Transistors without semiconductors

Breakthrough transistor design uses quantum tunneling at room temperature, solving the heat problem with existing FET transistor designs
June 29, 2013
gold quantum_dots_on_boron_nitride_nanotubes

Electrons (blue) flash across a series of gold quantum dots on boron nitride nanotubes (bottom). This breakthrough quantum-tunneling device acts like a transistor at room temperature — without using semiconducting materials. (Credit: Yoke Khin Yap)

Michigan Technological University scientists led by professor of physics Yoke Khin Yap have created a quantum tunneling device that acts like like an FET transistor and works at room temperature — without using semiconducting materials.

The trick was to use boron nitride nanotubes (BNNTs) with quantum dots made from gold.

When sufficient voltage is applied to the device, it switches from insulator to a conducting state. When the voltage is low or turned off, it reverts to its natural state as an insulator.

There is no leakage current of electrons escaping from the gold dots into the insulating BNNTs, thus keeping the tunneling channel cool. In contrast, silicon is subject to leakage, which wastes energy in electronic devices and generates a lot of heat, limiting miniaturization of transistors.

How it was created

Carpets of boron nitride nanotubes grown on a substrate (credit: Yoke Khin Yap)

Yap’s team had figured out in 2010 how to make virtual carpets of BNNTs, which are insulators, and thus highly resistant to electrical charge.

Using lasers, the team then placed quantum dots (QDs) of gold as small as three nanometers across on the tops of the BNNTs, forming QDs-BNNTs. BNNTs are the perfect substrates for these quantum dots due to their small, controllable, and uniform diameters, as well as their insulating nature. BNNTs confine the size of the dots that can be deposited.

In collaboration with scientists at Oak Ridge National Laboratory (ORNL), the team fired up electrodes on both ends of the QDs-BNNTs at room temperature.

An electron wavepacket directed at a potential barrier. The dim spot on the right represents tunneling electrons. (Credit: Wikimedia Commons)

Something interesting then happened: electrons jumped very precisely from gold dot to gold dot — a phenomenon known as quantum tunneling.

“Imagine that the nanotubes are a river, with an electrode on each bank. Now imagine some very tiny stepping stones across the river,” said Yap. “The electrons hop between the gold stepping stones.

“The stones are so small, you can only get one electron on the stone at a time. Every electron is hopping the same way, so the device is always stable.”

Other people have made transistors that exploit quantum tunneling, says Michigan Tech physicist John Jaszczak, who has developed the theoretical framework for Yap’s experimental research.

However, those tunneling FETs (field effect transistors) have only worked in conditions that would discourage the typical cellphone user: liquid-helium temperatures.

Image of BNNTs obtained by scanning transmission electron microscopy (credit: Yoke Khin Yap/Advanced Materials)

The secret to Yap’s gold-and-nanotube device is its sub-microscopic size: one micron long and about 20 nanometers wide.

”The gold islands have to be on the order of nanometers across to control the electrons at room temperature,” Jaszczak said. “If they are too big, too many electrons can flow.” In this case, smaller is truly better: “Working with nanotubes and quantum dots gets you to the scale you want for electronic devices.”

“Theoretically, these tunneling channels can be miniaturized into virtually zero dimension when the distance between electrodes is reduced to a small fraction of a micron,” said Yap.

Yap has filed for a full international patent on the technology.

The work was funded by the Office of Basic Energy Sciences of the US Department of Energy and was conducted in part at ORNL.