SiGe thermocouples in an RTG convert heat directly into electricity. Thermoelectric power generation requires a constantly maintained temperature difference among the junctions of the two dissimilar metals (i.e. Si and Ge) to produce a low power closed circuit electric current without extra circuitry or external power sources.
A large array of SiGe thermocouples/unicouples form a thermopile that was incorporated into the design of radioisotope thermoelectric generators (RTGs) used in the missions Voyager, Galileo, Ulysses, Cassini, and New Horizons. On these spacecraft, Pu-238 dioxide fuel undergoes natural decay. The SiGe thermocouples/unicouples convert this heat to hundreds of Watts of electrical power.