Abstract: A thermoelectric composite and a thermoelectric device and a method of making the thermoelectric composite. The thermoelectric composite is a semiconductor material formed from mechanically-alloyed powders of elemental constituents of the semiconductor material to produce nano-particles of the semiconductor material, and compacted to have at least a bifurcated grain structure. The bifurcated grain structure has at least two different grain sizes including small size grains in a range of 2-200 nm and large size grains in a range of 0.5 to 5 microns. The semiconductor material has a figure of merit ZT, defined as a ratio of the product of square of Seebeck coefficient, S2, and electrical conductivity ? divided by the thermal conductivity k, which varies from greater than 1 at 300 K to 2.5 at temperatures of 300 to 500K.