A quantum-tunneling device created without semiconducting materials by scientists at Michigan Technological University could find use as a nano-scale transistor in tiny future electronic devices.
As electronic devices continue to become progressively smaller and more sophisticated, scientists have successfully developed methods to place up to millions of transistors on a single silicon chip.
However, physicist Yoke Khin Yap of Michigan Technological University said, the size of transistors based on semiconductors is finite, which places a limit on the size of electronic devices.
“At the rate current technology is progressing, in 10 or 20 years, they won’t be able to get any smaller,” Yap said. “Also, semiconductors have another disadvantage: they waste a lot of energy in the form of heat.” Scientists have previously attempted to address these issues through experimentation with different materials and designs, but have always employed semiconductors like silicon in their research—a decision that has limited the success of these experiments, according to Yap.
Using an idea first conceived in 2007 in an attempt to develop a new structure capable of better handling heat dissipation at tiny levels, Yap and his research team grew “virtual carpets” of boron nitride nanotubes (BNNTs) and placed tiny quantum dots of gold on the tops of the BNNTs to form QDs-BNNTs. Their work is published online in the journal Advanced Materials.
“The idea was to make a transistor using a nanoscale insulator with nanoscale metals on top,” Yap said. “In principle, you could get a piece of plastic and spread a handful of metal powers 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, boron nitride nanotubes, or BNNTs, for the substrate.”
BNNTs were reportedly chosen as ideal substrates for these quantum dots due to their “small, controllable and uniform diameters”—a characteristic that enables the ability to confine the size of the dots that can be deposited—and their insulating nature.
Upon applying voltage to electrodes at both ends of the QDs-BNNTs at room temperature, Michigan Tech scientists observed electron movement between gold dots, suggesting that the application of sufficient voltage altered the device from its natural state as an insulator to a conducting state. Lowering or turning of the voltage entirely allowed the structure to revert back to an insulating state.
Yap’s team had successfully made a transistor without a semiconductor.
Researchers also noted that no “leakage”—the gradual loss of energy often caused by transistors or diodes—was observed. In contrast, leakage often occurs in silicon and contributes to energy loss and heat generation.
The success of Yap’s device, Michigan Tech physicist John Jaszczak says, is due to its “submicroscopic size” of one micron long and 20 nanometers wide. Larger transistors created previously by other researchers that exploit quantum tunneling have only worked in extremely low-temperature conditions not ideal for consumer electronics.
“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.”
“Working with nanotubes and quantum dots gets you to the scale you want for electronic devices.”
The research was completed in collaboration with researchers at Oak Ridge National Laboratory (ORNL) in Tennessee.
