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In recent papers conduction mechanism in tantalum pentoxide MIM capacitors was found to follow the space charge limited theory and the asymmetry between positive and negative polarization was attributed to an inhomogeneous spatial distribution of traps. Furthermore, the role of the top electrode geometry on the conduction was highlighted using a large range of test structures, allowing the complete characterization of the leakage current at time zero. However, other studies showed the instability of the leakage current in Ta2O5 during electrical stresses. In this study, a proof is given of the strong influence of the oxygen vacancies profile on the current instability as well as on the leakage current asymmetry between positive and negative polarization. Moreover, dedicated test structures are used to extract surface and peripheral component of the current during electrical stress, leading to a self consistent relation between the I-V and I-t characteristics.