Building Carbon Nanotube Transistors

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IBM scientists developed a breakthrough transistor technology that could preview how computer chips can be made smaller and faster than what is currently possible with silicon.

As reported in the April 27 issue of the journal Science, IBM researchers have built the world's first array of transistors out of carbon nanotubes -- tiny cylinders of carbon atoms that are 500 times smaller than today’s silicon-based transistors. This breakthrough is an important step in finding materials that can be used to build computer chips when silicon-based chips cannot be made any smaller -- a problem chip makers are expected to face in about 10-20 years.



Constructive Destruction -- Building Carbon Nanotube Transistors
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Depending on their size and shape, the electronic properties of carbon nanotubes can be metallic or semiconducting. The problem scientists had faced in using carbon nanotubes as transistors was that all synthetic methods of production yield a mixture of metallic and semiconducting nanotubes which “stick together” to form ropes or bundles. This compromises their usefulness because only semiconducting nanotubes can be used as transistors; and when they are stuck together, the metallic nanotubes overpower the semiconducting nanotubes. IBM scientists have pioneered a method called constructive destruction that allows them to destroy all the metallic nanotubes and leave the semiconducting nanotubes unharmed. Here are the steps:

  1. The scientists deposit ropes of “stuck together” metallic and semiconducting nanotubes on a silicon-oxide wafer,
  2. Then a lithographic mask is projected onto the wafer to form electrodes (metal pads) over the nanotubes. These electrodes act as a switch to turn the semiconducting nanotubes on and off,
  3. Using the silicon wafer itself as an electrode, the scientists "switch-off" the semiconducting nanotubes, which essentially blocks any current from traveling through them,
  4. The metal nanotubes are left unprotected and an appropriate voltage is applied to the wafer, destroying only the metallic nanotubes, since the semiconducting nanotubes are now insulated,
  5. The result: a dense array of unharmed, working semiconducting nanotube transistors that can be used to build logic circuits like those found in computer chips.



Selective Breakdown of a Multi-walled Carbon Nanotube
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IBM scientists can control the electricity flowing into a multi-walled carbon nanotube and break down the outer shells one-by-one. Using this selective breakdown technique, the scientists can control the size of the carbon nanotube, which allows them to build nanotubes with the electrical properties they want to build useful electronic devices.



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Combination of the above two photos.

Constructive Destruction -- Separating Semiconducting and Metallic Carbon Nanotubes to Build Transistors
  1. The scientists deposit ropes of “stuck together” metallic and semiconducting nanotubes on a silicon-oxide wafer,
  2. Then a lithographic mask is projected onto the wafer to form electrodes (metal pads) over the nanotubes. These electrodes act as a switch to turn the semiconducting nanotubes on and off,
  3. Using the silicon wafer itself as an electrode, the scientists "switch-off" the semiconducting nanotubes, which essentially blocks any current from traveling through them,
  4. The metal nanotubes are left unprotected and an appropriate voltage is applied to the wafer, destroying only the metallic nanotubes, since the semiconducting nanotubes are now insulated,
  5. The result: a dense array of unharmed, working semiconducting nanotube transistors that can be used to build logic circuits like those found in computer chips.
Click here to see Quicktime Animation:
"Constructive Destruction"


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