A method of manufacturing a biosensor semiconductor device in which copper electrodes at a major surface of the device are modified to form Au—Cu alloy electrodes. Such modification is effected by depositing a gold layer over the device, and then thermally treating the device to promote interdiffusion between the gold and the electrode copper. Alloyed gold-copper is removed from the surface of the device, leaving the exposed electrodes. The electrodes are better compatible with further processing into a biosensor device than is the case with conventional copper electrodes, and the process windows are wider than for gold capped copper electrodes. A biosensor semiconductor device having Au—Cu alloy electrodes is also disclosed.