Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 31

Conductance through a medium having lateral dimensions smaller than an
electron wavelength is quantized in units of 2 e 2 / h , where e is the charge on the
electron and h is Planck's constant, 38 as a consequence of the discrete energy
levels along with the Fermi velocity and electron density of states at the Fermi
level. Geometric imperfections and impurities in the conduction medium give
rise to many kinds of variations in the conduction behavior of nanostructures.
Interference effects have been the subject of extensive research (see, for
example, Agranovich). 39 The Aharonov-Bohm effect occurs when charge carri-
ers are passed through a ring, where two separate paths of almost equal length are
possible for these carriers. As they meet at the other side of the ring, if the
distance traversed is less than the coherence length, interference effects appear as
a function of magnetic field. This magnetoresistive behavior is fairly straightfor-
ward for regular geometric features. In more irregular shapes, however, the
prediction becomes increasingly difficult owing to the complexity of the wave
equation solutions in the presence of more complex boundary conditions. Mag-
netoresistive measurements of most objects with dimensions smaller than a co-
herence length demonstrate these “conductance fluctuations,” which have been
examined extensively. This serves to alert investigators to the sensitivity such
nanostructured devices are likely to have to the presence of geometric irregulari-
ties. These wave effects form the basis of quantum computing approaches (see
section on computing architectures) that might vastly increase the ability to solve
certain important classes of problems, such as prime number factorization.
Solid Electrochemical Switching
A new method of switching using a nanoscale device was recently de-
scribed. 40 A tip made of a solid electrolyte, silver sulfide, is positioned a few
nanometers above a flat platinum surface. When a bias voltage as low as 10 mV
is applied across the gap, atoms come out of solution and extend the tip toward
the surface, eventually making contact. Quantized conductance is observed with
conductance values of n (2 e 2 / h ), where n = 0 through 5, depending on the voltage
applied. It is expected that this reversible process can be controlled with switch-
ing rates of 100 megahertz and on-off impedance ratios of 1:1,000 in air at room
temperature. Simple logic gates have been constructed, which may have applica-
tions in information storage.
Vacuum Microelectronics: Back to the Future
Before transistors, vacuum tubes provided gain for electronic circuits. To-
day, vacuum tubes are still used as high-power radio frequency generators and
amplifiers and as display devices (most televisions and computer monitors are
still based on cathode ray tubes). Vacuum tubes utilize the free-space transmis-
sion of electrons from cathode to anode and are inherently radiation-hard and