Published on February 1, 2014
Different scales of Silicon Integration Technology Integration Type No. of components used on single chip SSI (small scale integration) 1-12 MSI (medium scale integration) 12-30 LSI (large scale integration) 30-300 VLSI (very large scale integration) 300-10000 ULSI (ultra large scale integration) beyond 10000
MOORE’S LAW Moore wrote in his original paper entitled ‘Cramming More Components Onto Integrated Circuit ’, “The complexity for minimum component costs has increased at the rate of roughly a factor of 2 per year. Certainly, over the short term, this rate can be expected to continue, if not to increase. Over the longer term, the rate of increase is a bit more uncertain, although there is no reason to believe that it will not remain constant for at least ten more years.”
MOLECULAR ELECTRONICS IS THE ONLY SOLUTION
SUBSTRATES USED UNDER THIS TECHNOLOGY
ORGANIC POLYMERS Discovered in mid 1970’s. Polymers are flexible, versatile and easy to process. Behave like a conventional inorganic semiconductor. Does not possess reasonable charge carrier mobility. Mobility obtained in polymers is rather low. Does not demonstrate the existence of controllable band gap of the order of 0.75 to 2 e V.
POLYPHENYLENE BASED CHAINS They are capable of carrying currents. They are also capable of switching small currents. Thus, they are used as molecular wires and switches. The current that passes through the molecular-wires is about 30 A, or about 30 n A per molecule. This works out to about 200 billion electrons per second being transmitted across the short polyphenylene-based molecular wire.
POLYPHENYLENE BASED CHAINS Fig. (a) Alkyldithiol; (b) Oligo(p-phenylene)-dithiol; (c) (p-phenylene ethynylene)-dithiol.
CARBON NANOTUBES A second type of molecule that can be used as molecular wires is the carbon nanotube or “bucky tube”. When used on micropatterned semiconductor surfaces, these nanotube structures make a very conductive wire. They differ in diameters and chiralities and come in a range of conductive properties ranging from excellent conduction to pretty good insulation. The most flexible polyphenylene backbone, is not the most conductive and the most conductive, the carbon nanotube, is not the most flexible chemically.
Carbon nanotubes: their structure
MOLECULAR ELECTRONIC COMPONENTS
MOLECULAR TRANSISTORS Prof. Francis Garnier and co-workers, in 1990 developed a total organic transistor known as organic FET. The transistor is a metal insulator semiconductor structure comprising an oxidized silicon substrate and a semiconductor polymer layer. It has great flexibility and can even function when it is bent.
Diode Switches A diode is a two terminal device in which current may pass in one direction through the device, but not the in the other direction, and in which the conduction of current may be switched on or/off. Two important types of molecular-scale diode switches have been demonstrated: rectifying diodes and resonant tunneling diodes.
Rectifying Diodes Rectifying diodes, also called molecular rectifiers, use structures that make it more difficult for an electric current to go through them in one direction, usually termed “reverse” direction, than it is to go the opposite “forward” direction. Rectifying diodes have been elements of analog and digital circuits since the beginning of the electronic revolution. The first theoretical paper on molecular electronics was a paper entitled “Molecular Rectifiers” by A. Aviram and M.A. Ratner that appeared in the journal Chemical Physics Letters in November 1974.
Resonant Tunneling Diodes (RTDs) The RTD uses electron energy quantization to permit the amount of voltage bias across the source and drain to control the diode so as to switch current on and off, and so as to keep electrical current going from the source to the drain. An experimental RTD of a working electronic device has been recently synthesized by Tour and demonstrated by Reed. The device is a molecular analog of a larger solid-state RTD that has commonly been fabricated in III-V semiconductors and used in solid-state, quantum-effect circuitry.
REALIZATION OF BASIC CIRCUITS
Similarly following basic circuits can be derived from the above circuits Molecular XOR Gates Using Molecular RTDs and Rectifying Diodes Molecular Electronic Half Adder Molecular Electronic Full Adder Combining Individual Devices
CHARACTERISTICS OF MOLECULAR DEVICES
Nonlinear I-V Behavior Energy Dissipation Gain in Molecular Electronic Circuits Speeds
Advantages of Molecular Electronics
Size Power Assembly Manufacturing Cost Low Temperature Manufacturing Stereochemistry Synthetic flexibility
i) Molecular electronics must still be integrated with Silicon. ii)The determination of the resistance of a single molecule. iii) It is difficult to perform direct characterization . iv) Interconnection of two components at molecular level also creates hindrances. v) One of the biggest problems is with measuring on single molecules. vi) Another big hindrance is to connect a molecular sized circuit to bulk electrodes in a way that gives reproducible results. vii) Also problematic is the fact that some measurements on single molecules are carried out in cryogenic temperatures (close to absolute zero) which is very energy consuming.
Techniques for electrical characterization of molecules
Various techniques to measure electronic properties of molecules. (A) Hg drop junction. (B) Mechanically controlled break junctions. (C) Nanopore. (D) Nanowire. (E) Nanoparticle bridge. (F) Crossed wires. (G) STM. (H) Contact CP-AFM. (I) Nanoparticle coupled CP-AFM.
FUTURE DEVELOPMENTS & CONCLUSION
“The Next Big Thing is very, very small. Picture trillions of transistors, processors so fast their speed is measured in terahertz, infinite capacity, zero cost. It's the dawn of a new technological revolution - and the death of silicon.”
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