We report electrical characterisation of NiFe/SiO2/Si and NiFe/Al2O3/Si tunnel diodes that are of potential interest for spin injection into silicon. The effect of the nature of the insulator on the electrical properties of the diode has been studied. Impedance measurements have shown a small difference in charge trap densities between SiO2/Si and Al2O3/Si interfaces. Analyses of the Current-Voltage characteristics of NiFe/SiO2/Si sample have evidenced assisted tunnelling through defects in the oxide bulk in contrast to Fowler-Nordheim tunnelling in the NiFe/Al2O3/Si structures. This result shows that Al2O3 is a more efficient barrier against diffusion of magnetic atoms towards the silicon substrate; which is promising for spin injection into silicon. (C) 2010 Published by Elsevier Ltd.