The Architecture Of Moletronics Computer

ABSTRACT

Moletronics means molecular computers in which single molecules serve as switches, “quantum wires” a few atoms thick serve as wiring, and the hardware is synthesized chemically from the bottom up.

The central thesis of moletronics is that almost any chemically stable structure that is not specifically disallowed by the laws of physics can be built.

Moletronics is expected to touch almost every aspect of our lives, right down to the water we drink and the air we breathe. Experimental work has already resulted in the production of molecular tweezers, a carbon nanotube transistor and logic gates.

The Architecture of a Moletronics Computer

Introduction

Recently, there have been some significant advances in the fabrication and demonstration of individual molecular electronic wires and diode switches. This means in coming future, this technology could be a replacement for VLSI.

However, currently, this technology is only available under lab condition. How to mass product moletronic chips is still a big problem.
Currently, integrated circuits by etching silicon wafers using beam of light. It's the VLSI lithography-based technology makes mass production of Pentium III processor possible. But as the size of logic block goes to nano-scale, this technology no long available. As wavelength get too short, they tend to become X-rays and can damage the micro structure of molecules. On the other hand, the mask of lithography of Pentium III is so complex, and the shape and the dimension of its logic block varies so much.

Moletronic circuit--QCA basics

We discuss an approach to computing with quantum dots, Quantum-dot Cellular Automata (QCA), which is based on encoding binary information in the charge configuration of quantum-dot cells. The interaction between cells is Coulombic, and provides the necessary computing power. No current flows between cells and no power or information is delivered to individual internal cells. Local interconnections between cells are provided by the physics of cell-cell interaction. The links below describes the QCA cell and the process of building up useful computational elements from it.

Fundamental Aspects of QCA

A QCA cell consists of 4 quantum dots positioned at the vertices of a square and contains 2 extra electrons. The configuration of these electrons is used to encode binary information. The 2 electrons sitting on diagonal sites of the square from left to right and right to left are used to represent the binary "1" and "0" states respectively. For an isolated cell these 2 states will have the same energy. However for an array of cells, the state of each cell is determined by its interaction with neighboring cells through the Coulomb interaction.

$\begin{figure} \centerline{ \psfig{figure=fig1.ps ,width=3in} } \end{figure}$

If the barriers between cells are sufficiently high, the electrons will be well localized on individual dots. The Coulomb repulsion between the electrons will tend to make them occupy antipodal sites in the square. For an isolated cell there are two energetically equivalent arrangements of the extra electrons which we denote as a cell polarization P = +1 and P = -1. The term "cell polarization" refers only to this arrangement of charge and does not imply a dipole moment for the cell. The cell polarization is used to encode binary information - P = +1 represents a binary 1 and P = -1 represents a binary 0.

$\begin{figure} \centerline{ \psfig{figure=fig3.ps ,width=3in} } \end{figure}$ $\begin{figure} \centerline{ \psfig{figure=fig2.ps ,width=3in} } \end{figure}$ The two polarization states of the cell will not be energetically equivalent if other cells are nearby. Consider two cells close to one another. The figure inset illustrates the case when cell 2 has a polarization of +1. It is clear that in that case the ground-state configuration of cell 1 is also a +1 polarization. Similarly if cell 2 is in the P = -1 state, the ground state of cell 1 will match it. The figure shows the nonlinear response of the cell-cell interaction.

