Architectures for Molecular Electronic Computers: 1. Logic Structures and an Adder Built from Molecular Electronic Diodes

Recently, there have been some significant advances in the fabrication and demonstration of individual molecular electronic wires and diode switches. This paper shows how these demonstrated molecular devices might be combined to design molecular-scale electronic digital computer logic. The design for the demonstrated rectifying molecular diode switches is refined and made more compatible with the demonstrated wires through the introduction of intramolecular dopant groups chemically bonded to modified molecular wires. Quantum mechanical calculations are performed to characterize some of the electrical properties of the proposed molecular diode switches. Explicit structural designs are displayed for AND, OR, and XOR gates that are built from molecular wires and molecular diode switches. These diode-based molecular electronic logic gates are combined to produce a design for a molecular-scale electronic half adder and a design for a molecular-scale electronic full adder. These designs correspond to conductive monomolecular circuit structures that would be one million times smaller in area than the corresponding micron-scale digital logic circuits fabricated on conventional solid-state semiconductor computer chips. It appears likely that these nanometer-scale molecular electronic logic circuits could be fabricated and tested in the foreseeable future. At the very least, these molecular circuit designs constitute an exploration of the ultimate limits of electronic computer circuit miniaturization. In that connection, examination of these designs suggests that diode switches alone probably will not be sufficient to operate such extended molecular electronic circuitry. Three-terminal switching devices that produce power gain probably are required. Nonetheless, molecular diode switches and diode-based logic are likely to be important components of future molecular electronic digital circuits, if only because they demand many fewer interconnects and, consequently, should make such ultra-dense circuitry much easier to design and to fabricate. In its treatment of the foregoing innovations and analysis, this work addresses two of the architectural issues that will be encountered in the fabrication and operation of a molecular electronic computer: the issues of design and interconnects. A more complete enumeration of such issues is discussed, with a view to considering the assembly of a molecular electronic computer using logic gates similar to or based upon those designs proposed here.