A solid-state heat engine which employs the electron gas as a working fluid. It directly converts heat energy into electrical energy using the Seebeck effect. This phenomenon can be demonstrated using a thermocouple which comprises two legs (thermoelements) of dissimilar conducting materials joined at one end to form a junction. If this junction is maintained at a temperature which differs from ambient, a voltage is generated across the open ends of the thermoelements. When the circuit is completed with a load, a current flows in the circuit and power is generated. In practice the thermocouples are fabricated generally from n- and p-type semiconductors, and several hundred are connected electrically in series to form a module which is the active component of a thermoelectric generator. Provided a temperature difference is maintained across the device, it will generate electrical power. Heat is provided from a variety of sources depending on the application, and they include burning fossil fuels in terrestrial and military applications, decaying long-life isotopes in medical and deep-space applications, and waste heat. The performance of the thermoelectric generator, in terms of efficiency, output power, and economic viability, depends upon its temperature regime of operation; the materials used in the module construction; its electrical, thermal, and geometrical design; and the generator load. The power output spectrum of thermoelectric generators spans 14 orders of magnitude and ranges from nanowatt generators fabricated using integrated circuit technology to the nuclear reactor–powered 100-kW SP-100 generator intended to provide electrical power to orbiting space stations.