PORTLAND, Ore. Carbon nanotube transistor channels are so small--a single nanometer thick--that heat has a hard time transferring to a nanotube substrate. The gap between nanotube and its silicon oxide substrate thermally insulates the nanotube, meaning heat builds up in the structure rather than dissipating.
IBM researchers claim to have found a possible means of cooling carbon-based transistors by directly coupling their electric current fields with those of a silicon substrate. Solving a heat-dissipation problem brings carbon-based transistors one step closer to commercialization, they said.
"We measured how a nanotube transistor channel heats up when we crank up the current," said Mathias Steiner, research scientist at IBM Research (Yorktown Heights, N.Y.) "We wanted to know how the heat gets dissipated in the vibrational modes of the nanotube and the substrate beneath it, and in the process made an important new discovery."
|Artist's conception of a nanotube being heated (excited) by passing through it a current of electrons (e-'s). The energetic electrons excite preferentially the mode of vibration of the carbon atoms (labeled "K".) Energy then flows to other vibrational modes labeled "F," "IFM" and "RBM," with different rates schematically indicated by the width of the arrows. A non-equilibrium energy distribution is established.|
The problem with cooling nanotube transistors is that they do not form a chemical bond with silicon-based substrates as does a silicon transistor. Without a chemical bond to transfer the vibrational energy to the substrate, the heat quickly builds up in the nanotube causing device failure. IBM believes is has discovered a mechanism for directly coupling the electric fields of the nanotube and the substrate, enabling the efficient transfer of heat.
"It is assumed that when energy reaches the gap between the carbon nanotube and the silicon substrate, there is nothing to help it to propagate to the other side, so it piles up there," said Phaedon Avouris, IBM Fellow and manager of nanoscale science and technology. "But this new mechanism allows the heat to jump the gap."
The mechanism had been observed before in transistors. Since its effect falls off exponentially with distance, however, scientists considered it a negligible effect for the relatively larger size of silicon transistor channels. However, because carbon nanotube transistor channels are so thin, the effect may be the best means of cooling future carbon-based transistors.