Researchers reduce transistor gate length

The semiconductor industry has long regarded five nanometers as the limit for transistor gate length. Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) claim to have successfully shrunk the transistor gate to one nanometer.

Transistors are semiconductor devices that can be used to either amplify or switch electronic signals. As switches, they exist in two clear forms. They can be denoted as “on” or “off” depending on whether they allow electrons to pass through a gate. These stored ons and offs can be translated as information for electronic devices.

Transistors are found in virtually every electronic device, from smartphones to radios. In these devices, there are up to billions of transistors compacted in chips. Intuitively, there can be more transistors packed into chips if their sizes are reduced. More transistors in a chip theoretically means faster, more efficient processing.

“We made the smallest transistor reported to date,” Ali Javey, lead investigator of the Electronic Materials program in Berkeley Lab’s Materials Science Division, said according to the Berkeley Lab News Center.

The research team led by Javey, was able to reduce the length of the gate by using carbon nanotubes and molybdenum disulfide (MoS2). Conventional transistors that use silicon as semiconductor material can be as short as seven nanometers.

Silicon transistors are limited to seven nanometers because electrons in a sub-seven-nanometer gate because they experience the phenomenon known as quantum tunneling. Quantum tunneling allows electrons to flow across a gate even if it is intended to stay or be in the off state. At smaller lengths, silicon can’t be used because the quantum tunneling disrupts the transistor to function as a switch.

Using molybdenum disulfide as the semiconductor in the transistor is the key to producing a shorter transistor gate. Electrons flowing through molybdenum disulfide is heavier compared to silicon, so there is no quantum tunneling. This allows for the use of shorter gates to control electron flow.

Atomic molybdenum disulfide sheets can be as thin as 0.65 nanometers with a low dielectric constant. The success of the one nanometer transistor gate can be attributed to the properties of molybdenum disulfide and carbon nanotubes.

“This research shows that sub-five-nanometer gates should not be discounted. By changing the material from silicon to MoS2, we can make a transistor with a gate that is just one nanometer in length, and operate like a switch,” Sujay Desai, a graduate student in Javey’s lab, said according to the Berkeley Lab News Center.

The notion that the number of transistors on a chip will double every two years is an observation called Moore’s Law, named after Gordon Moore, the co-founder of Fairchild Semiconductor and Intel. This trend, which originated in 1965 has been mostly accurate over the years. Recently, Moore’s Law is running into more limitations posed by nanophysics. Eventually, the reduction of transistor gates must terminate at the atomic level.

Currently, no five nanometer transistors have been commercially made. High-end transistors on the market typically contain 14 nanometer transistor gates. As predicted by Moore’s law, innovative technology companies such as Intel are working to commercially manufacture smaller transistors.

The researchers admit to the scope of their research.

“This work demonstrated the shortest transistor ever. However, it’s a proof of concept,” Javey said. “We have not yet packed these transistors onto a chip and we haven’t done this billions of times over.”

The researchers still face problems in the development, stability, enhancement and manufacturing of small-scale transistors.

“However, large-scale processing and manufacturing of TMD [transition metal semiconductor] devices down to such small gate lengths are existing challenges requiring future innovations,” the researchers conclude in their published Science article.