Silicon transistors are a critical component in most electronic devices. However, they have limitations. Expansion of silicon semiconductor technology is held back by a fundamental physical limit that prevents transistors from operating below a certain voltage. This limit, known as “Boltzmann tyranny”, hinders the energy efficiency of computers and other electronics, especially with the rapid development of artificial intelligence technologies that demand faster computation.

To overcome this fundamental limit of silicon, MIT researchers fabricated a different type of three-dimensional transistor using a unique set of ultrathin semiconductor materials. Their devices, featuring vertical nanowires only a few nanometres wide, can deliver performance comparable to state-of-the-art silicon transistors while operating efficiently at much lower voltages than conventional devices.

“This is a technology with the potential to replace silicon so you could use it with all the functions that silicon currently has but with much better energy efficiency,” says Yanjie Shao, an MIT postdoc and lead author of a paper on the new transistors.

The transistors leverage quantum mechanical properties to simultaneously achieve low-voltage operation and high performance within an area of just a few square nanometres. Their extremely small size would enable more of these 3D transistors to be packed onto a computer chip, resulting in fast, powerful electronics that are also more energy-efficient.

“With conventional physics, there is only so far you can go. The work of Yanjie shows that we can do better than that but we must apply different physics. There are many challenges yet to be overcome for this approach to become commercial but, conceptually, it really is a breakthrough,” says senior author Jesús del Alamo, donner professor of Engineering in the MIT Department of Electrical Engineering and Computer Science (EECS).

They are joined on the paper by Ju Li, Tokyo Electric Power Company professor in nuclear engineering and materials science and engineering at MIT, EECS graduate student Hao Tang, MIT postdoc Baoming Wang and professors Marco Pala and David Esseni of the University of Udine in Italy. The research appears in Nature Electronics.

Surpassing silicon

In electronic devices, silicon transistors often operate as switches. Applying a voltage to the transistor causes electrons to move over an energy barrier from one side to the other, switching the transistor from “off” to “on”. By switching, transistors represent binary digits to perform computation.

A transistor’s switching slope reflects the sharpness of the “off” to “on” transition. The steeper the slope, the less voltage is needed to turn on the transistor and the greater its energy efficiency.

But because of how electrons move across an energy barrier, Boltzmann tyranny requires a certain minimum voltage to switch the transistor at room temperature.

To overcome the physical limit of silicon, the MIT researchers used a different set of semiconductor materials – gallium antimonide and indium arsenide – and designed their devices to leverage a unique phenomenon in quantum mechanics called quantum tunnelling.

Quantum tunnelling is the ability of electrons to penetrate barriers. The researchers fabricated tunnelling transistors, which leverage this property to encourage electrons to push through the energy barrier rather than going over it. “Now, you can turn the device on and off very easily,” Shao says.

But, while tunnelling transistors can enable sharp switching slopes, they typically operate with low current, which hampers the performance of an electronic device. Higher current is necessary to create powerful transistor switches for demanding applications.

Fine-grained fabrication

Using tools at MIT.nano, MIT’s state-of-the-art facility for nanoscale research, the engineers were able to carefully control the 3D geometry of their transistors, creating vertical nanowire heterostructures with a diameter of only six nanometres. They believe these are the smallest 3D transistors reported to date.

Such precise engineering enabled them to achieve a sharp switching slope and high current simultaneously. This is possible because of a phenomenon called quantum confinement.

Quantum confinement occurs when an electron is confined to a space that is so small that it can’t move around. When this happens, the effective mass of the electron and the properties of the material change, enabling stronger tunnelling of the electron through a barrier.

Because the transistors are so small, the researchers can engineer a very strong quantum confinement effect while also fabricating an extremely thin barrier.

“We have a lot of flexibility to design these material heterostructures so we can achieve a very thin tunnelling barrier, which enables us to get very high current,” Shao says.

Precisely fabricating devices that were small enough to accomplish this was a major challenge. “We are really into single-nanometre dimensions with this work. Very few groups in the world can make good transistors in that range. Yanjie is extraordinarily capable of crafting such well-functioning transistors that are so extremely small,” says Del Alamo.

When the researchers tested their devices, the sharpness of the switching slope was below the fundamental limit that can be achieved with conventional silicon transistors. Their devices also performed about 20 times better than similar tunnelling transistors.

“This is the first time we have been able to achieve such sharp switching steepness with this design,” Shao adds.

The researchers are now striving to enhance their fabrication methods to make transistors more uniform across an entire chip. With such small devices, even a one nanometre variance can change the behaviour of the electrons and affect device operation. They are also exploring vertical fin-shaped structures, in addition to vertical nanowire transistors, which could potentially improve the uniformity of devices on a chip.

“This work definitively steps in the right direction, significantly improving the broken-gap tunnel field effect transistor (TFET) performance. It demonstrates steep slope together with a record drive current. It highlights the importance of small dimensions, extreme confinement and low-defectivity materials and interfaces in the fabricated broken-gap TFET. These features have been realised through a well-mastered and nanometre-size controlled process,” says Aryan Afzalian, a principal member of the technical staff at the nanoelectronics research organisation imec, who was not involved with this work.