Power dissipation in clocking wires for clocked molecular quantum-dot cellular automata

Abstract

In the molecular quantum-dot cellular automata (QCA) paradigm clocking wires are used to produce an electric field which is perpendicular to the device plane of surface-bound molecules and is sinusoidally modulated in space and time. This clocking field guides the data flow through the molecular QCA array. Power is dissipated in clocking wires due to the non-zero resistance of the conductors. We analyze quantitatively the amount of power dissipated in the clocking wires and find that in the relevant parameter range it is fairly small. Dissipation in the molecular devices themselves will likely dominate the energy budget. Quantum-dot cellular automata (QCA)QCA is a novel approach to computing at the nanoscale [1, 2] which is enabled by quantum-mechanical tunneling and simple Coulomb interactions but does not rely on the flow of current [3]. Information is encoded in the charge configuration of cells comprised of quantum dots. Coulomb interaction between cells creates device-device coupling that has been shown to support general-purpose computation.QCA operation has been demonstrated in several materials systems at low temperatures . Circuit-level QCA devices composed of metal tunnel junctions have been fabricated. Inverters, logic gates, and shift registers have been demonstrated. These experiments have also demonstrated fan-out, power gain, and significant error tolerance in QCA systems [20,26,28]. The major drawback is the need for cryogenic operation.Molecular QCA offers the promise of nanometer-scale devices with accompanying ultra-high device densities as well as room-temperature operation, yet without suffering the crippling heat dissipation which prevents achieving such densities in current-switching technologies [29][30][31]. Room temperature operation at the molecular scale has been predicted theoretically [32] and confirmed experimentally [33].Small assemblies of QCA cells operate properly as circuits by simply relaxing to the ground state configuration. Large circuits require clocked operation [34]. Clocking is effected by controllably varying the relative energies of cell charge configurations. This has been demonstrated in (cryogenic) metal-dot cells. Clocking in the QCA paradigm provides the mechanism for power gain, which is crucial for the restoration of weakened signals, and minimizes power dissipation [28,35] by enabling quasi-adiabatic switching.In metal-dot and semiconductor-dot QCA clocking signals can be directly applied to the individual QCA dots. For molecular devices, this would be impractical. However, it has been shown that molecular QCA devices attached to a surface can be clocked using an electric field, E z (x, y, t), perpendicular to the surface, that varies smoothly in space and time [36]. Locally, this field is usually simply a traveling sinusoidal wave which slides across the surface. This time-varying field guides data flow through arrays of cells. It does not affect the value of the data itself, which is contained in the cellular state and moves by cell-cell coupling

Design of Sequential Circuit Using Quantum-Dot Cellular Automata (QCA)

“…[7], in the clock. Switch phase initially QCA cells are unpolarized and the potential barriers are low and they are polarize in switch phase and their barriers become high; computation occurs in this phase .in hold phase of the clock barriers remain at high.…”

Designing efficient QCA logical circuits with power dissipation analysis

“…Therefore, there is a negligible power dissipation in a clocking circuit called P diss [5][6][7]. Fig.…”

“…Nevertheless, it is necessary to characterize all aspects of a new technology, so several studies have been performed in the area of QCA power [4][5][6][7][8]. One of the most accurate power dissipation models has been proposed by Timler and Lent [4] and an upper bound power dissipation for QCA circuits is estimated by Srivastava et al [6] based on it.…”

Introduction

“…In particular, [3,20] give an account of the clocking mechanism while [18] describes aspects pertaining to power dissipation in QCA. A more recent work [21] indicates that QCA can operate at densities of 10 14 devices/cm 2 and at this density, if each device dissipated k B T (where k B is the Boltzmann constant given by 1.38 × 10 −23 J K −1 and T is the temperature in Kelvin), the power dissipated per unit area would be 4 kW/cm 2 . The authors in [22] explore the relation between computation and energy dissipation.…”