In this work, we investigated the effect of W doping on the resistance switching behavior of NiOx films. The W doping is expected to produce cation-rich NiOx film because W has high valence state compared to Ni in NiOx. To measure resistance switching behavior, Pt/NiOx/Pt and Pt/NiOx/TiN MIM stacks were fabricated by reactive dc magnetron sputtering with various W doping contents in NiOx. Also, physical properties such as atomic density and chemical bonding states of NiOx were characterized in addition to the resistance switching characteristics. Increasing W doping into NiOx produced the higher resistance value of high resistance states (HRS) compared to the undoped NiOx films with large memory window. Even though Pt/NiOx with W/Pt stacks showed a unipolar resistance switching behavior, Pt/NiOx with W/TiN stacks showed bipolar resistance switching. Composition and chemical bonding states in the deposited films were investigated by x-ray photoelectron spectroscopy (XPS). W doping into NiOx films produced higher density of metallic Ni (Ni-0) than that of NiOx films deposited at the same conditions. This work demonstrated that the resistive switching behavior of NiOx can be enhanced by doping NiOx with W of different oxidation valence. (C) 2011 Elsevier B. V. All rights reserved.