A redox-based three-terminal device was fabricated using Ta 2O5 as the ionic transfer material, and its operation was investigated. We found that application of a negative polarity gate bias, which increases oxygen anions in the channel regions, can make a conductive path between a source electrode and a drain electrode. The insulating state of the pristine device is turned on to a semiconductor state by the application of a negative polarity gate bias. Since turning off to the insulating state could not be achieved, the switching process resembles the soft breakdown of the first turning-on process of oxygen vacancy controlled resistive random access memories, although the polarity of the bias is opposite to that used in the first turning-on process. Further application of a gate bias causes a transition from the semiconductor state to a metal state. Accordingly, there are two types of on-state. It is possible to switch between the semiconductor and metal states.