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Nanotube transistors

Carbon nanotubes hold great promise due to their extraordinary electrical, mechanical, optical, thermal, and chemical properties. Their current applications range from improving consumer electronics, to medicine delivery to cells, to strengthening airplane components. Carbon nanotubes come in many different forms and purities. They range from flexible, thin, few-walled or single-walled nanotubes (SWNTs) to rigid, long, thick, multi-walled nanotubes (MWNTs), with a spectrum of characteristics.

Nanotubes transistor developments
Researchers at Stanford University, Cornell University and Purdue University have jointly produced a carbon nanotube transistor that they claim has better properties than silicon transistors of an equivalent size. The device uses zirconium oxide rather than silicon dioxide, which has a lower dielectric constant, as the gate insulator. Highest performance carbon nanotube field-effect transistors were made to date by integrating zirconia gate insulators. They obtained 70 mV/decade sub threshold swings approaching the theoretical limit for transistors. The scientists used semi conducting single-walled nanotubes (SWNTs) to make p-type field-effect-transistors (FETs). They formed the zirconia gate insulators by atomic layer deposition, creating zirconia films of about 8 nm thick without significantly degrading key transistor performance parameters of the nanotubes, such as mobility. The team converted p-type ZrO2/SWNT-FETs to n-type transistors by heating them in molecular hydrogen at 400°C for one hour. The properties of the n-type transistors, although good, were not as ideal as the p-type FETs. The researchers also made a NOT logic gate, i.e. an inverter, by connecting a p- and n-type ZrO2/SWNT-FET. The device had a high voltage gain.
Zurich researchers have built a transistor whose crucial element is a carbon nanotube, suspended between two contacts, with outstanding electronic properties. A novel fabrication approach allowed the scientists to construct a transistor with no gate hysteresis. This opens up new ways to manufacture nano-sensors and components that consume particularly little energy.
Researchers of University of California at Irvine developed a device which consists of a single-walled carbon nanotube sandwiched between two gold electrodes to operate at extremely fast microwave frequencies. This has resulted is an important effort to develop nano electronic components that could be used to replace silicon in a range of electronic applications.
Researchers from the University of California, San Diego and Clemson University synthesized Y-shaped carbon nanotubes to make transistors. The nanotube transistors were initially grown as straight nanotube elements. Titanium-modified iron catalyst particles added to the synthesis mixture were then attached to the straight nanotubes, nucleating additional growth, which continued in a fashion similar to branches growing from a tree trunk. The nascent nanotubes assumed a Y-shape with the catalyst particle gradually becoming absorbed at the junction of the stem and two branches. When electrical contacts are attached to the nanotube structures, electrons travel into one arm of the Y, hop onto the catalyst particle, and then hop to the other arm and flow outward. The movement of electrons through the Y-junction can be finely controlled, or gated, by applying a voltage to the stem, a replication of the function of existing transistors.
Printable transistors
The semi conducting properties of carbon nanotubes can be exploited to create printable transistors with extremely high performance. Specifically, researchers have shown CNT-based transistors employing a sparse nanotube network to achieve mobility of 1 cm2/V-s, while those using an aligned array of single-walled nanotubes can reach as high as 480 cm2/V-s. Nanotubes also prove to be useful additives to polymer-based TFTs and help to overcome some of the shortcomings of those devices. Beyond their performance, such devices are compatible with solution-based printing techniques, which enable dramatic cost savings in such devices as LCDs and OLED-based displays.

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