  We present an analysis of the thermoelectric properties of of $n$-type GeTe
and SnTe in relation to the lead chalcogenides PbTe and PbSe. We find that the
singly degenerate conduction bands of semiconducting GeTe and SnTe are highly
non-ellipsoidal, even very close to the band edges. This leads to isoenergy
surfaces with a strongly corrugated shape that is clearly evident at carrier
concentrations well below 0.005 $e$ per formula unit (7 - 9 $\times$
10$^{19}$cm$^{-3}$ depending on material) Analysis within Boltzmann theory
suggests that this corrugation may be favorable for the thermoelectric
transport. Our calculations also indicate that values of the power factor for
these two materials may well exceed those of PbTe and PbSe. As a result these
materials may exhibit $n$-type performance exceeding that of the lead
chalcogenides.
