What dictates the efficiency of a semiconductor?

Question

Semiconductors can be used for a heat exchange but are less efficient than a Freon air-conditioning system. What dictates this efficiency?

Answer 1

The efficiency of semiconductors for refrigeration (Peltier junctions) is established by the ratio of the junction's thermal conductivity to its electrical conductivity. Here is why:

For best performance, you would like the junction to conduct heat poorly, so the cold generated on one side of the junction is not immediately cancelled by the heat generated on the other. At the same time, you also want the electrical conductivity of the junction to be as high as possible, so $I^2R$ losses in the junction are minimized.

Unfortunately, the thermal and electrical conductivities are tied together in a way that makes it very difficult to maximize one while simultaneously minimizing the other.

Materials scientists try to overcome this effect by coming up with weird junction compositions from rarely-visited corners of the periodic table, but with limited success.