© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.2.2 Nanoelectronic Computers

In 1998, the metal-oxide-semiconductor field-effect transistor (MOSFET) was still the most common type of transistor in general use. These solid-state devices are characterized by an electrical source, an electrical drain, and a gate that controls the flow. Since its inception, the FET has scaled well to lower sizes. Individual working transistors with 40-nm gate lengths have been demonstrated in silicon,¹⁷⁵¹ and 25-nm gate-length transistors have been fabricated in gallium arsenide.¹⁷⁵² However, as feature sizes shrink below <~100 nm, a number of obstacles to stable operation begin to appear and the cost-effective downscaling of dense circuitry may not persist.^1753-1757 A number of solid-state replacements for the bulk-effect semiconductor transistor have been suggested that could overcome these obstacles by taking advantage of nanometer-scale quantum mechanical effects.¹⁸¹³ Such replacements include quantum dots (QDs),^1784-1787 resonant tunneling devices (RTDs),^{1787-1791,1747} electron turnstiles,²⁷⁰⁹ and single-electron transistors (SETs).^{1794-1799,1855} However, while hybrid RTD-FET circuits¹⁷⁹¹ and SETs^1814,1815 have been successfully switched at room temperature, many of these devices must be operated at cryogenic temperatures¹⁸³⁰ and other practical challenges remain;^{1747,1784,1791-1794} such top-down approaches to nanoelectronics are not considered further here.

In contrast, molecular nanoelectronics uses primarily covalently bonded molecular structures, electrically isolated from a bulk substrate, producing wires, switches and devices composed of individual molecules and nanometer-scale supramolecular structures.¹⁷⁴⁷ While it is relatively difficult and expensive to sculpt trillions of identical nanoscale structures in bulk materials (e.g., using 2-nm resolution electron-beam lithographic techniques¹⁸¹⁶), individual molecules are natural nanometer-scale structures that can be manufactured to be exactly the same in near-mole quantities.¹⁸⁶⁷ The great versatility of organic chemistry offers more options for designing and fabricating nanoelectronic devices than are available in silicon.^1773-1778 Investigators are designing, modeling, fabricating, and testing individual molecules^{1760-1769,1871} and nanometer-scale supramolecular structures^1811,1817 that act as electrical switches and even exhibit some of the same properties as small solid-state transistors.¹⁷⁶⁸ For example, three and four terminal devices with ~0.3-nm feature sizes have been examined computationally.¹⁸⁷⁹

Molecular electronics is a rapidly growing area in the literature.^{1769-1776,1832,1924} J.M. Tour expects experimental demonstration of molecular-sized transistor devices by 2000-2001, and commercial high-level computers using molecular electronics by 2008-2013.¹⁵¹⁷ A current list of major research groups is maintained by the International Society for Molecular Electronics and Biocomputing. Some of the following discussion is appreciatively drawn from an extensive 1997 review by MITRE Corporation.¹⁷⁴⁷

Last updated on 23 February 2003