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 an insulating material layer made of a rare earth oxide or a nitride 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, said memory layer is such that 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, a conduction path is formed in said memory layer and said memory layer has a relatively low resistance value, said conduction path being stable in the absence of the application of any electric field across the first and second electrodes, said memory layer is such that 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, the conductive path dissolves and said memory layer has a relatively high resistance value, said ion source layer has a resistance value lower than that of said memory layer when said memory layer does not contain therein said ionized metal atoms, and an overall resistance value between said electrodes is mainly determined by a resistance value of said memory layer.