This paper provides an overview of research developments toward nanometer-scale electronic switching devices for use in building ultra-densely integrated electronic computers. Specifically, two classes of alternatives to the field-effect transistor are considered: 1) quantum-effect and single-electron solid-state devices and 2) molecular electronic devices. A taxonomy of devices in each class is provided, operational principles are described and compared for the various types of devices, and the literature about each is surveyed. This information is presented in nonmathematical terms intended for a general, technically interested readership

1 Introduction In the past 40 years, the metal-oxide semiconductor field effect transistor (MOSFET) has become the basic building block for almost all computing devices. The steady growth of their popularity is due to the steady shrinking of the feature size which at present has reached 0.1 micron. However, the laws of quantum mechanics and limitations of fabrication techniques may soon prevent the further decrease of feature size. Hence, researchers are investigating several alternatives to the transistor for ultra-dense circuitry. These new devices whose dimensions are on the order of tens of nanometers are called nano-devices and their science is termed nano-technology.

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