Silicon–Germanium: The Legacy Lives On

Alloy systems comprised of silicon with germanium, lead with tellurium, and bismuth with antimony have constituted a majority of thermoelectric applications during the last half-century. These legacy materials are primarily covalently bonded with a maximum ZT near one. Silicon–germanium alloys have provided the thermal to electrical conversion for many of NASA’s radioisotope thermoelectric generator (RTG) configurations and for nearly all of its deep space and outer planetary flights, such as Pioneer I and II, Voyager I and 11, Ulysses, Galileo, and Cassini. The remarkable success of these materials and their respective devices is evidenced by the fact that there has never been a failure of the RTG systems even after over 1 billion cumulative mission-hours. The history of this alloy system as a thermoelectric conversion material spans over six decades and research to further improve its performance continues to this day. Si-Ge alloys have long been a mainstay of thermoelectric research because of a fortuitous combination of a sufficiently high melting temperature, reasonable energy band gap, high solubility for both n- and p-type dopants, and the fact that this alloy system exhibits complete miscibility in the solid state, which enable tuning of both electrical and thermal properties. This article reviews the history of silicon–germanium as a thermoelectric material and its use in NASA’s RTG programs. Since the device technology is also a critical operational consideration, a brief review of some of the unique challenges imposed by the use in an RTG is also discussed.