Chemically assembled electronic nanotechnology (CAEN) is a promising alternative to CMOS for constructing circuits with device sizes of only a few nanometers. CAEN takes advantage of chemical synthesis techniques to construct molecular-sized circuit elements that are only a few nanometers in size and exploit quantum mechanical properties to control voltage and current levels across the terminals of the device. However, CAEN based circuits have a key limitation from a circuit designer's point of view: three-terminal devices, such as transistors, will be practically impossible at the nanoscale. In this paper we present a molecular-based latch that overcomes this limitation.
Molecular-scale devices cannot by themselves overcome the constraints that will prove limiting for CMOS unless a low-cost manufacturing technique such as thermodynamic self-assembly and self-alignment is used for composing them into circuits. Self-assembly imposes a severe limitation on nanoscale architectures: precise device alignment will be very difficult to achieve. The most reliable way to make device connections will be to incorporated into the circuit out of topological necessity, i.e., only include devices at the intersection of two wires coated with the appropriate molecular device. However, this technique rules out three-terminal devices and in practical, near-term nanocomputer designs, the active components will be two-terminal devices such as diodes, configurable switches, and molecular RTDs. From these two-terminal devices a transistor replacement must be created.
The molecular latch shown in Figure 1 is the first example of a transistor replacement constructed out of two terminal devices. The molecular latch is based on a pair of molecular negative differential resistors (NDRs). The state of the latch is determined by the voltage at the node between the two RTDs. The latch has two stable states (logic "0" and logic "1") and a third metastable state. Small voltage fluctuations in the metastable state will push the circuit into one or the other of the stable states. The state of the latch is changed by temporarily disrupting and restoring this bistable equilibrium state. h In molecular circuits, latches are used to buffer and condition the output of a combinational circuit so that it can serve as input to the next one. We therefore need to consider the interactions of multiple latches. It is clear that the lack of I/O isolation provided by transistors is problematic for a latch consisting solely of an RTD pair. To provide the necessary isolation characteristics, we have incorporated several additional devices into the latch. To allow the latches to be reset when the output was high in a previous state we also bring the pull-up voltage source low for the combinational logic at the output of the latch being set. This removes the forward influence and allows the latches to be set properly.
Using SPICE we have simulated many multi-level logic circuits successfully. The latch appears to be stable against variability brought about by manufacture. Because of the relatively low current required to switch states the latch can have moderate fan-out. Finally, the latch also provides the necessary I/O isolation to ensure proper calculation.
S. C. Goldstein
School of Computer Science, Carnegie Mellon University
5000 Forbes Ave, Pittsburgh, PA 15213 USA
Email: email@example.com http://www.cs.cmu.edu/~seth