Silicon transistors, used to amplify and switch signals, are an important component in most electronic devices, from smartphones to cars. However, silicon semiconductor technology is hampered by a fundamental physical limit that forestalls transistors from operating below a certain voltage.
This limit, generally known as “Boltzmann tyranny,” hinders the energy efficiency of computers and other electronic devices, especially given the rapid development of artificial intelligence technologies that require faster calculations.
To overcome this fundamental limit of silicon, MIT researchers fabricated a special form of three-dimensional transistor using a singular set of ultrathin semiconductor materials.
Their devices, using vertical nanowires just a number of nanometers wide, can deliver performance comparable to modern silicon transistors while operating efficiently at much lower voltages than traditional devices.
“This is a technology with the potential to switch silicon, so you can use it with all of the functions that silicon currently has, but with a lot better energy efficiency,” says Yanjie Shao, a postdoctoral researcher at MIT and lead writer of a paper on the brand new transistors.
The transistors use quantum mechanical properties to concurrently achieve low-voltage operation and high performance in an area of just a number of square nanometers. Their extremely small size would allow more of those 3D transistors to be packed onto a pc chip, leading to fast, powerful electronics which are also more energy efficient.
“There are only limited options with conventional physics. Yanjie's work shows that we are able to do higher, but we now have to make use of different physics. There are still many challenges that have to be overcome for this approach for use commercially in the long run, but conceptually it is really a breakthrough,” says lead writer Jesús del Alamo, Donner Professor of Engineering in MIT’s Department of Electrical Engineering and Computer Science ( EEC).
Ju Li, professor of nuclear engineering at Tokyo Electric Power Company and professor of materials science and engineering at MIT, assists within the paper; EECS graduate student Hao Tang; MIT postdoc Baoming Wang; and Professors Marco Pala and David Esseni from the University of Udine in Italy. The research appears today in
Surpasses silicon
In electronic devices, silicon transistors often act as switches. By applying a voltage to the transistor, electrons move backward and forward across an energy barrier, switching the transistor from “off” to “on.” By switching, transistors represent binary digits to perform calculations.
The switching fringe of a transistor reflects the sharpness of the transition from “off” to “on”. The steeper the slope, the less voltage is required to activate the transistor and the upper its energy efficiency.
However, resulting from the best way electrons move across an energy barrier, Boltzmann tyranny requires a certain minimum voltage to modify the transistor at room temperature.
To overcome the physical limitations of silicon, MIT researchers used a special group of semiconductor materials – gallium antimonide and indium arsenide – and designed their devices to make the most of a singular phenomenon in quantum mechanics called quantum tunneling.
Quantum tunneling is the power of electrons to penetrate barriers. The researchers created tunneling transistors that use this property to encourage electrons to interrupt through the energy barrier slightly than overcome it.
“Now you may easily turn the device on and off,” says Shao.
Although tunnel transistors can provide sharp switching edges, they typically operate at low current, which degrades the performance of an electronic device. To develop high-performance transistor switches for demanding applications, higher current is required.
Fine-grain processing
Using tools from MIT.nano, MIT's state-of-the-art facility for nanoscience, engineers were capable of fastidiously control the 3D geometry of their transistors, creating vertical nanowire heterostructures just 6 nanometers in diameter. They consider these are the smallest 3D transistors reported up to now.
This precise technique allowed them to attain a steep switching ramp and high current at the identical time. This is feasible resulting from a phenomenon called quantum confinement.
Quantum confinement occurs when an electron is confined in an area so small that it cannot move. When this happens, the effective mass of the electron and the properties of the fabric change, allowing more tunneling of the electron through a barrier.
Because the transistors are so small, the researchers can create a really strong quantum confinement effect while creating an especially thin barrier.
“We have plenty of flexibility within the design of those material heterostructures, allowing us to attain a really thin tunnel barrier that enables us to acquire very high currents,” says Shao.
A significant challenge was producing devices sufficiently small to attain this.
“With this work we’re really moving into the one-nanometer range. Very few firms on this planet can produce good transistors on this area. “Yanjie is incredibly capable of creating such well-performing transistors which are so extremely small,” says del Alamo.
When the researchers tested their devices, the steepness 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 comparable tunnel transistors.
“This is the primary time we now have been capable of achieve such a robust switching steepness with this design,” adds Shao.
The researchers are actually working to enhance their manufacturing methods to make the transistors more uniform across the chip. In such small devices, a deviation of just 1 nanometer can change the behavior of the electrons and affect device operation. In addition to vertical nanowire transistors, also they are exploring vertical rib-shaped structures that might potentially improve the uniformity of devices on a chip.
“This work is certainly in the proper direction and significantly improves the performance of tunnel field effect transistors (TFETs) with a broken radio link. It contains a steep climb and record driving current. It highlights the importance of small size, extreme confinement, and low-defectivity materials and interfaces within the fabricated broken-gap TFET. “These features were realized through a well-mastered and nanometer-sized controlled process,” says Aryan Afzalian, a senior technical fellow on the nanoelectronics research organization imec, who was not involved on this work.
This research is funded partially by Intel Corporation.
