A gallium arsenide device (11) has a GaAs substrate (14) and a copper contact layer (21) for making electrical ground contact with a pad (16) of a target device. The copper contact layer is isolated from the GaAs substrate via a diffusion barrier layer (23), such as a nickel vanadium (NiV) layer. An organic solder preservative may coat the exposed copper to reduce oxidation effects. A gold or copper seed layer may be deposited prior to depositing the copper contact layer. The copper contact layer (21) can be directly soldered (18) to the contact pad (16), implying that the contact pad does not require an adhesive overflow area and can be made relatively small.