7511 Molecular AND and OR Gates Using Diode Diode Logic

Circuits for AND and OR digital logic gates based upon diodes, so-called diode-diode logic structures, have been known for decades. Schematic diagrams for these two circuits are shown, respectively, in Figures 7.12(a) and 7.13(a).106,107 The operation of these logic circuits is explained in Appendix B, Sections B.1 and B.2.

Part (b) of each of these figures shows a structure for a corresponding novel molecular implementation of each of these logic gates. It is noteworthy that these molecular logic gates each would measure only about 3 nm x 4 nm, which is approximately one million times smaller in area than the corresponding logic element fabricated on a semiconductor chip using transistor-based circuits. (See Appendix C for a detailed size comparison.)

In the molecular implementations of both logic gates, the doped polyphenylene-based rectifying diode structures proposed above and depicted schematically in Figure 7.10 are connected together via conductive polyphenylene-based Tour wires, respecting the geometric constraints imposed by the chemical bonding behavior of organic molecules. Insulating aliphatic chains, as described in connection with Figure 7.1(e), are used for the purely resistive elements of the circuit. In particular, the three-methylene chains at the lower right of the molecular structure diagrams for both the AND gate and the OR gate correspond to the large resistances R specified in the schematics immediately above them in Figures 7.12 and 7.13, respectively. As is explained further in Appendix B, these large resistances serve to reduce power dissipation and to maintain a distinct output voltage signal at C when the inputs at A and B cause the diodes in either logic gate to be forward biased so that a current flows through them.

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FIGURE 7.12 Diode-diode-type molecular electronic AND gate incorporating polyphenylene-based molecular rectifying diodes embedded in Tour wires.

The primary structural difference between the AND gate in Figure 7.12(b) and the OR gate in Figure 7.13(b) is that the orientation of the molecular diodes is reversed. Figures 7.12(d) and 7.13(d) show the truth tables of the inputs and outputs for each gate.

Similar diode-diode logic gates might be built from molecules like that shown in Figure 7.4(a), which recently were demonstrated by Metzger as having the properties of rectifying diodes. Metzger's experiment was conducted using a Langmuir-Blodgett film of such molecules, as sketched in part (b) of the same figure.

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