Yesterday, IEEE Spectrumpublished a feature “Changing the Transistor Channel” that chronicles the laborious migration from the ubiquitous silicon in transistors to new materials, primarily compound semiconductors known as III-Vs.
These efforts to replace the semiconducting silicon in the channels of transistors is being pursued by all the big chip manufacturers and international research labs.. Various nanomaterials from graphene to nanowires made from III-V materials are being experimented with to help achieve that aim.
As momentum builds in this field, researchers at Michigan Technological University (MTU) are looking ahead not only beyond silicon but also to when semiconductors will not even be needed for transistors.
Yoke Khin Yap, a physicist at MTU, and his colleagues, including those at Oak Ridge National Laboratory (ORNL), have developed a method by which they use an insulator—boron nitride nanotubes—coupled with quantum dots to create a path for electrons to travel between electrodes in a transistor. No semiconductor material is used in the design.
“The idea was to make a transistor using a nanoscale insulator with nanoscale metals on top,” Yap said in a press release. “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, boron nitride nanotubes, or BNNTs for the substrate.”
Two years ago, Yap and his team developed a way to make a virtual carpet out of BNNTs. In this latest research, which was published in the journal Advanced Materials (“Room Temperature Tunneling Behavior of Boron Nitride Nanotubes Functionalized with Gold Quantum Dots”), the MTU team devised a method for depositing gold quantum dots on the BNNT carpet using a laser. The BNNTs turn out to be perfect for the job. They have controllable and uniform diameters so they can confine the size of the quantum dots.
When Yap and his colleagues, along with scientists at ORNL, put a voltage on the electrodes, they observed that the electrons jumped from one gold quantum dot to the next in an orderly fashion. This phenomenon is known as quantum tunneling. One benefit of this device is that the quantum tunneling effect is achieved at room temperature conditions.
“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 in a press release. “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.”
This design allowed for the creation of a transistor that did not require a semiconductor. When sufficient charge was applied, the material was in a conducting state. When the charge was removed, it reverted back to being an insulator. An additional benefit to the design was that it didn’t suffer any “leakage” of electrons that plagues silicon, creating overheating problems and wasted energy.
Yap notes: “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.”
Image: Yoke Khin Yap
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.