27-03-2012, 03:54 PM
Carbon nanotube field-effect transistor (CNTFET) with ultra-short gate width
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Field-effect transistors based on semiconducting carbon nanotubes (CNTs) have been a focus of active research in recent years. Similarly to silicon metal-oxide-semiconductor field-effect transistors, various strategies have been employed to scale down CNTFETs and maximize device performances, such as using a top gate with high-κ dielectric material or short channel length.
In a recent work (Nanotechnology 18, 095202), an ultimate scaling in the width of a gate electrode is demonstrated by a CNTFET with another CNT (diameter ∼3 nm) as a gate electrode. It is shown that such a short gate width is enough to turn the CNTFET on and off, and full channel operation is possible, independent of properties of the CNT–metal contacts. The device characteristics, such as transconductance and subthreshold swing, show an order of magnitude improvement with a CNT gate compared to the global back gate. A CNTFET with an ultra-short gate width is expected to have marked advantages in terms of operational frequency and will be a subject of future studies.
Motivations for transistor applications
A carbon nanotube’s bandgap is directly affected by its chirality and diameter. If those properties can be controlled, CNTs would be a promising candidate for future nano-scale transistor devices. Moreover, because of the lack of boundaries in the perfect and hollow cylinder structure of CNTs, there is no boundary scattering. CNTs are also quasi-1D materials in which only forward scattering and back scattering are allowed, and elastic scattering mean free paths in carbon nanotubes are long, typically on the order of micrometers. As a result, quasi-ballistic transport can be observed in nanotubes at relatively long lengths and low fields.[7] Because of the strong covalent carbon-carbon bonding in the sp2 configuration, carbon nanotubes are chemically inert and are able to transport large amounts of electric current. In theory, carbon nanotubes are also able to conduct heat nearly as well as diamond or sapphire, and because of their miniaturized dimensions, the CNTFET should switch reliably using much less power than a silicon-based device.[8]
CNTFET material considerations
There are general decisions one must make when considering what materials to use when fabricating a CNTFET. Semiconducting single-walled carbon nanotubes are preferred over metallic single-walled and metallic multi-walled tubes since they are able to be fully switched off, at least for low source/drain biases. A lot of work has been put into finding a suitable contact material for semiconducting CNTs;
Comparison to MOSFETs
CNTFETs show different characteristics compared to MOSFETs in their performances. In a planar gate structure, the p-CNTFET produces ~1500 A/m of the on-current per unit width at a gate overdrive of 0.6 V while p-MOSFET produces ~500 A/m at the same gate voltage.[23] This on-current advantage comes from the high gate capacitance and improved channel transport. Since an effective gate capacitance per unit width of CNTFET is about double that of p-MOSFET, the compatibility with high- k gate dielectrics becomes a definite advantage for CNTFETs.[21] About twice higher carrier velocity of CNTFETs than MOSFETs comes from the increased mobility and the band structure. CNTFETs, in addition, have about four times higher transconductance.
Heat dissipation
The decrease of the current and burning of the CNT can occur due to the temperature raised by several hundreds of kelvins. Generally, the self-heating effect is much less severe in a semiconducting CNTFET than in a metallic one due to different heat dissipation mechanisms. A small fraction of the heat generated in the CNTFET is dissipated through the channel. The heat is non-uniformly distributed, and the highest values appear at the source and drain sides of the channel.[24] Therefore, the temperature significantly gets lowered near the source and drain regions. For semiconducting CNT, the temperature rise has a relatively small effect on the I-V characteristics compared to silicon.
Disadvantages
• Lifetime (degradation)
Carbon nanotube degrades in a few days when exposed to oxygen. There has been several works done on passivating the nanotubes with different polymers and increasing their lifetime