1. A two-terminal memory device, comprising: a first metal electrode above a first oxide layer over a semiconductor substrate; a pillar structure comprising: a contact material layer on a top surface of the first metal electrode, wherein the contact material layer is semiconductive; and a switching layer on top of the contact material layer, wherein the switching layer comprises a portion of the contact material layer that is ionized, and wherein the switching layer has a lower conductivity than a portion of the contact material layer that is not ionized; and a second metal electrode above and in contact with a top surface of the pillar, wherein the second metal electrode comprises: an active metal layer in contact with the top surface of the switching layer of the pillar; and a conductor layer in contact with the active material.