where S is the Seekbeck coefficient, σ is the electrical conductivity, S 2 σ is the power factor (PF), T is the absolute temperature, and κ el and κ latt are the electronic and lattice (phonon) contributions to the thermal conductivity. To first approximation, S, σ and κ el are interrelated through the carrier concentration n such that the best balance of a large PF and minimal κ el is typically obtained for heavily-doped semiconductors. On the other hand, κ latt is largely independent of the electrical transport and should be minimised to obtain a large ZT .