Nanoscale 3D transistors constructed from extremely thin semiconductor materials demonstrate higher efficiency compared to traditional silicon-based devices, utilizing quantum mechanics properties which could make ultra-low-power AI applications feasible.
Silicon transistors play a vital role in amplifying and switching signals, essential for numerous electronic products ranging from smartphones to cars. However, the technology behind silicon semiconductors faces a physical limitation that restricts transistors from functioning below a certain voltage.
This limitation, termed “Boltzmann tyranny,” adversely impacts the energy efficiency of computers and other electronic devices, particularly with the swift evolution of artificial intelligence technologies that require enhanced computational speeds.
To address this core restriction of silicon, researchers at MIT developed a novel type of three-dimensional transistor utilizing a unique array of ultrathin semiconductor materials.
Their creations, which include vertical nanowires measuring only a few nanometers in width, are capable of achieving performance akin to cutting-edge silicon transistors while functioning efficiently at significantly lower voltages compared to standard devices.
“This technology has the potential to replace silicon and can maintain all current functionalities of silicon but with superior energy efficiency,” remarks Yanjie Shao, a postdoctoral researcher at MIT and the lead author of the study on these new transistors.
The transistors exploit properties of quantum mechanics to attain both low-voltage operation and high performance within an area of just a few square nanometers. Their diminutive size permits a larger number of these 3D transistors to be integrated into a single computer chip, which results in fast and powerful electronics that also use less energy.
“Conventional physics has its limits. Yanjie’s research illustrates that we can surpass those limits by employing different physics principles. Although there are numerous challenges that need to be addressed for this technology to become commercially viable in the future, it represents a significant conceptual breakthrough,” states Jesús del Alamo, a senior author and the Donner Professor of Engineering in the MIT Department of Electrical Engineering and Computer Science (EECS).
The paper’s contributors include Ju Li, the Tokyo Electric Power Company Professor in Nuclear Engineering and a materials science and engineering professor at MIT; EECS graduate student Hao Tang; MIT postdoc Baoming Wang; and professors Marco Pala and David Esseni from the University of Udine in Italy. The findings are published in Nature Electronics.
Exceeding Silicon
In electronic devices, silicon transistors primarily function as switches. By applying a voltage to a transistor, electrons can traverse an energy barrier from one side to the other, switching the transistor from “off” to “on.” This switching action enables transistors to represent binary digits for computation.
The efficiency with which a transistor switches is determined by its switching slope; a steeper slope requires less voltage to activate the transistor, enhancing its energy efficiency.
However, due to the nature of electron movement across an energy barrier, Boltzmann tyranny mandates a minimum voltage to switch silicon transistors at room temperature.
To sidestep the physical constraints of silicon, the MIT researchers utilized alternative semiconductor materials such as gallium antimonide and indium arsenide, designing their devices to utilize a unique quantum mechanical phenomenon known as quantum tunneling.
Quantum tunneling allows electrons to penetrate barriers. The team created tunneling transistors that harness this principle, encouraging electrons to push through energy barriers instead of going over them.
“This allows us to easily turn the device on and off,” Shao explains.
However, while tunneling transistors can provide sharp switching slopes, they generally operate at lower currents, which limits the performance of electronic devices. To cater to demanding applications, higher currents are necessary for generating powerful transistor switches.
Precision in Fabrication
At MIT.nano, MIT’s advanced facility for nanoscale research, the engineers exercised precise control over the 3D geometry of their transistors, fabricating vertical nanowire heterostructures with diameters as small as 6 nanometers. They believe these are the smallest 3D transistors documented to date.
Such meticulous engineering allowed them to achieve both a sharp switching slope and high current simultaneously, made possible by quantum confinement.
Quantum confinement transpires when an electron is restricted to such a minuscule space that it cannot move freely. This confinement leads to a change in the effective mass of the electron and the material’s properties, enabling more efficient tunneling through a barrier.
The minuscule size of these transistors empowers the researchers to foster strong quantum confinement effects while also fabricating exceptionally thin barriers.
“We can strategically design these material heterostructures to establish very thin tunneling barriers, allowing for elevated current levels,” states Shao.
Crafting devices at such small dimensions posed a significant challenge.
“We are delving into single-nanometer precision with this work. Few research groups worldwide can fabricate such high-functioning transistors at this scale. Yanjie has demonstrated remarkable skill in creating such efficient transistors that are extremely small,” comments del Alamo.
In testing their devices, the researchers noted that the sharpness of the switching slope surpassed the fundamental limits achievable with standard silicon transistors. Furthermore, their devices exhibited performance approximately 20 times superior to similar tunneling transistors.
“Never before have we achieved such sharp switching steepness with this design,” adds Shao.
The team is now focused on enhancing their fabrication techniques to ensure greater uniformity across an entire chip. At such small scales, even a 1-nanometer difference can significantly alter electron behavior and impact device operation. They are also investigating vertical fin-shaped structures, alongside the vertical nanowire transistors, which may enhance chip uniformity.
This study is partly funded by Intel Corporation.
