Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> Limitations, innovations, and challenges of circuits and devices in silicon integrated circuits when they are scaled down to a half micrometer and smaller are reviewed. The major directions of innovation are given. Then, device innovations in advanced MOSFET devices featuring sizes of a half micrometer, quarter micrometer, and even smaller than one tenth of a micrometer are reviewed. Major issues are signal-to-noise ratio and speed of operation at low supply voltages. It is pointed out that miniaturized MOSFETs with channel lengths even shorter than one tenth of a micrometer could probably be realized with superior performance, but the circuit technique of using them at low-voltage power supplies still remains a challenge. Finally, it is pointed out that the real challenges in the future will be design and test limits of huge systems, and the real goals of engineers in the future will be functional integration rather than scale integration. > <s> BIB001 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> The authors formulate a new paradigm for computing with cellular automata (CAS) composed of arrays of quantum devices-quantum cellular automata. Computing in such a paradigm is edge driven. Input, output, and power are delivered at the edge of the CA array only; no direct flow of information or energy to internal cells is required. Computing in this paradigm is also computing with the ground state. The architecture is so designed that the ground-state configuration of the array, subject to boundary conditions determined by the input, yields the computational result. The authors propose a specific realization of these ideas using two-electron cells composed of quantum dots. The charge density in the cell is very highly polarized (aligned) along one of the two cell axes, suggestive of a two-state CA. The polarization of one cell induces a polarization in a neighboring cell through the Coulomb interaction in a very non-linear fashion. Quantum cellular automata can perform useful computing. The authors show that AND gates, OR gates, and inverters can be constructed and interconnected. <s> BIB002 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> An overview of nanocomputers is given, including a discussion of reversible computing techniques and an explanation of why they are necessary. The design of a nanocomputer will specify every atom and covalent bond present in the device. We can reasonably expect the switches, gates, or other embodiment of the logical elements to be on the order of a nanometer in size. (They may have to be further apart than that if electrical, due to electron tunneling.) In any case, it is quite reasonable to expect the entire computer to be smaller than a cubic micron, which contains a billion cubic nanometers. There are two new design rules that the nanocomputer designer must adopt: erase as few bits as possible, and eliminate entropy loss in operations that do not erase bits. The macroscopic strictures of the Second Law are consequences of the microscopic reversibility of the laws of physics. There are some hints that the oddities of quantum mechanics may have a similar source. One speculates that under the proper formulation, information, like energy, is a conserved quantity in the universe. <s> BIB003 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> We discuss the fundamental limits of computing using a new paradigm for quantum computation, cellular automata composed of arrays of coulombically coupled quantum dot molecules, which we term quantum cellular automata (QCA). Any logical or arithmetic operation can be performed in this scheme. QCA's provide a valuable concrete example of quantum computation in which a number of fundamental issues come to light. We examine the physics of the computing process in this paradigm. We show to what extent thermodynamic considerations impose limits on the ultimate size of individual QCA arrays. Adiabatic operation of the QCA is examined and the implications for dissipationless computing are explored. > <s> BIB004 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> Nanometer scale electronics present a challenge for the computer architect. These quantum devices have small gain and are difficult to interconnect. I have analyzed current device capabilities and explored two general design requirements for the design of computers: error correction and long range connections. These two principles follow when Turing machines are implemented as integrated circuits. I consider the roles of electromigration through thin wires, circuit layout, and error rates for devices with small gain. The analysis brings into sharp focus the future of nanocomputers and suggests solutions to some of its difficulties. It gives a theoretical model for a nanocomputer, separating the roles of devices and algorithms. Within the model one can implement a stochastic computer, which operates despite quantum device limitations. <s> BIB005 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> The era of low power microelectronics began with the invention of the transistor in the late 1940's and came of age with the invention of the integrated circuit in the late 1950's. Historically, the most demanding applications of low power microelectronics have been battery operated products such as wrist watches, hearing aids, implantable cardiac pacemakers, pocket calculators, pagers, cellular telephones and prospectively the hand-held multi-media terminal However, in the early 1990's low power microelectronics rapidly evolved from a substantial tributary to the mainstream of microelectronics. The principal reasons for this transformation were the increasing packing density of transistors and increasing clock frequencies of CMOS microchips pushing heat removal and power distribution to the forefront of the problems confronting the advance of microelectronics. The distinctive thesis of this discussion is that future opportunities for low power gigascale integration (GSI) will be governed by a hierarchy of theoretical and practical limits whose levels can be codified as: (1) fundamental, (2) material, (3) device, (4) circuit, and (5) system. > <s> BIB006 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> We formulate and discuss principles for designing computationally useful networks of Coulomb‐blockade devices. Our particular focus is on locally interconnected synchronous networks in which the numerical discreteness of a computation is represented directly by the quantization of electron charge, i.e., electrons represent bits. To highlight our emphasis on circuits and architectural issues, and on performing locally interconnected computation rather than traditional logic as has been the interest heretofore (single‐electron logic), we refer to our networks as