1. A self-rectifying RRAM cell structure, comprising a first electrode layer formed of a first metal element; a second electrode layer formed of a second metal element different from the first metal element; and a first resistive-switching layer and a second resistive-switching layer sandwiched between the first electrode layer and the second electrode layer, wherein the first resistive-switching layer and the second switching layer form an ohmic contact, and the first resistive-switching layer has a first bandgap lower than a second bandgap of the second resistive-switching layer.