A composite wafer comprising: a semiconductor-on-diamond wafer comprising at least one layer of single-crystal semiconductor and at least one layer of diamond; and a carrier plate; wherein said at least one layer of diamond is proximal to said carrier plate, wherein said carrier plate comprises a material having a coefficient of linear thermal expansion α at 20°C in at least one plane of no more than 10.0 x 10 -6 K -1, wherein an exposed surface of single-crystal semiconductor has a surface flatness of better than 30 μm over a total area of the exposed surface of single-crystal semiconductor, wherein the composite wafer has a longest linear dimension of at least 50 mm, and wherein said at least one layer of single-crystal semiconductor comprises at least one semiconductor selected from the group containing gallium nitride, aluminum nitride, and indium nitride; characterised in that: a porous ceramic adhesive (391, 404) is disposed between said semiconductor-on-diamond wafer and said carrier plate; wherein said porous ceramic adhesive (391, 404) comprises a material having a melting point of at least 900°C; wherein said porous ceramic adhesive (391, 404) is capable of withstanding a temperature change of at least 900°C in no more than 1.5 minutes while maintaining bonding between the semiconductor-on-diamond wafer and the carrier plate; wherein the composite wafer has a bow of no more than 30 μm over the total area of the composite wafer; and wherein the composite wafer has a thickness variation of no more than 30 μm over the total area of the composite wafer. a semiconductor-on-diamond wafer comprising at least one layer of single-crystal semiconductor and at least one layer of diamond; and a carrier plate; wherein said at least one layer of diamond is proximal to said carrier plate, wherein said carrier plate comprises a material having a coefficient of linear thermal expansion α at 20°C in at least one plane of no more than 10.0 x 10 -6 K -1, wherein an exposed surface of single-crystal semiconductor has a surface flatness of better than 30 μm over a total area of the exposed surface of single-crystal semiconductor, wherein the composite wafer has a longest linear dimension of at least 50 mm, and wherein said at least one layer of single-crystal semiconductor comprises at least one semiconductor selected from the group containing gallium nitride, aluminum nitride, and indium nitride; characterised in that: a porous ceramic adhesive (391, 404) is disposed between said semiconductor-on-diamond wafer and said carrier plate; wherein said porous ceramic adhesive (391, 404) comprises a material having a melting point of at least 900°C; wherein said porous ceramic adhesive (391, 404) is capable of withstanding a temperature change of at least 900°C in no more than 1.5 minutes while maintaining bonding between the semiconductor-on-diamond wafer and the carrier plate; wherein the composite wafer has a bow of no more than 30 μm over the total area of the composite wafer; and wherein the composite wafer has a thickness variation of no more than 30 μm over the total area of the composite wafer. a porous ceramic adhesive (391, 404) is disposed between said semiconductor-on-diamond wafer and said carrier plate; wherein said porous ceramic adhesive (391, 404) comprises a material having a melting point of at least 900°C; wherein said porous ceramic adhesive (391, 404) is capable of withstanding a temperature change of at least 900°C in no more than 1.5 minutes while maintaining bonding between the semiconductor-on-diamond wafer and the carrier plate; wherein the composite wafer has a bow of no more than 30 μm over the total area of the composite wafer; and wherein the composite wafer has a thickness variation of no more than 30 μm over the total area of the composite wafer.