In this work we calculate the I-V characteristics of
Fe/MgO/Fe(100) tunneling junctions from first principles. First we investigate the
zero-bias transmission coefficients T(E) as a function of the energy for either
collinear parallel or antiparallel configurations of the magnetic leads. The zero
bias calculations are then compared to the self-consistent calculations at finite
bias. For small bias voltage there is a significant contribution from the minority
spin channel to the current in the parallel configuration, which is due to resonant
surface states. As a bias of about 25 mV is applied the surface states get out of
resonance and the current through the minority spin channel saturates, so that the
main current flows through the majority spin.