  Using first principles calculations, we study the conduction band alignment,
effective mass, and Fermi surface complexity factor of n-type Mg3Sb2-xBix (x =
0, 1, and 2) from the full ab initio band structure. We find that with
increasing the Bi content the K and M band minima moves away from the
conduction band minimum CB1 while the singly-degenerate $\Gamma$ band minimum
shifts rapidly downward and approaches the conduction band minimum. But the
favorable six-fold degenerate CB1 band minimum keeps dominating the conduction
band minimum and there is no band crossing between the $\Gamma$ and CB1 band
minima. In addition, we show that the connection of the CB1 carrier pockets
with the energy level close to the band minimum M can strongly enhance the
carrier pocket anisotropy and Fermi surface complexity factor, which is likely
the electronic origin for the local maximum in the theoretical power factor.
Our calculations also show that the band gap, density of states effective mass,
Seebeck coefficient, and Fermi surface complexity factor decrease with
increasing the Bi content, which is unfavorable to the electrical transport.
However, reducing the conductivity effective mass with increasing the Bi
content is beneficial to the electrical transport by improving carrier mobility
and weighted mobility as long as the detrimental bipolar effect is
insignificant. As a result, in comparison with n-type Mg3Sb2, n-type Mg3SbBi
shows higher power factors and a much lower optimal carrier concentration for
the theoretical power factor at 300 K, which can be easily achieved by the
experiment.
