The laws of physics have set a 5-nanometer threshold on the size of transistor gates among conventional semiconductors, about one-quarter the size of high-end, 20-nanometer-gate transistors now on the market. Researchers from the Department of Energy's Lawrence Berkeley National Laboratory have created a transistor with a working 1-nanometer gate.

Schematic of a transistor with a molybdenum disulfide channel and 1-nanometer carbon nanotube gate. (Credit: Sujay Desai/UC Berkeley)

The key was to use carbon nanotubes and molybdenum disulfide (MoS2), an engine lubricant commonly sold in auto parts shops. MoS2 is part of a family of materials with immense potential for applications in LEDs, lasers, nanoscale transistors, solar cells, and more. By changing the material from silicon to MoS2, a transistor with a gate that is just 1 nanometer in length is possible, and it can be operated like a switch.

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. Both silicon and MoS2 have a crystalline lattice structure, but electrons flowing through silicon have a smaller effective mass compared with MoS2. That is a boon when the gate is 5 nanometers 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 that the transistors cannot be turned off.

Because electrons flowing through MoS2 have a higher effective mass, their flow can be controlled with smaller gate lengths. MoS2 can also be scaled down to atomically thin sheets about 0.65 nanometers thick, with a lower dielectric constant — a measure reflecting the ability of a material to store energy in an electric field. Both properties, in addition to the effective mass of the electron, help improve the control of the flow of current inside the transistor when the gate length is reduced to 1 nanometer.

Making a 1-nanometer structure was not possible with conventional lithography techniques, which don't work well at that scale, so the researchers turned to carbon nanotubes. They then measured the electrical properties of the devices to show that the MoS2 transistor with the carbon-nano-tube gate effectively controlled the flow of electrons.

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NASA Tech Briefs Magazine

This article first appeared in the March, 2017 issue of NASA Tech Briefs Magazine.

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