  In this paper, general relationships that allow evaluating the figure of
merit of both two-dimensional superlattices and three-dimensional crystals with
a quadratic and isotropic energy spectrum of free charge carriers are derived.
It is shown that with the same values of the so-called dimensionless
temperature and scattering coefficient, the figure of merit of superlattices is
always lower than that of three-dimensional crystals. The figure of merit of
superlattices can become higher that that of three-dimensional crystals if the
dimensionless temperature of free charge carrier gas and (or) scattering
coefficient in them is considerably higher than in three-dimensional crystals.
