The I-V relations found for the system metal vertical bar semiconductor vertical bar metal with chemically inert electrodes are not always as one would expect from the common theory of semiconductivity. The problem can be traced back to motion of ionic defects, i.e. the acceptors (or donors) in the semiconductor. We here discuss the parameters, equations and physical constraints that have to be taken into consideration when evaluating the I-V relations of that system in the presence of motion of the ionic defects. For comparison another model, that of electron hopping in the band of dopant impurity states, is also discussed. I-V relations and the defect distributions depend on three main factors: the contact potentials at the semiconductor/metal-electrode, the thickness of the sample and the degree of ionization of the dopant. A variety of I-V relations are obtained for different values of the controlling parameters. This explains the diversity in the I-V relations reported for systems of the form metal vertical bar semiconductor vertical bar metal. (c) 2008 Elsevier B.V. All rights reserved.