The convention adopted here makes n and p dimensionless as in Ref. 32. zT as a function of ⌘ in a two-band model is shown in Figure for three representative cases of E g : (a) a semiconductor with a wide band gap, (b) a narrowgap semiconductor, and (c) a semimetal. As expected from a single parabolic band model, p determines the zT for a given doping level ⌘ and temperature, where higher p corresponds to higher TE performance. In general, high n and p are beneficial for n-type and p-type TE transport, respectively. However, in narrow-gap semiconductors and semimetals, bipolar conduction is significant, which lowers the maximum zT for smaller E g . When there is significant bipolar conduction, the maximum zT is largely determined by the asymmetry in the electron and hole transport properties. We notice that the conductivity ratio e h appears in the Seebeck coe cient (Eq. 10) and the electronic thermal conductivity (Eq. 12). From single-band electrical conductivity (Eq. 4), the conductivity ratio can be expressed as