A Majority Gate

$\begin{figure} \centerline{ \psfig{figure=fig4.ps ,width=3in} } \end{figure}$ The figure shows the fundamental QCA logical device, a three-input majority gate, from which more complex circuits can be built. The central cell, labeled the device cell, has three fixed inputs, labeled A, B, and C. The device cell has its lowest energy state if it assumes the polarization of the majority of the three input cells. The output can be connected to other wires from the output cell. The difference between input and outputs cells in this device, and in QCA arrays in general, is simply that inputs are fixed and outputs are free to change. The inputs to a particular device can come from previous calculations or be directly fed in from array edges. The schematic symbol used to represent such a gate is also shown in Fig. 4. It is possible to "reduce" a majority logic gate by fixing one of its three inputs in the 1 or 0 state. If the fixed input is in the 1 state, the OR function is performed on the other two inputs. If it is fixed in the 0 state, the AND function is performed on the other two inputs. In this way, a reduced majority logic gate can also serve as a programmable AND/OR gate. Combined with the inverter shown above, this AND/OR functionality ensures that QCA devices provide logical completeness

Programmable Logic Devices and Field Programmable Gate Array basics

$\begin{figure} \centerline{ \psfig{figure=fig7.ps ,width=3in} } \end{figure}$ The Programmable Logic Devices(PLD) are nothing new, they have been around for almost 20 years. It is well known that in order to design a digital system, besides microprocessors and peripheral ICs there are needed several other devices, such as lots of logic gates to glue these chips together. This circuits make our life and our printed boards very hard and complex. It exists a way to dramatically improve this way of design digital devices that brings the desired results more efficiently: in a shorter time and with fewer expenses. The way abovementioned is Programmable logic devices (PLD), they permit the customizing of one or more logic functions on a chip in contrast to the designer being restricted to defining a logic function with specific chips. The programmability aspect permits the logic designer to spend more time on the development and validation of high level functionality. The simplest Integrated circuit of the PLD is PAL/GAL. PAL consists of an AND array followed by an OR array, either (or both) of which is programmable. Inputs are fed into the AND array, which performs the desired AND functions and generates product terms. The products terms are then fed into the OR array. In OR array, the output of various product terms are combined to produced the desired output. With PAL, we can implement any combinational logic circuit.

How about the sequential logic circuits? There exists another kind of customized IC: Field Programmable Gate Array.

Unlike the traditional fully customised VLSI circuits, Field Programmable Gate Array(FPGAs) represent a technical breakthrough in the corresponding industry.

A programmable device is a general-purpose device capable of implementing the logic of tens or hundreds of discrete devices. It is programmed by users at their site using programming hardware. The size of a PLD is limited by the power consumption and time delay. In order to implement designs with thousands or tens of thousands of gates on a single IC, MPGA can be used. An MPGA consists of a base of pre-designed transistors with customised wiring for each design.

The availability of FPGAs offer the benefits of both PLD and MPGA. FPGAs can implement thousand of gates of logic in a single IC and it can be programmed by users at their site in a few seconds or less depending on the type device used. The risk is low and the development time is short. These advantages have made FPGAs very popular for prototype development, custom computing, digital signal processing, and logic emulation.

Interconnection: nanotube

$\begin{figure} \centerline{ \psfig{figure=fig8.ps ,width=3in} } \end{figure}$ Today, one way to pack transistors more densely on a chip is to make the already microscopic wires smaller and thinner. But the wires are approaching the thickness of a few hundred atoms. Once wires get down to only several atoms thick they blow up when you try to send electrical signals through them. Nanotubes don't. The race has begun to use nanotubes to make the first carbon chips, perhaps the successor to silicon chips. A carbon nanotube is a tubular form of carbon with a diameter as smaller as 1 nm. The length can be from a few nanometres to several microns. It is made of only carbon atoms.

Carbon nanotubes exhibit extraordinary mechanical properties as well. The nanotube along the axis is as stiff as a diamond. The estimated tensile strength is about 200 Gpa, which is an order of magnitude higher than that of any other material. The metallic and semiconducting nature described previously has given rise to the possibilities of metal-semiconductor or semiconductor junctions. These junctions may form nanoelectronic devices based entirely on single atomic species such as carbon.

Conclusion

Even a lot of approach has been proposed in moletronic computer. But there still exists critical problem: most of the technologies are valid only in laboratory condition, and cannot be produced massively.

BIBLIOGRAPHY

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Friday, June 17, 2011