(Abbreviations fixed – h/t to reader Bob Gottschall)
The use of graphene as a full-function transistor is a step closer. A team at the Berkeley National Lab led by Feng Wang has figured out a way to create a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts at room temperature. This is a smaller bandgap than typical semiconductors and could open the door to new kinds of optoelectronic devices for working with and detecting infrared light, including nano-LEDs.
Heretofore, single layer graphene’s lack of a bandgap has limited its potential in electronic applications. In a LBL press release, Wang said, “You can build field-effect transistors with graphene, but if there’s no bandgap you can’t turn them off! If you could achieve a graphene bandgap, however, you should be able to make very good transistors.”
Wang’s approach is to induce a bandgap by applying an electrical field between two layers of graphene, i.e., bilayer graphene. In 2006, the group had found a way to create a crude bandgap by doping bilayer graphene with metal atoms. The bandgap, however, was uncontrollable and of little practical use. After coming across some theoretical work about the use of an electrical field, they decided to focus on using that approach.
They went on to build a dual-gated field-effect transistor device with an independently adjustable bandgap. This two-gated device can controls the flow of electrons from a source to a drain with electric fields shaped by gate electrodes. Wang’s group used a silicon substrate as the bottom gate with an insulating layer of silicon dioxide between it and the stacked graphene layers. It was topped off with sapphire on the graphene bilayer and a top gate of platinum. In essence, all of these layers were transparent.
The group shined a synchrotron light beam generated at LBL’s Advanced Light Source facility through the nano-FET. Wang and the other researchers discovered that as they tuned the electrical fields (by varying the voltage of the gate electrodes), they could detect changes in the light absorbed by the gated graphene layers. IR spectrographic measurements of the absorption peak provide a measurement of the bandgap that could be mapped against each change in gate voltage.
Wang and his colleagues work is published in the June 11 issue of Nature.
Adding . . . the work was funded by DOE’s Office of Science, Office of Basic Energy Sciences.