Presented on Feb 2009
Carbon nanotubes (CNTs) are nanometer-diameter carbon cylinders consisting of graphene sheet wrapped up to form tube.C-60 allotropes, also called fullerenes are used to make CNT. All carbon atoms are involved in hexagonal aromatic rings only and are therefore in equivalent position, except at the nanotube tips where 6×5 = 30 atoms at each tip are involved in pentagonal rings.
CNT has unique electrical and mechanical properties that it can be shaped to act as conductor, semiconductor and insulator depending on the armchair, chiral and zig-zag structure respectively.
It may be composed of a single shell to form single wall nanotubes (SWNTs) or of several concentric shells to form multi-wall nanotubes (MWNTs).
MWNT was discovered earlier than the latter, which is comprised of 2 to 30 concentric graphitic layers, whose diameter ranges from 10 to 50 nm and more than 10 mm in length.
SWNT, on the other hand, is a lot thinner due to its single graphite layer and has diameter from 1.0 to 1.4 nm.
Single-Walled CNT
Multi-Walled CNT
Comparision of CNT with other
| Property | Carbon Nanotubes | Comparatively |
| Size | 0.6-1.8 nm in diameter | Si wires at least 50nm thick |
| Strength | 45 Billion Pascals | Steel alloys have 2 Billion P. |
| Resilience | Bent and straightened without damage | Metals fracture when bent and restraightened |
| Conductivity | Estimated at 109 A/cm2 | Cu wires burn at 106 A/cm2 |
| Cost | $2500/gram by BuckyUSA in Houston | Gold is $30/gram |
Progress of transistor technology
CNTFET features
Main role of CNT as the conducting channel of a MOSFET.
- These new devices are very similar to the CMOS FETs.
- All CNFETs are pFETs by nature.
- nFETs can be made through
- Annealing
- Doping
- Very low current and power consumption
- Although tubes are 3nm thick , CNFETs are still the size of the contacts, about 20nm.
Analysis : Transfer Characteristics
•As generally CNT FETs are of p-FET, so the origin of the holes is an important question to address.
•One possibility is that the carrier concentration is inherent to the NT.
•Another possibility is that the majority of carriers are injected at the gold–nanotube contacts. The higher work function of gold leads to the generation of holes in the NT by electron transfer from the NT to the gold electrodes.
Challenges
- CNTs are flexible tubes that can be made conducting or semiconducting.
- Nano-scale, strong and flexible.
- Multilevel interconnects not available
- Chip density still limited to the density of contacts.
- Tube density not entirely exploited
- Fabrication is still a stochastic process
- Alternatives to gold contacts need to be found.
- Great potential for storage memory (116 Gb/cm2 )
- Small size offers faster switching speeds (100GHz ) and low power
- Easy to fabricate: standard semiconductor process
- Bistability gives well defined on & off states
- Nonvolatile nature: no need to refresh.
- Faster than SRAM, denser than DRAM, cheaper than flash memory.
- Have an almost unlimited life, resistant to radiation and magnetism—better than hard drive.
- CMOS technology is approaching saturation – problems in the nanometer range
- Carbon nanotube based FET is one of the best alternative for CMOS technology.
References
Ravi's Blog 