a first conductive layer in the base of the blind via in the substrate using first deposition parameters including a first electrochemical bath comprising a first chemical additive system such that the first conductive layer fills the base of the blind via to a depth of 10% to 70% of a total depth of the blind via, forming a conductive plug; and afterwards forming a second conductive layer on the first conductive layer in a second portion of the opening using second deposition parameters including a second electrochemical processing bath including a second chemical additive system that is different from the first chemical additional system such that the second conductive layer lines the sidewalls of said opening, defining a gap in the second portion, wherein the first deposition parameters are selected so as to favour growth of the first conductive layer in a vertical direction with respect to the front surface of the substrate and the second deposition parameters are selected so as to favour growth of the second conductive layer in a lateral direction with respect to the front surface of the substrate (130; 191), wherein the second conductive layer is positioned directly on the first layer and forms an interface with the first conductive layer and extends along side walls of the blind via to the open end of the blind via at the front surface, exposing a portion of said conducive plug at the rear surface of the substrate, coupling a source electrode of the Group III nitride-based transistor to the rear surface of the substrate through said conductive material in said opening. forming an opening in a front surface of a substrate comprising at least one Group III nitride-based transistor on the front surface; wherein the opening is a blind via with a base and an open end at the front surface, inserting conductive material into the opening from the front surface, wherein inserting the conductive material into the opening from the front surface comprises: forming a first conductive layer in the base of the blind via in the substrate using first deposition parameters including a first electrochemical bath comprising a first chemical additive system such that the first conductive layer fills the base of the blind via to a depth of 10% to 70% of a total depth of the blind via, forming a conductive plug; and afterwards forming a second conductive layer on the first conductive layer in a second portion of the opening using second deposition parameters including a second electrochemical processing bath including a second chemical additive system that is different from the first chemical additional system such that the second conductive layer lines the sidewalls of said opening, defining a gap in the second portion, wherein the first deposition parameters are selected so as to favour growth of the first conductive layer in a vertical direction with respect to the front surface of the substrate and the second deposition parameters are selected so as to favour growth of the second conductive layer in a lateral direction with respect to the front surface of the substrate (130; 191), wherein the second conductive layer is positioned directly on the first layer and forms an interface with the first conductive layer and extends along side walls of the blind via to the open end of the blind via at the front surface, exposing a portion of said conducive plug at the rear surface of the substrate, coupling a source electrode of the Group III nitride-based transistor to the rear surface of the substrate through said conductive material in said opening. forming a first conductive layer in the base of the blind via in the substrate using first deposition parameters including a first electrochemical bath comprising a first chemical additive system such that the first conductive layer fills the base of the blind via to a depth of 10% to 70% of a total depth of the blind via, forming a conductive plug; and afterwards forming a second conductive layer on the first conductive layer in a second portion of the opening using second deposition parameters including a second electrochemical processing bath including a second chemical additive system that is different from the first chemical additional system such that the second conductive layer lines the sidewalls of said opening, defining a gap in the second portion, wherein the first deposition parameters are selected so as to favour growth of the first conductive layer in a vertical direction with respect to the front surface of the substrate and the second deposition parameters are selected so as to favour growth of the second conductive layer in a lateral direction with respect to the front surface of the substrate (130; 191), wherein the second conductive layer is positioned directly on the first layer and forms an interface with the first conductive layer and extends along side walls of the blind via to the open end of the blind via at the front surface, exposing a portion of said conducive plug at the rear surface of the substrate, coupling a source electrode of the Group III nitride-based transistor to the rear surface of the substrate through said conductive material in said opening.