A team of scientists headed by Lawrence Berkeley National Laboratory researcher Prof. Ali Javey has used carbon nanotubes and a compound called molybdenum disulfide to create a transistor with a working 1-nm (nanometer) gate. The team’s findings will appear in the October 7 issue of the journal Science.
Transistors consist of three terminals: a source, a drain, and a gate. Current flows from the source to the drain, and that flow is controlled by the gate, which switches on and off in response to the voltage applied.
“The semiconductor industry has long assumed that any gate below 5 nm wouldn’t work, so anything below that was not even considered,” said Sujay Desai, the lead author on the study and a graduate student in Prof. Javey’s lab.
“We made the smallest transistor reported to date,” Prof. Javey said.
“The gate length is considered a defining dimension of the transistor. We demonstrated a 1-nm-gate transistor, showing that with the choice of proper materials, there is a lot more room to shrink our electronics,” he added.
“Our research shows that sub-5-nm gates should not be discounted. Industry has been squeezing every last bit of capability out of silicon,” Desai said.
“By changing the material from silicon to molybdenum disulfide (MoS2), we can make a transistor with a gate that is just 1 nm in length, and operate it like a switch.”
Both silicon and molybdenum disulfide have a crystalline lattice structure, but electrons flowing through silicon are lighter and encounter less resistance compared with molybdenum disulfide. That is a boon when the gate is 5 nm or longer. But below that length, a quantum mechanical phenomenon called tunneling kicks in, and the gate barrier is no longer able to keep the electrons from barging through from the source to the drain terminals.
“This means we can’t turn off the transistors. The electrons are out of control,” Desai explained.
Because electrons flowing through molybdenum disulfide are heavier, their flow can be controlled with smaller gate lengths.
Molybdenum disulfide can also be scaled down to atomically thin sheets, about 0.65 nm thick, with a lower dielectric constant, a measure reflecting the ability of a material to store energy in an electric field.
Both of these properties, in addition to the mass of the electron, help improve the control of the flow of current inside the transistor when the gate length is reduced to 1 nm.
Once they settled on molybdenum disulfide as the semiconductor material, it was time to construct the gate.
Making a 1-nm structure, it turns out, is no small feat. Conventional lithography techniques don’t work well at that scale, so the team turned to carbon nanotubes, hollow cylindrical tubes with diameters as small as 1 nm.
The researchers then measured the electrical properties of the devices to show that the MoS2 transistor with the carbon nanotube gate effectively controlled the flow of electrons.
“This work is important to show that we are no longer limited to a 5-nm gate for our transistors,” Prof. Javey said.
“Moore’s Law can continue a while longer by proper engineering of the semiconductor material and device architecture.”
S.B. Desai, S.R. Madhvapathy, A.B. Sachid, J.P. Llinas, Q. Wang, G.H. Ahn, G. Pitner, M.J. Kim, J. Bokor, C. Hu, H.-S.P. Wong, A. Javey. 2016. Science, in press;
This article is based on a press-release from Lawrence Berkeley National Laboratory.