A novel composite metal material comprises a ferrous metal of relatively low thermal expansion properties and a silver metal substantially free of ferrous constituents having a relatively high thermal conductivity which cooperate in a novel way to provide the composite material with an effective thermal expansion coefficient corresponding to that of various silicon or gallium arsenide semiconductor devices and the like for reliably mounting the devices while also providing paths of high conductivity silver metal extending through the composite material to provide improved heat-dissipation from the semiconductor devices. A circuit system mounts a semiconductor device using the novel composite metal material alone, bonded to other support materials, or formed into a selected shape. In one preferred embodiment, the composite material is used in a novel heat-dissipating member having components of different shape which are bonded together. One component formed of the novel composite material has a selected shape for compactly mounting the semiconductor device and the second component is formed of high thermal conductivity metal and has a different shape particularly adapted to receive heat from the silver paths and to facilitate heat-dissipation from the member. Novel methods for making the member and the composite material are shown.