Infineon Becomes the First Company to Demonstrate Carbon Nanotube Transistors for Power Applications: Carbon-based Transistors Drive Motors and LEDs
Power semiconductors such as power MOSFETs (metal-oxide-semiconductor field-effect transistors) drive electronic loads in numerous applications, such as motors and controllers. Here, it is crucial that the power switches cause as little loss of power as possible, despite the fact that they switch high voltages and currents. Consequently, the switching resistance and the current density are the most important characteristic values for power transistors. Modern MOSFETs, such as Infineons CoolMOS products, reach switching resistances of 20mΩ /mm 2 and current densities of 2000A/cm 2. The high conductance and current-carrying capacity that carbon nanotubes offer can increase these values considerably. Infineons researchers were able to show that the typical switching resistance for power transistors built with carbon nanotubes is 20 times lower than for conventional transistors, which results in a correspondingly lower loss of power. Furthermore, carbon-based transistors withstand current densities that are approximately 200 times higher than the levels that their silicon counterparts can handle. One important characteristic of nanotubes is that they can be metallic or semiconducting, said Dr. Franz Kreupl, project manager in Infineons carbon nanotube research section. This makes it possible to build active switching elements, such as field-effect transistors for controlling electronic loads, and we are the first to successfully demonstrate this.
Since a single carbon nanotube with a diameter of 1nm can only deliver about 24 microamperes, the challenge is to arrange hundreds or thousands of the tiny tubes in parallel to achieve the desired current-carrying capacity. The first prototype developed by Infineon consists of approximately 300 parallel tubes, and it supplies 2mA at 2.5V. As Infineon has successfully demonstrated, this is already sufficient to drive LEDs or small motors, which represents a milestone in the field of molecular electronics. Moreover, the prototype can easily be scaled for higher power ratings.
Several recent advances in nanotechnology were integrated into the production of the nanotube power transistor. Among other things, Infineon researchers were able to grow high-quality, single-walled carbon nanotubes at the low temperature of 600 °C; until now, this required temperatures of approximately 900 °C. Only one single lithographic step is required to build the entire transistor with the drain, source, and gate contacts. With Infineons demonstrator, the drain and source contacts were made of palladium. Silicon was used as the substrate, although any conductive material would suffice. The researchers then grew the carbon nanotubes on a high-k aluminum dioxide gate dielectric. The carbon nanotubes are distributed randomly in a relatively simple process, although a sufficient number are arranged in parallel to use for the connection between the drain and source.
Background Information on Carbon NanotubesCarbon nanotubes (CNTs) are macromolecules made of carbon atoms arranged in a cage-like sheet of hexagons that form tiny hollow cylinders (like rolled chicken wire) that are long and stretched-out. They can reach lengths of up to 1mm, and have diameters of 0.4nm to more than 100nm, depending on how many concentric tubes form inside each other. There are two basic types: single-walled CNTs (SWCNTs), and multi-walled CNTs (MWCNTs). SWCNTs have diameters between 0.4 and 5nm. In comparison, traditional silicon process technology is currently transitioning to structures with minimum diameters reaching 90nm. Depending on the tube geometry, nanotubes can be either metallic or semiconducting.
Besides their metallic properties, their most important advantage can be seen in the extremely high charge-carrier mobility of semiconducting SWCNTs, which exceeds that of silicon by a factor of 200. Nevertheless, CNTs can withstand current densities of up to 10 10A/cm². This is an extremely high value considering the fact that copper already begins to melt at a current density of approximately 10 7A/cm².
Given these properties and the fact that as with polymer electronics the production processes are inexpensive, carbon nanotubes are potentially well suited for diverse applications. The spectrum ranges from serving as an alternative to silicon and metalization in semiconductor chips, to use in displays and sensors, all the way to employment in the power semiconductors described here.
Infineons research activities in the field of carbon nanotubes are subsidized by the German Federal Ministry of Education and Research (BMBF).
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