Wednesday, April 11

AC Performance Of Nanoelectronics


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.

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.

AC Performance Of Nanoelectronics Seminar Doc

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. 


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.


Raj said...

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