# What dictates the efficiency of a semiconductor?

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

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.