1. A memory device, comprising: a substrate; first and second electrodes carried on said substrate; an ion source layer between said first and second electrodes; and a memory layer between said ion source layer and said second electrode, wherein, said memory layer is made of an insulating material or a semiconductor material, said ion source layer includes (a) a metal atom that can be ionized therein and (b) at least one of Te, S and Se, and being conductive with a lower conductivity than that of said memory layer, when a first electric field of a first polarity is applied across said first and second electrodes, ionized metal atoms from said ion source layer migrate into said memory layer and a conduction path is formed in said memory layer, said conduction path being stable in the absence of the application of an electric across the first and second electrodes, and when a second electric field of a second polarity opposite the first polarity is applied across the first and second electrodes, the ionized metal atoms in said memory layer migrate back into said ion source layer and the conductive path dissolves.