In this paper, we present the analysis of the results of I(V) characteristics of n-GaAs/Co Schottky structures and of spin-LED structures of the type [p-GaAs/p-AlGaAs/AlGaAs/GaAs/AlGaAs/n-AlGaAs]/Co where the semiconductor heterostructure acts as a quantum well LED. We have focused this study on tunneling through the space charge layer at the metal-semiconductor interface. We report strong differences for the I(V) characteristics of the two structures which can have important implications in the understanding of the exact mechanism of spin injection between a metal and a semiconductor. (C) 2004 Published by Elsevier Ltd.