Different scales of Silicon Integration Technology
No. of components used on single chip
SSI (small scale integration)
MSI (medium scale integration)
LSI (large scale integration)
VLSI (very large scale integration)
ULSI (ultra large scale integration)
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.”
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.
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.
Francis Garnier and co-workers, in 1990
developed a total organic transistor known as organic
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
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, 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”
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
An experimental RTD of a working electronic device has
been recently synthesized by Tour and demonstrated by
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
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
i) Molecular electronics must still be integrated with
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
vi) Another big hindrance is to connect a molecular sized
circuit to bulk electrodes in a way that gives reproducible
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
Techniques for electrical
characterization of molecules
Various techniques to
electronic properties of
(A) Hg drop junction.
(E) Nanoparticle bridge.
(F) Crossed wires.
(H) Contact CP-AFM.
(I) Nanoparticle coupled
“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.”