21-06-2013, 04:25 PM
IEEE Spectrum: How We Found the Missing Memristor
IEEE Spectrum.pdf (Size: 1.61 MB / Downloads: 29)
It’s time to stop shrinking
Moore’s Law, the semiconductor industry’s
obsession with the shrinking of transistors and their commensurate steady
doubling on a chip about every two years, has been the source of a 50-year
technical and economic revolution. Whether this scaling paradigm lasts for five
more years or 15, it will eventually come to an end. The emphasis in electronics
design will have to shift to devices that are not just increasingly infinitesimal but
increasingly capable.
The story of the memristor is truly one for the history books. When Leon Chua,
now an IEEE Fellow, wrote his seminal paper predicting the memristor, he was a
newly minted and rapidly rising professor at UC Berkeley. Chua had been fighting
for years against what he considered the arbitrary restriction of electronic circuit
theory to linear systems. He was convinced that nonlinear electronics had much
more potential than the linear circuits that dominate electronics technology to this
day.
Chua discovered a missing link in the pairwise mathematical equations that relate
the four circuit quantities—charge, current, voltage, and magnetic flux—to one
another. These can be related in six ways. Two are connected through the basic
physical laws of electricity and magnetism, and three are related by the known
circuit elements: resistors connect voltage and current, inductors connect flux and
current, and capacitors connect voltage and charge. But one equation is missing
from this group: the relationship between charge moving through a circuit and
the magnetic flux surrounded by that circuit—or more subtly, a mathematical
doppelgänger defined by Faraday’s Law as the time integral of the voltage across
the circuit. This distinction is the crux of a raging Internet debate about the
legitimacy of our memristor [see sidebar, “Resistance to Memristance ”
Resistance To Memristance
Introducing a new fundamental circuit element earned R. Stanley Williams
some grief along with his newfound fame. After the Nature article appeared in
May, online comments pages boiled over with skepticism. “Is this a hoax?”
someone asked on the Wikipedia memristor page on 30 April 2008, the day
the news broke, in one of the milder statements of disbelief. Seven months
later, the debate continues.
THE CROSSBAR ARCHITECTURE:
The crossbar architecture is a fully
connected mesh of perpendicular wires. Any two crossing wires are connected
by a switch. To close the switch, a positive voltage is applied across the two
wires to be connected. To open the switch, the voltage is reversed.
THE SWITCH:
A switch is a 40-nanometer cube of titanium dioxide (TiO2) in
two layers: The lower TiO2 layer has a perfect 2:1 oxygen-to-titanium ratio,
making it an insulator. By contrast, the upper TiO2 layer is missing 0.5
percent of its oxygen (TiO2-x), so x is about 0.05. The vacancies make the
TiO2-x material metallic and conductive.