which represents the relative contribution of electrons and holes to the overall charge transport. In a single-band scenario (one dominant carrier), the quality factor determines the maximum attainable zT at a given temperature. On the other hand, the maximum zT in a two-band system at a given temperature is governed by three material-dependent parameters: R , E g , and either p or n . In Figures and, we contrast zT and the electronic transport coe cients when the bands are nearly symmetric (R ⇡ 1) vs. highly asymmetric (| log(R )| >> 0). When R ⇡ 1, zT varies nearly symmetrically about the mid gap (Figures ). As the band gap decreases (0.5 eV ! 0.05 eV ! -0.05 eV), the TE performance worsens due to the detrimental e↵ects of bipolar conduction (see also Figure ). This behavior can be understood from the individual transport coe cients. The electrical conductivity is higher when the band gap is smaller or negative (Figure ), because the Fermi-Dirac distribution samples conduction and valence band states, leading to additive contributions from both electrons and holes (Eq. 9, Figure ). However, the magnitude of the Seebeck coe cient decreases (Figure ). This is due to the partial cancellation of the positive and negative contributions above and below E F (Figure ). Moreover, the higher electrical conductivity contributes to a higher electronic thermal conductivity in materials with small and negative band gaps (Figure ). Therefore, bipolar conduction in materials with low asymmetry (R close to 1) lead to lower zT for narrow-gap semiconductors and semimetals. A high asymmetry parameter R , on the other hand, will result in comparatively higher zT for the same p , as illustrated in Figures . Generally, the maximum zT for a given E g and p will be higher when R > 1 as shown in Figure . Analogously, maximum zT for a given E g and n will be higher when 1/R > 1.