Silicon transistors, essential for amplifying and switching signals in electronic devices from smartphones to automobiles, face a fundamental physical limit known as “Boltzmann tyranny,” which prevents them from operating below a certain voltage and hinders energy efficiency. This is particularly problematic with the rapid development of AI technologies that require faster computation. To overcome this limit, MIT researchers have developed a new type of three-dimensional transistor using ultrathin semiconductor materials. These devices, featuring vertical nanowires only a few nanometers wide, can match the performance of state-of-the-art silicon transistors while operating efficiently at much lower voltages.
“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 researcher.
The transistors leverage quantum mechanical properties to simultaneously achieve low-voltage operation and high performance within an area of just a few square nanometers. 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.
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 another, 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.
However, because electrons move across an energy barrier, Boltzmann tyranny requires a certain minimum voltage to switch the transistor at room temperature.
To overcome silicon’s physical limit, 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 electrons’ ability to penetrate barriers. The researchers fabricated tunneling transistors, which leverage this property to encourage electrons to push through the energy barrier rather than go over it.
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.
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.
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 tunneling 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.
Precisely fabricating devices that were small enough to accomplish this was a major challenge.
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 tunneling transistors.
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 1-nanometer variance can change the behavior 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.
For More Information: Nature Electronics
Image: Ultra-scaled vertical-nanowire device design.
