1. A threshold switching device comprising: a first electrode layer; a second electrode layer; an insulating layer interposed between the first and second electrode layers and containing a plurality of neutral defects; and an additional insulating layer interposed between the insulating layer and one or each of the first and second electrode layers, and being substantially free from neutral defects, and wherein the threshold switching device has an ON or OFF state according to whether electrons are ejected from the plurality of neutral defects.