Abstract
The
phenomenological predictions for the cutoff frequency of carbon nanotube
transistors and the predictions of the effects of parasitic capacitances on AC
nanotube transistor performance are presented. The influence of quantum
capacitance, kinetic inductance, and ballistic transport on the high-frequency
properties of nanotube transistors is analyzed. The challenges of impedance
matching for ac nano-electronics in general, and how integrated nanosystems can
solve this challenge, are presented.
Nanotube
Interconnects Quantum Impedances
The
first step towards understanding the high frequency electronic properties of
carbon nanotubes is to understand the passive ac impedance of a Id quantum
system. In the presence of a ground plane below the nanotube or top gate above
the nanotube, there is electrostatic capacitance between the nano tube and the
metal. Due to quantum properties of Id systems there are two additional
components to the ac impedance: the quantum capacitance and the kinetic
inductance.
Transconductance
The
transconductance is the most critical parameter the underlying mechanism is the
least understood. Transconductances upto 20µS have been measured using aqueous
gate geometry. A transconductance of 60 µS was recently predicted by
simulation.
Effects Of
Ballistic Transport In Mosfet's
If
carrier transport in a device can be assumed to be completely ballistic,
analysis of MOSFET current voltage characteristics reduces to carrier
transmission over the channel potential barrier. As shown in figure , the
potential energy distribution in the channel of the transistor has a maximum,
Emax, near the source end of the device. Carriers with a higher energy than
Emax can be transmitted over the barrier through the process of thermionic
emission. Carriers with lower energies can travel from source to drain only by
tunneling quantum mechanically through the channel potential barrier. Such
transport phenomena is markedly different from that generally associated with
mobility-limited diffusive transport. As a result, the current-voltage
characteristics of MOSFET's operating in the ballistic regime will be
different.
Parasitic Capacitance
The
parasitic capacitance is due to fringing electric fields between the electrodes
for the source, drain and gate. While these parasitic capacitance are generally
small, they may comparable to the intrinsic device capacitances and hence must
be considered. In order to estimate the order of magnitude of the parasitic
capacitance, we can use known calculations for the capacitance between two thin
metal films, spaced by a distance w, as drawn in Fig. For this geometry, if
w is l^im, the capacitance is ~ 10A-16 F/lm of electrode length . For a length
of l µm, this gives rise to ~10A-16 F. Thus, typical parasitic capacitances are
of the same order of magnitude as typical intrinsic capacitances.
Beyond
Microelectronics
Nanoelectronics
is not simply a smaller version of microelectronics; things change at the
nanoscale. At the device level, silicon transistors may give way to new
materials such as organi molecules or inorganic nanowires. At the interconnect
level, microelectronics uses long, fat wires, but nanoelectronics seeks to use
short nanowires. Finally, fundamentally new architectures will be needed to
make use of simple, locally connected structures that are imperfect and are
comprised of devices whose performance varies widely. I have argued in this
paper that 21st century silicon technology is rapidly evolving into a true
nanotechnology. Critical dimensions are already below 100 nm. The materials
used in these silicon devices have properties that differ from the bulk.
Nanoscale silicon transistors have higher leakage, lower-drive current, and
exhibit more variability from device to device. New circuits and architectures
will need to be developed to accommodate such devices. It matters little
whether the material is silicon or something else, the same issues face any
nanoelectronics technology. It's likely that many of the advances and
breakthroughs at the circuits and systems levels that will be needed to make
nanoelectronics successful will come from the silicon design community. One
reason, of course, is that 20 years is not a long time to develop fundamentally
new technologies, so that we need to start now, but there are other reasons.
The most compelling practical reason is that the fabrication and assembly processes
and the materials, device, circuit, and system understanding that we develop by
examining radically new technologies are almost certain to be useful in silicon
nanotechnology.
Conclusion
The
phenomenological predictions for the ac performance of nanotube Transistors
were presented. It was predicted that carbon nanotube transistors may be faster
than conventional semiconductor technologies. There are many Challenges that
must be overcome to meet this goal, which can be best be achieved by the
integration of Nanosystems.
1 comments:
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