Silicon transistors, that are used to amplify and switch signals, are a critical component in most electronic devices, from smartphones to automobiles. But silicon semiconductor technology is held back by a fundamental physical limit that stops transistors from operating below a certain voltage.
This limit, referred to 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.
In an effort to beat this fundamental limit of silicon, MIT researchers fabricated a unique kind of three-dimensional transistor using a novel set of ultrathin semiconductor materials.
Their devices, featuring vertical nanowires only just a few nanometers wide, can deliver performance comparable to state-of-the-art silicon transistors while operating efficiently at much lower voltages than conventional devices.
“This can be a technology with the potential to exchange silicon, so you possibly can use it with all of the functions that silicon currently has, but with a lot better energy efficiency,” says Yanjie Shao, an MIT postdoc and lead creator of a paper on the brand new transistors.
The transistors leverage quantum mechanical properties to concurrently achieve low-voltage operation and high performance inside an area of just just a few square nanometers. Their extremely small size would enable more of those 3D transistors to be packed onto a pc chip, leading to fast, powerful electronics which might be also more energy-efficient.
“With conventional physics, there is simply to date you may go. The work of Yanjie shows that we will do higher than that, but we’ve to make use of different physics. There are a lot of challenges yet to be overcome for this approach to be industrial in the long run, but conceptually, it truly is a breakthrough,” says senior creator Jesús del Alamo, the Donner Professor of Engineering within the MIT Department of Electrical Engineering and Computer Science (EECS).
They’re 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 today in
Surpassing silicon
In electronic devices, silicon transistors often operate as switches. Applying a voltage to the transistor causes electrons to maneuver over an energy barrier from one side to the opposite, 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 required to activate the transistor and the greater its energy efficiency.
But due to how electrons move across an energy barrier, Boltzmann tyranny requires a certain minimum voltage to modify the transistor at room temperature.
To beat the physical limit of silicon, the MIT researchers used a unique set of semiconductor materials — gallium antimonide and indium arsenide — and designed their devices to leverage a novel phenomenon in quantum mechanics called quantum tunneling.
Quantum tunneling is the power of electrons to penetrate barriers. The researchers fabricated tunneling transistors, which leverage this property to encourage electrons to push through the energy barrier reasonably than going over it.
“Now, you may 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 vital to create powerful transistor switches for demanding applications.
Effective-grained fabrication
Using tools at MIT.nano, MIT’s state-of-the-art facility for nanoscale research, the engineers were in a position to fastidiously control the 3D geometry of their transistors, creating vertical nanowire heterostructures with a diameter of only 6 nanometers. They consider these are the smallest 3D transistors reported thus far.
Such precise engineering enabled them to attain a pointy switching slope and high current concurrently. This is feasible due to a phenomenon called quantum confinement.
Quantum confinement occurs when an electron is confined to an area that’s so small that it will possibly’t move around. When this happens, the effective mass of the electron and the properties of the fabric change, enabling stronger tunneling of the electron through a barrier.
Since the transistors are so small, the researchers can engineer a really strong quantum confinement effect while also fabricating a particularly thin barrier.
“We have now numerous flexibility to design these material heterostructures so we will achieve a really thin tunneling barrier, which enables us to get very high current,” Shao says.
Precisely fabricating devices that were sufficiently small to perform this was a serious challenge.
“We’re really into single-nanometer dimensions with this work. Only a few groups on this planet could make good transistors in that range. Yanjie is awfully capable to craft such well-functioning transistors which might be so extremely small,” says del Alamo.
When the researchers tested their devices, the sharpness of the switching slope was below the basic limit that might be achieved with conventional silicon transistors. Their devices also performed about 20 times higher than similar tunneling transistors.
“That is the primary time we’ve been in a position to achieve such sharp switching steepness with this design,” Shao adds.
The researchers are actually striving to boost their fabrication methods to make transistors more uniform across a whole chip. With such small devices, even a 1-nanometer variance can change the behavior of the electrons and affect device operation. Also they are exploring vertical fin-shaped structures, along with vertical nanowire transistors, which could potentially improve the uniformity of devices on a chip.
“This work definitively steps in the fitting direction, significantly improving the broken-gap tunnel field effect transistor (TFET) performance. It demonstrates steep-slope along with a record drive-current. It highlights the importance of small dimensions, extreme confinement, and low-defectivity materials and interfaces within the fabricated broken-gap TFET. These features have been realized through a well-mastered and nanometer-size-controlled process,” says Aryan Afzalian, a principal member of the technical staff on the nanoelectronics research organization imec, who was not involved with this work.
This research is funded, partially, by Intel Corporation.
