[en] In this report, we study factors that dominate the mode transformation of resistive switching (RS) in yttria based memristive devices. It is found that amorphous yttria films are more suitable for RS whereas highly crystalline films are counterproductive for RS. The transformation from unipolar to bipolar resistive switching mode is demonstrated in our devices via moving from a system of single Schottky barrier diode (SBD) to double SBD. The conduction mechanism behind these transformation mechanisms is found to be predominantly interfacial. We also report a forming-free Al/Y₂O₃/Al based memristor fabricated by the dual ion beam sputtering without any post-processing steps for the first time. It shows stable switching behavior for  >29 000 cycles with good retention (10⁵ s) characteristics. (paper)