1. An oxide-based negative differential resistance comparator circuit comprising: a composite NDR device comprising: a first electrode; a first thin film negative differential resistance (NDR) layer in contact with the first electrode; a central conductive portion; a second thin film NDR layer over the central conductive portion; and a second electrode disposed on the second thin film NDR layer; a resistor in series with the composite NDR device; and an electrical energy source to apply a voltage across the first electrode and second electrode, in which the composite NDR device is to produce a threshold based stepping transfer function in the comparator circuit.