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After calculating the ballistic electrical and thermal parameters, one can evaluate
the thermoelectric figure of merit using the Eq. 2.1. The
values are shown in Fig. 3.6 for the ribbon's widths between
and
and temperature up to
. By increasing the ribbon's width,
decreases
drastically. It is due to the fact that the lattice thermal
conductance strongly increases with
, whereas the power factor
slightly increases. On the other hand, for a given ribbon's width,
peaks both at very low and very high temperatures. At high
temperature, the contribution of the second conduction subband results
in a high power factor. On the other hand, the lattice thermal
conductance strongly decreases with temperature, whereas the power
factor can be kept by adjusting the Fermi energy. As a result, a high
value of
can be obtained at very low temperature as well, although the
value is smaller than
for various ribbons and temperatures.