High-quality, c-axis oriented YBa2Cu3O7-x/SrTiO3/Au (YBCO/STO/Au) planar structures were fabricated in situ by direct current/radiofrequency inverted-cylinder magnetron sputtering on (001) STO oriented substrates. The sandwich-type structures were patterned to transistor dimensions by standard ultraviolet-photolithography and At etching. The current transport mechanism in the very thin STO barriers (2-30 nm) was examined by measuring the tunneling G as function of temperature (T), and bias voltage (V). It was found that resonant tunneling and hopping via a small number of localized states (LS) are responsible for electronic conduction in the insulating material. Elastic tunneling was observed for the case of a nominal 2 nm thick STO-barrier with an energy gap Delta approximate to 20 meV in the (001) direction of YBCO. On the other hand, inelastic hopping transport via n-LS dominated for STO barrier thickness d>2 nm. G of the lowest-order hopping channel (hopping via two LS) exhibits the characteristic T and V dependences: G(2)(hop)(T)proportional to T-4/3, G(2)(hop)(V)proportional to V-4/3, respectively. Increasing the thickness of the STO barriers, hopping channels of higher order contribute more and more to the current transport as proven by measuring the T and V dependences. A crossover to variable range hopping behavior has been observed for junctions with thicker barriers (d >= 20 nm) in the high-V or high-T regime. By fitting the experimental data to theoretical models, physical parameters of the LS could be determined. For instance, the value of the localization length or radius of the localized state was determined to be similar to 4.6x10(-8) cm which corresponds to the lattice constant of the STO unit cell. A value of similar to 6x10(19) (eV)(-1) cm(-3) was calculated for the density of LS and the average barrier height was estimated as similar to 0.4 eV. (C) 2008 Elsevier B.V. All rights reserved.