A team of researchers from Infineon Technologies AG has now been able to show, for the first time, that tiny carbon nanotubes not only represent an interesting alternative for silicon integration or for the metalization of chips, but that they can also provide the basis for power components. The results that have now been achieved were obtained from a test chip for which the nanotubes were produced using a regular Chemical Vapor Deposition (CVD) procedure. The power transistor prototype can control small motors as well as light emitting diodes (LEDs). All process parameters, such as the temperature and materials, are suitable for use in conjunction with standard semiconductor manufacturing processes. The growth process for the carbon nanotubes takes just a few minutes. As early as mid 2002, Infineon’s researchers had drawn public attention when they demonstrated growth of carbon nanotubes in a controlled manner at predefined locations on silicon wafers.
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 Infineon’s CoolMOS products, reach switching resistances of 20mΩ /mm2 and current densities of 2000A/cm2. The high conductance and current-carrying capacity that carbon nanotubes offer can increase these values considerably. Infineon’s 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 Infineon’s 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 Infineon’s 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.