Carrier transport across Fe3 O4 GaAs interfaces has been studied for n - and p -type GaAs(001) substrates with medium (7.7× 1017 cm-3) to high (3.5× 1018 cm-3) carrier concentrations. Current-voltage (I-V) measurements on medium-doped substrates show a rectifying behavior that is characteristic for thermionic emission/diffusion across a Schottky barrier. The n -type structure exhibits a low ideality factor of 1.3 and a Schottky barrier height of 0.58-0.63 eV. The Schottky barrier height of the p -type sample is 0.51 eV. For Fe3 O4 GaAs structures with higher doping levels the I-V dependence is nearly symmetric. In this case, tunneling of electrons and holes through the Schottky barrier dominates transport between the Fe3 O4 layer and the GaAs substrate.