1. A semiconductor device, comprising: an electrode; a first stack of material layers, wherein the first stack is carried by the electrode; a second stack of material layers positioned on the first stack, the first and second stacks having different widths; and first and second control terminals coupled to the first and second stacks, respectively, so that the first and second stacks each operate as an electronic device; wherein one of the first and second stacks operates as a transistor and the other one operates as a negative differential resistance device.