A neuromorphic device includes a synapse. The synapse includes a first electrode, a second electrode spaced apart from the first electrode, an oxygen-containing layer disposed between the first electrode and the second electrode, a reactive metal layer disposed between the oxygen-containing layer and the second electrode, and an oxygen diffusion-retarding layer disposed between the reactive metal layer and the oxygen-containing layer. The oxygen-containing layer includes a P-type material and oxygen ions. The reactive metal layer reacts with the oxygen ions of the oxygen-containing layer. The oxygen diffusion-retarding layer includes an N-type material and interferes with a movement of the oxygen ions from the oxygen-containing layer to the reactive metal layer. An interface between the oxygen-containing layer and the oxygen diffusion-retarding layer is a P-N junction.