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We have fabricated Nb/Al2O3/Al/Al2O3/Nb devices with different current densities using a conventional fabrication process, varying pressure and oxidation time. Patterning of the multilayers was done using standard photolithography and electron-beam lithography. The current density of SINIS junctions was changed in the range from 0.5 kA/cm2 to 20 kA/cm2. We achieved characteristic voltages up to 0.35 mV. By fabricating sub-μm junction with a width from 0.1 μm to 0.5 μm, we have studied the influence of the asymmetry of barriers on transport properties. By comparing the experimental and theoretical temperature dependence of the characteristic voltage we estimated the barrier transparency and its asymmetry. The comparison shows a good agreement of experimental data with the theoretical model of tunnelling through double-barrier structures in the dirty limit. A new approach for determination of the asymmetry of both barriers based on the measurement of the electrostatic field distribution in the SINIS structure has been developed.