Harnessing the potential of quantum tunneling: Transistors without semiconductors

June 21, 2013 by Marcia Goodrich, Michigan Technological University
Electrons flash across a series of gold quantum dots deposited on a boron nitride nanotubes. Scientists at Michigan Technological University made the quantum-tunneling device, which behaves like a transistor at room temperature, without using any semiconducting materials. Credit: Yoke Khin Yap

(Phys.org) —For decades, electronic devices have been getting smaller, and smaller, and smaller. It's now possible—even routine—to place millions of transistors on a single silicon chip.

But transistors based on semiconductors can only get so small. "At the rate the current technology is progressing, in 10 or 20 years, they won't be able to get any smaller," said physicist Yoke Khin Yap of Michigan Technological University. "Also, semiconductors have another disadvantage: they waste a lot of energy in the form of heat."

Scientists have experimented with different materials and designs for transistors to address these issues, always using semiconductors like silicon. Back in 2007, Yap wanted to try something different that might open the door to a new age of electronics.

"The idea was to make a transistor using a nanoscale insulator with nanoscale metals on top," he said. "In principle, you could get a piece of plastic and spread a handful of metal powders on top to make the devices, if you do it right. But we were trying to create it in nanoscale, so we chose a nanoscale insulator, nanotubes, or BNNTs for the substrate."

Yap's team had figured out how to make virtual carpets of BNNTs, which happen to be insulators and thus highly resistant to . Using lasers, the team then placed quantum dots (QDs) of gold as small as three across on the tops of the BNNTs, forming QDs-BNNTs. BNNTs are ideal 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), they fired up electrodes on both ends of the QDs-BNNTs at room temperature, and something interesting 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 on each bank. Now imagine some very tiny stepping stones across the river," said Yap. "The electrons hopped 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 passing the same way, so the device is always stable."

Yap's team had made a transistor without a semiconductor. When sufficient voltage was applied, it switched to a conducting state. When the voltage was low or turned off, it reverted to its natural state as an .

Furthermore, there was no "leakage": no electrons from the gold dots escaped into the insulating BNNTs, thus keeping the tunneling channel cool. In contrast, silicon is subject to leakage, which wastes energy in and generates a lot of heat.

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 devices have only worked in conditions that would discourage the typical cellphone user.

"They only operate at liquid-helium temperatures," said Jaszczak.

The secret to Yap's gold-and-nanotube device is its submicroscopic 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 and 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.

Explore further: Harnessing the Divas of the Nanoworld

More information: Their work is described in the article "Room Temperature Tunneling Behavior of Boron Nitride Nanotubes Functionalized with Gold Quantum Dots," published online on June 17 in Advanced Materials: onlinelibrary.wiley.com/doi/10 … a.201301339/abstract

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1.4 / 5 (10) Jun 21, 2013
Very cool. I wonder if we will have light based quantum computer powered cell phones before this tech is developed?
5 / 5 (3) Jun 21, 2013
This is an advance -- real questions -- density, scalability, cost, and switching speed.
1 / 5 (6) Jun 21, 2013
Good...The more the better.
not rated yet Jun 22, 2013
I'd like to get a sense of how much power this technology consumes. it would be a real revolution if we could do the equivalent of today's computing without having to lug around batteries that only last for a few hours.
1 / 5 (1) Jun 22, 2013
We'll only know how efficient it is after someone builds a processor from it so that we can compare them.

It's probably a LOT more efficient though.

It kind of concerns me that the method of controlling whether or not is conducts is through voltage on the same path that the data would have to flow(or the voltage of the data itself?). I was under the impression that transistors are generally controlled by a second path. Can someone who is better versed explain if this is an issue?
Andrew Palfreyman
1 / 5 (8) Jun 23, 2013
I must have missed something, because I see a diode, but no transistor. It's a two-terminal device with a threshold, and unity gain.
not rated yet Jun 23, 2013
Yeah, you missed something. It's called TFA. From the Abstract:
" We demonstrate that tunneling currents can be modulated at room temperature by tuning the lengths of QD-BNNTs and the gate potentials."
1 / 5 (3) Jun 24, 2013
Agreed Andrew, I think it's a strange duck, not even a diode because it doesn't limit current flow to only one direction. Still though it could be a very interesting duck. A modern NPN or PNP transistor is simply two diodes connected back-to-back with a third connection to a controlling gate. FET's, on which CPU's are based, are simple conductors which can be switched on or off by a voltage applied to a gate placed near the conductor by insulated from it. What is needed for this technology is to figure out how to use one of these units as a gate on a second unit. If gain is required, then the second unit could be larger, or perhaps several in parallel. How about wrapping one unit, the control unit, around a second controlled unit in a way that a current flow on the control unit would create a limiting field which would push the controlled unit below conduction threshold? Then there's still the question of how much power flow (energy loss) in the gate would be needed to control it
1 / 5 (1) Jun 28, 2013
You missed it too? It has a gate-source like channel with a gate that can modulate the currents. Can't anyone read (the abstract)? It is just like a FET. Channel length and all. Wake up, folks.
Andrew Palfreyman
1.5 / 5 (8) Jun 28, 2013
A FET is a 3-terminal device with over-unity gain.
1 / 5 (1) Jun 29, 2013
Someone obviously does not know what gain is, much less a FET.
1 / 5 (3) Jul 11, 2013
Someone obviously does not know what gain is, much less a FET.
I challenge you to show where the gate connection on this technology is. Until then, and with no unidirectional conduction, you've got nothing more than a fancy (expensive and useless) conductor.
1 / 5 (1) Jul 12, 2013
this is called negative resistance because current decreases with increasing voltage.
-- http://en.wikiped...el_diode

Calling this circuit a tunnel diode simply shows your lack of knowledge.
1 / 5 (2) Jul 18, 2013
What do you think a gate is? Again, and Again, from the Abstract:
" We demonstrate that tunneling currents can be modulated at room temperature by tuning the lengths of QD-BNNTs and the gate potentials."

It has a gate --- they call it a gate --- It controls the flow --- what more do you need? A hammer to drive it into your thick skull? Sheesh.

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