Silicon transistors play a vital role in modern electronics, serving as essential elements for amplifying and switching signals in devices ranging from smartphones to electric vehicles. However, the capabilities of silicon semiconductor technology are restricted by a fundamental obstacle known as “Boltzmann tyranny,” which limits the operation of transistors below a specific voltage threshold.
This constraint significantly impacts energy efficiency in computing and other electronic applications, particularly in light of the rapid advancements in artificial intelligence (AI) technologies that require increasingly faster processing speeds.
To address this challenge, researchers at MIT have developed an innovative type of three-dimensional transistor utilizing a novel combination of ultrathin semiconductor materials. These transistors are constructed with vertical nanowires that measure just a few nanometers in width, allowing them to perform on par with state-of-the-art silicon transistors while operating efficiently at much lower voltages.
“This technology has the potential to supplant silicon, maintaining all its existing functionalities while offering significantly improved energy efficiency,” states Yanjie Shao, a postdoctoral researcher at MIT and lead author of this groundbreaking study.
The newly designed transistors harness quantum mechanical properties to enable low-voltage operation combined with high performance in a remarkably compact area of just a few square nanometers. Their small size allows for a higher density of transistors to be integrated into computer chips, leading to electronics that are not only faster and more potent but also more energy-efficient.
According to senior author Jesús del Alamo, the Donner Professor of Engineering at MIT’s Department of Electrical Engineering and Computer Science (EECS), “With conventional physics, advancements are limited. Yanjie’s work demonstrates that we can achieve superior outcomes by exploring different physics. Although there are hurdles to overcome before this technology becomes commercially viable, it represents a conceptual breakthrough.”
The research team includes Ju Li, the Tokyo Electric Power Company Professor in Nuclear Engineering and a materials science professor at MIT, along with EECS graduate student Hao Tang, MIT postdoc Baoming Wang, and professors Marco Pala and David Esseni from the University of Udine in Italy. Their findings are published in Nature Electronics.
Breaking Silicon Barriers
In typical electronic devices, silicon transistors function as switches. Applying a voltage prompts electrons to overcome an energy barrier, transitioning the transistor from an “off” state to an “on” state, which in turn enables computation through binary representation.
The slope of a transistor’s switching curve illustrates the efficiency of its “off” to “on” transition: a steeper slope necessitates less voltage, yielding greater energy efficiency.
However, due to electron behavior at energy barriers, Boltzmann tyranny imposes a minimum voltage requirement for transistors operating at room temperature. To sidestep this limitation, the MIT team employed alternative semiconductor materials, namely gallium antimonide and indium arsenide, which enabled them to exploit a unique quantum phenomenon called quantum tunneling.
Quantum tunneling allows electrons to bypass barriers, thus the researchers fabricated tunneling transistors that effectively encourage electrons to penetrate energy barriers rather than surmount them, enhancing switching capabilities significantly.
“With this design, we can easily toggle the device on and off,” remarks Shao.
However, while tunneling transistors are known for their sharp switching characteristics, they generally yield low current, which could limit the overall performance in demanding applications. High current levels are essential for creating robust transistor switches.
Advancing Fabrication Techniques
Utilizing state-of-the-art tools at MIT.nano, the researchers meticulously engineered the three-dimensional geometry of their transistors, achieving vertical nanowire heterostructures with diameters measuring only 6 nanometers—an advancement believed to represent the smallest 3D transistors developed to date.
This exceptional precision led to the realization of a steep switching slope in conjunction with high current capabilities, attributed to a process known as quantum confinement.
Quantum confinement occurs when an electron is confined to a diminutive space, modifying its effective mass and the material’s characteristics, which facilitates stronger tunneling through barriers.
The minute dimensions of the transistors allow the researchers to generate a robust quantum confinement effect while fabricating an extremely thin barrier.
“Our capability to design material heterostructures enables us to produce a thin tunneling barrier that promotes high current levels,” explains Shao.
Creating devices that are consistently small enough to achieve these specifications posed significant challenges. Del Alamo notes, “We are pushing the boundaries into single-nanometer dimensions with this project. Very few research groups globally can fabricate effective transistors at such a scale. Yanjie’s skill in producing high-functioning, exceedingly small transistors is exceptional.”
The testing of the new devices revealed that their sharp switching slope surpassed the fundamental limits typical of conventional silicon transistors, delivering performance approximately 20 times greater than similar tunneling transistors.
“This achievement marks the first instance where we have succeeded in realizing such a steep switching slope with this design,” concludes Shao.
The research team aims to improve their fabrication methods to enhance the uniformity of transistors across entire chips. Given the minuscule size of the devices, even a one-nanometer variation can significantly alter electron behavior and, consequently, device performance. Additionally, they are looking into vertical fin-shaped structures as a way to further refine device uniformity on chips.
Aryan Afzalian, a principal staff member at imec’s nanoelectronics research organization, who was not involved in this study, remarked, “This research takes a significant step forward in enhancing the broken-gap tunnel field-effect transistor (TFET) performance. It highlights the essentiality of small dimensions, extreme confinement, and high-quality materials in achieving such impressive results.”
This innovative research is supported in part by Intel Corporation.
Photo credit & article inspired by: Massachusetts Institute of Technology