single‐electron digital circuits (SEDCs). In addition to being single‐electron and locally interconnected, the SEDCs we propose have a regular ‘‘cellular’’ structure with occupancy‐independent biasings and with electron‐electron interactions carefully controlled. The chief virtue of SEDCs is their scalability, both as devices (because of their Coulomb blockade basis) and as circuits (because of their local interconnectivity), perhaps even to molecul... <s> BIB007 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> We describe a new class of nanoelectronic circuits where circuit functions are derived from cooperative, quantum mechanical interactions between single electrons confined in arrays of quantum dots. Two specific architectures are examined: (i) quantum coupled logic in which Boolean logic functions are implemented by quantum mechanical spin-spin interactions between single electrons in arrays of quantum dots, and (ii) quantum neuromorphic networks that exploit the complex spatial and temporal evolution of discrete charge in an ensemble of non-linearly interacting quantum dots to elicit collective computational behavior. The first class of circuits includes combinational and sequential digital systems. Both logically irreversible elements such as half-adders, S-R flip-flops, shift registers, ring counters, etc., and reversible Feynman gates for quantum computation have been designed in this paradigm. These circuits can be endowed with the required "unidirectional" (non-reciprocal) character that previous (flawed) designs of similar circuits lacked. The second class of circuits comprises discrete Hopfield networks which utilize single electron tunneling events in arrays of metallic islands to perform neuromorphic computation. They can solve NP-complete optimization problems (such as the traveling salesman problem), produce associative memory effects and also exhibit rudimentary image-processing capability. <s> BIB008 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> We describe a paradigm for computing with interacting quantum dots, quantum-dot cellular automata (QCA). We show how arrays of quantum-dot cells could be used to perform useful computations. A new adiabatic switching paradigm is developed which permits clocked control, eliminates metastability problems, and enables a pipelined architecture. <s> BIB009 </s> Overview of Nanoelectronic Devices <s> V. DISCUSSION AND CONCLUSIONS <s> The long-continued advance of the performance of information processing technologies has been based on miniaturization of components. The history of miniaturization is presented through examples. They suggest that limits proposed by Landauer in the 1960s will be reached in two or three decades. <s> BIB010
In considering prospects for continuing the exponential rate of miniaturization of electronics well into the next century, one always must be cognizant of the obstacles. These include the fundamental limitations of thermodynamics and quantum mechanics BIB010 ,  , BIB006 ,  , BIB001 , BIB003 , as well as the practical limitations arising from the cost and difficulty of fabrication  ,  , [80] , [171] . However, as explained in this overview, progress is being made in harnessing the principles of quantum mechanics to design and to build solid-state and molecular devices that can function well on smaller and smaller scales, even after aggressive miniaturization of solid-state FET's has ceased to be feasible and cost-effective. This makes it possible to envision the stages of a natural evolution from microelectronic to nanoelectronic devices and circuits. In parallel with the aggressive miniaturization of CMOS FET's, industry could begin to employ hybrid quantum-effect/bulk-effect devices similar to the one diagramed in Fig. 6 . This could permit the present technology for microelectronic digital circuitry to be leveraged to increase the logic density of two-dimensional digital circuitry by as much as 100 or 1000 times. After that, emerging methods of nanofabrication such as mechanosynthesis and chemosynthetic self-assembly may be used to manufacture purely quantum-effect, nanometer-scale solid-state circuitry or molecular electronics. This might achieve digital processing logic 10 000 or even 100 000 times as dense as is presently feasible. Memories, which are organized in more regular structures, might be made even more dense. The problems of achieving such increases in computing power and information storage capacity still are formidable. As discussed elsewhere in this volume  , BIB009 , new architectures BIB007 , BIB008 , like cellular automata BIB004 , BIB002 , must be devised to reduce the amount of wire and number of interconnects on a circuit while accommodating billions or even trillions of devices in the same area now occupied by only a few million. These architectures also must compensate for the intrinsically lower reliability of very small quantum-effect devices BIB005 . Materials must be found to maximize device reliability, by better isolating the operation of closely spaced devices from each other, and by desensitizing individual devices to background charges. Less power must be used per device, and it must be used more efficiently, with less heat dissipation  , BIB003 . Nonetheless, as described in this paper, there is already a range of design options and prototypes for next-generation nanoelectronic switching and amplification devices. These can be used to experiment with solutions to the fundamental problems outlined above. The exciting, world-wide enterprise of engineering nanometer-scale electronic computers is well under way and growing. Nanocomputers will arrive as a result of breakthroughs on many fronts. The excitement of standing on the threshold of such an innovation is enhanced by the multidisciplinary nature of nanotechnology. It is impossible to predict from which traditional discipline will come the impetus or key breakthrough necessary to construct these new, much tinier computers with much greater speed and power. One only can be confident that such dramatically smaller computational engines, along with the methods devised to fabricate them, will transform electronic computing and our technological infrastructure as well. like to thank Prof. K. K. Likharev for helping them to clarify certain discussions in [145] . The authors gratefully acknowledge valuable discussions with and technical materials generously provided by many of our colleagues working in the field of nanoelectronics. Thanks also are due to Dr. M. Ancona and to Prof. K. K. Likharev, who read an earlier version of this article and provided detailed comments that were of great assistance to the authors in preparing the present version. They thank J. Schleier-Smith and D. Mumzhiu of The MITRE Nanosystems Group, who assisted with the editing and production of this paper.