ZnO nanowire field-effect transistors (NW-FETs) employing single nanowires were fabricated, using a self-assembled superlattice (SAS) as the gate insulator. Both depletion-mode and enhancement-mode ZnO NW-FETs were fabricated and characterized. An electrostatic model is proposed to describe observed threshold voltage shift upon optimum ozone treatment. Temperature-dependent current-voltage characteristics of depletion-mode ZnO NW-FETs verify this model, indicating the existence of body current through ZnO nanowires with low activation energy. In addition, NW-FETs that use multiple ZnO nanowires and a SiO2 gate insulator were fabricated to achieve higher on-current without significant degradation in on-off current ratio, threshold voltage shift, and subthreshold slopes.