A process of fabrication of a semiconductor substrate structure, comprising: (i) fabricating a first intermediate structure comprising: a silicon carbide, SiC, wafer substrate; a microcrystalline diamond, MCD, layer bonded to the SiC wafer substrate, the MCD layer having a first MCD surface and a second MCD surface; a first bonding material layer disposed between the MCD layer and the SiC wafer substrate, said first bonding material layer being bonded to the SiC wafer substrate and to the first MCD surface of the first MCD layer; and a second bonding material layer disposed on the second MCD surface of the MCD layer and having an outer surface being exposed; (ii) fabricating a second intermediate structure comprising: a gallium nitride, GaN, active layer, a Si carrier substrate, and an adhesive layer therebetween; the GaN active layer having a first GaN surface bonded to the Si carrier substrate via the adhesive layer, and a second GaN surface being exposed; (iii) bonding said first intermediate structure to said second intermediate structure, where the second GaN surface of the GaN active layer of the structure is bonded to the outer surface of the second bonding material layer; (iv) removing the Si carrier substrate together with the adhesive layer formed between the Si carrier substrate and the first GaN surface of the GaN active layer; thereby obtaining said semiconductor substrate structure comprising: the SiC wafer substrate; the GaN active layer; and the MCD layer disposed between the SiC wafer substrate and the GaN active layer, and bonded to the SiC wafer substrate via the first bonding material layer and to the GaN active layer via the second bonding material layer. (i) fabricating a first intermediate structure comprising: a silicon carbide, SiC, wafer substrate; a microcrystalline diamond, MCD, layer bonded to the SiC wafer substrate, the MCD layer having a first MCD surface and a second MCD surface; a first bonding material layer disposed between the MCD layer and the SiC wafer substrate, said first bonding material layer being bonded to the SiC wafer substrate and to the first MCD surface of the first MCD layer; and a second bonding material layer disposed on the second MCD surface of the MCD layer and having an outer surface being exposed; a silicon carbide, SiC, wafer substrate; a microcrystalline diamond, MCD, layer bonded to the SiC wafer substrate, the MCD layer having a first MCD surface and a second MCD surface; a first bonding material layer disposed between the MCD layer and the SiC wafer substrate, said first bonding material layer being bonded to the SiC wafer substrate and to the first MCD surface of the first MCD layer; and a second bonding material layer disposed on the second MCD surface of the MCD layer and having an outer surface being exposed; (ii) fabricating a second intermediate structure comprising: a gallium nitride, GaN, active layer, a Si carrier substrate, and an adhesive layer therebetween; the GaN active layer having a first GaN surface bonded to the Si carrier substrate via the adhesive layer, and a second GaN surface being exposed; a gallium nitride, GaN, active layer, a Si carrier substrate, and an adhesive layer therebetween; the GaN active layer having a first GaN surface bonded to the Si carrier substrate via the adhesive layer, and a second GaN surface being exposed; (iii) bonding said first intermediate structure to said second intermediate structure, where the second GaN surface of the GaN active layer of the structure is bonded to the outer surface of the second bonding material layer; (iv) removing the Si carrier substrate together with the adhesive layer formed between the Si carrier substrate and the first GaN surface of the GaN active layer; thereby obtaining said semiconductor substrate structure comprising: the SiC wafer substrate; the GaN active layer; and the MCD layer disposed between the SiC wafer substrate and the GaN active layer, and bonded to the SiC wafer substrate via the first bonding material layer and to the GaN active layer via the second bonding material layer. the SiC wafer substrate; the GaN active layer; and the MCD layer disposed between the SiC wafer substrate and the GaN active layer, and bonded to the SiC wafer substrate via the first bonding material layer and to the GaN active layer via the second bonding material layer.