Some embodiments include selectively conducting devices having a first electrode, a second electrode, and dielectric material between the first and second electrodes. The dielectric material may be configured to conduct current from the first electrode to the second electrode when a first voltage is applied across the first electrode and the second electrode. Furthermore, the dielectric material may be configured to inhibit current from flowing from the second electrode to the first electrode when a second voltage having a polarity opposite that of a polarity of the first voltage is applied across the first electrode and the second electrode. The diode material may comprise a plurality of layers of different dielectric materials arranged in order of increasing barrier height. Quantum wells may form at junctions of layers of the plurality responsive to the first voltage. Some embodiments include diode forming methods.