Nanoscale 3D transistors composed of ultrathin semiconductor materials are more efficient than silicon-based devices, potentially enabling ultra-low-power AI applications.
Silicon transistors, which are used to amplify and switch signals, are an essential component in most electronic devices, including smartphones and automobiles. However, silicon semiconductor technology is constrained by a fundamental physical constraint that prevents transistors from working below a specific voltage. This limit, known as “Boltzmann tyranny,” reduces the energy efficiency of computers and other devices, particularly with the rapid development of artificial intelligence technologies that require quicker calculation.
In an effort to circumvent silicon’s fundamental constraint, MIT researchers created a new form of three-dimensional transistor utilizing a novel collection of ultrathin semiconductor materials. Their devices, which include vertical nanowires only a few nanometers broad, can give performance equivalent to cutting-edge silicon transistors while operating at significantly 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.
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.
Yanjie Shao
The transistors use quantum mechanical features to achieve low-voltage operation and high performance within a few square nanometers. Their extraordinarily small size would allow for more of these 3D transistors to be crammed onto a computer chip, resulting in faster, more powerful electronics that are also more energy efficient.
“With conventional physics, you can only go so far. The work of Yanjie demonstrates that we can do better, but we must employ new physics. Senior author Jesús del Alamo, the Donner Professor of Engineering at the MIT Department of Electrical Engineering and Computer Science, believes that while there are many obstacles to overcome before this approach can be commercialized, it represents a significant conceptual accomplishment.
They are joined on the paper by Ju Li, the Tokyo Electric Power Company Professor in Nuclear Engineering and professor of 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
Silicon transistors are commonly used as switches in electronic equipment. When a voltage is applied to a transistor, electrons cross an energy barrier from one side to the other, switching the transistor from “off” to “on.” Transistors execute computation by representing binary digits through switching.
The switching slope of a transistor represents the sharpness of the transition from “off” to “on”. The steeper the slope, the lower the voltage required to switch on the transistor and the higher the energy efficiency. However, owing of how electrons migrate over an energy barrier, Boltzmann tyranny necessitates a specific minimum voltage to switch the transistor at ambient 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 tunneling. Quantum tunneling is the ability of electrons to penetrate barriers. The researchers fabricated tunneling 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 tunneling 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 6 nanometers. They believe these are the smallest 3D transistors reported to date.
With such exact engineering, they were able to accomplish a sharp switching slope while also maintaining a high current. This is feasible due to the phenomenon known as quantum confinement. Quantum confinement happens when an electron is confined to such a small place that it is unable to move. When this occurs, the electron’s effective mass and material characteristics change, allowing for increased electron tunneling through a barrier. Because the transistors are so small, the researchers can build a powerful quantum confinement effect while also creating an exceedingly thin barrier.
“We have a lot of flexibility to design these material heterostructures so we can achieve a very thin tunneling 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-nanometer dimensions with this work. Very few groups in the world can make good transistors in that range. Yanjie is extraordinarily capable to craft 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 less than the basic limit achievable with traditional silicon transistors. In addition, their devices outperformed similar tunneling transistors by nearly 20 times.
“This is the first time we have been able to achieve such sharp switching steepness with this design,” Shao elaborates.
The researchers are currently working to improve their fabrication procedures to make transistors more homogeneous across the entire chip. With such small devices, even a 1-nanometer variation can alter electron behavior and interfere with device operation. They are also investigating vertical fin-shaped structures, as well as vertical nanowire transistors, which could increase the uniformity of devices on a chip.
