Carbon Nanotubes and Carbon Nanotube Transistors
Carbon nanotubes are a single layer or multiple layers of carbon sheet formed into a cylinder or "nanotube." Single walled carbon nanotubes (CNTs) naturally range from 0.8 to 3 nanometers in diameter with one-third being metallic. Carbon Technology, Inc. (CTI) has developed methods to grow just semiconducting tubes with the diameter needed for high performance electronics.
The electrical and thermal properties of semiconducting carbon nanotubes make them among the best bets to replace current bulk semiconductors. Electron transit along the tube's axis shows 50x the mobility (how fast an electron moves with voltage) of silicon electrons and 200x that of silicon holes. For RF, the electronic structure of CNTs results in inherently linear amplifiers. Carbon nanotubes also exhibit superb robustness to heat and radiation.
Carbon Nanotube Field Effect Transistors (CNTFETs) need to start with the right material and add robust, low resistance contacts and the right protection to keep everything clean. CTI has demonstrated each of these key elements and the high yield manufacturing needed to produce in volume.
High Linearity RF Transistors based on Carbon Nanotubes
The Linearity Problem
As mobile data applications proliferate, the demands for device level performance put extraordinary pressure on the Radio Frequency (RF) front-end components to deliver higher data rates at the same or lower power levels. The key to solving these obstacles lies in improving the linearity of analog RF components.
Current bulk semiconductors cannot deliver more bits/spectrum without power consumption becoming an issue. Their linear regions of operation are limited, consequently, when used as amplifiers, they are operated at less than optimal levels and often require the addition of costly linearization techniques to extend the range of linear performance as noted in the figure below. Power amplifiers are most energy efficient when operated near saturation, but this is where non-linearities often appear, so "backing off" on the transmit power level is often the chosen solution.
Unfortunately, that means lower efficiency, so battery life is reduced in mobile wireless devices and power consumption in base stations/access points is unnecessarily high. As an example, even the most up to date 4G devices operate at very low efficiencies when delivering the highest data rates. Similarly, the limited linearity of bulk semiconductors means the market transiton to 5G and its much higher bandwidths will result in network designs that require 4x the number of basestations and will consume much more power. Without a solution, the total investment will be measured in the trillions of dollars. The intrinsically higher linearity of carbon nanotube based amplifiers can extend the useful range of amplifier operation as indicated in the figure in orange. It can accomplish this without linearization techniques that drive up cost and processing requirements.
Higher data rates and channel capacity at lower power requires increased linearity in front end components to cleanly amplify more data dense and complex encoding (modulation) in-band and not waste energy out-of-band (which of course also reduces data rates there).
Through vastly improved linearity, CTI's advanced RF components will support further coding advances in the digital domain and generate less out-of-band interference (and intermodulation), delivering More Data for Less Power.
Theoretical calculations confirmed by recent publications make clear that carbon nanotubes have much lower susceptibility to radiation than traditional semiconductors. Bulk semiconductor devices are prone to catastrophic failure when exposed to radiation while CTI's CNTFETs incorporate a massively parallel CNT array avoiding catastrophic failure and amplifying the material's intrinsic radiation hardness many times over. This, with more data for less power, make CNTFETs ideal for use in space and nuclear applications. The Carbon Tech team drove the earliest space tests of CNTFETs.
As a single layer of carbon with surface electrons just "a ballistic flight" away, carbon nanotubes have demonstrated the ability to sense a single molecule. With material control, their properties will be exploited to revolutionize sensing from high value industrial processes and biotech to mass volume diagnostics on your cell phone. For additional information about the chemical sensing properties of carbon nanotubes click on the link below.
CTI is working with partners for sensing applications of its material. If you are interested in partnering with us, please contact us.
CNTFET Technology for RF Applications: Review and Future Perspective