  The success of a ferroelectric tunnel junction (FTJ) depends on the asymmetry
of electron tunneling as given by the tunneling electroresistance (TER) effect.
This characteristic is mainly assessed considering three transport mechanisms:
direct tunneling, thermionic emission, and Fowler-Nordheim tunneling. Here, by
analyzing the effect of temperature on TER, we show that taking into account
only these mechanisms may not be enough in order to fully characterize the
performance of FTJ devices. We approach the electron tunneling in FTJ with the
non-equilibrium Green function (NEGF) method, which is able to overcome the
limitations affecting the three mechanisms mentioned above. We bring evidence
that the performance of FTJs is also affected by temperature, in a non-trivial
way, via resonance (Gamow-Siegert) states, which are present in the electron
transmission probability and are usually situated above the barrier. Although
the NEGF technique does not provide direct access to the wavefunctions, we show
that, for single-band transport, one can find the wavefunction at any given
energy and in particular at resonant energies in the system.
