  Several models of thermionic energy nanoconverters have been proposed to
study the transport phenomena that take place in electronic devices. For
example, in resonant tunneling junctions those phenomena are manifested through
the thermoelectric effects. The coupling between the electron flux and the heat
flux in this type of semiconductor heterostructures, not only allows to obtain
transport coefficients (electrical and thermal conductivities, and a
Seebeck--like and Peltier--like coefficients), but also to study its operation
as a thermionic generator or as a refrigerator within the context of
irreversible thermodynamics. The existence of the characteristic steady states
that can be reached by any linear energy converter led us to characterize a
family of Seebeck--like coefficients, as well as establish bounds for the
values of a kind of figure of merit $(Tz'_{D,I})$, both associated with the
well-known operating regimes: minimum dissipation function, maximum power
output, maximum efficiency and maximum compromise function. By taking as
example an $Al_{x}GaAs/GaAs$ junction, we found that the transport coefficients
depend strongly on temperature and the conduction band height, which can be
modulated according to the selected operation mode.
