Silicon transistors, which are used to amplify and switch signals, are a key component of most electronic devices, from smartphones to cars. However, silicon semiconductor technology is held back by a fundamental physical limitation that prevents transistors from operating below a certain voltage.
This limitation, known as the “tyranny of Boltzmann”, hampers the energy efficiency of computers and other electronics, especially with the rapid development of artificial intelligence technologies that require faster computations.
In an attempt to overcome this fundamental limitation of silicon, MIT researchers have produced a different type of three-dimensional transistor using a unique set of ultra-thin semiconductor materials.
Their devices, composed of vertical nanowires just a few nanometers wide, can provide performance comparable to state-of-the-art silicon transistors while operating efficiently at much lower voltages than conventional devices.
“This is a technology that can replace silicon, so you can use it with all the functionality that silicon currently has, but with much better energy efficiency,” says Yanjie Shao, a postdoc at MIT and lead author of the paper on the up-to-date transistors.
The transistors apply quantum mechanical properties to simultaneously achieve low-voltage operation and high efficiency in an area of just a few square nanometers. Their extremely diminutive size would enable more 3D transistors to be placed on a computer chip, resulting in quick, capable electronics that are also more energy capable.
“You can only go so far with conventional physics. Yanjie’s work shows that we can achieve more, but we need to apply different physics. Many challenges remain before this approach becomes commercially viable in the future, but conceptually it is truly a breakthrough, says senior author Jesús del Alamo, Donner Professor of Engineering in the MIT Department of Electrical Engineering and Computer Science (EECS).
Joining them in the article are Ju Li, professor of nuclear engineering at Tokyo Electric Power Company and professor of materials science and engineering at MIT; EECS graduate Hao Tang; MIT postdoctoral fellow Baoming Wang; and Professors Marco Pala and David Esseni from the University of Udine in Italy. Tests is published today in
Superior silicone
In electronic devices, silicon transistors often act as switches. Applying a voltage to a transistor causes electrons to move across the energy barrier from one side to the other, switching the transistor from “off” to “on”. Through switching, transistors represent binary digits to perform calculations.
The switching slope of a transistor reflects the sharpness of the transition from “off” to “on”. The steeper the slope, the less voltage is needed to turn on the transistor and the greater its energy efficiency.
However, due to the way electrons move through the energy barrier, Boltzmann tyranny requires a certain minimum voltage to switch the transistor at room temperature.
To overcome the physical limitations of silicon, MIT researchers used a different set of semiconductor materials—gallium antimonide and indium arsenide—and designed their devices to take advantage of a unique phenomenon in quantum mechanics called quantum tunneling.
Quantum tunneling is the ability of electrons to penetrate barriers. Scientists have produced tunneling transistors that use this property to encourage electrons to push through the energy barrier rather than crossing it.
“Now you can turn the device on and off very easily,” says Shao.
Although tunnel transistors can enable pointed switching slopes, they typically operate at low current, which impairs the performance of the electronic device. To create powerful transistor switches for demanding applications, higher current is necessary.
Fine-grained version
Using tools available at MIT.nano, MIT’s state-of-the-art nanoscale research facility, engineers were able to precisely control the 3D geometry of their transistors, creating vertical nanowire heterostructures as diminutive as 6 nanometers in diameter. They believe these are the smallest 3D transistors reported to date.
This precise design enabled them to achieve a steep switching slope and high current simultaneously. This is possible thanks to a phenomenon called quantum confinement.
Quantum confinement occurs when an electron is confined in a space so diminutive that it cannot move. When this happens, the electron’s effective mass and material properties change, allowing the electron to tunnel more strongly through the barrier.
Because transistors are so diminutive, researchers can engineer a very forceful quantum confinement effect while also producing an extremely slim barrier.
“We have a lot of flexibility in designing the heterostructures of these materials, so we can get a very thin tunnel barrier that allows us to get very high current,” Shao says.
Precisely making devices diminutive enough to achieve this was a significant challenge.
“In this work, we really focus on the dimensions of single nanometers. Very few groups in the world can produce good transistors in this range. Yanjie has an extraordinary ability to create such well-functioning transistors that are so very small,” says del Alamo.
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 tunnel transistors.
“With this design, we were able to achieve such a sharp switching steepness for the first time,” adds Shao.
Scientists are now trying to improve their manufacturing methods to make the transistors more uniform throughout the chip. For devices this diminutive, even a difference of 1 nanometer can change the behavior of electrons and affect the operation of the device. In addition to vertical nanowire transistors, they are also investigating vertical fin-shaped structures that could potentially improve on-chip device uniformity.
“This work is definitely moving in the right direction, significantly improving the performance of the gap-broken tunnel field effect transistor (TFET). It exhibits a steep gradient along with a record driving current. It highlights the importance of small dimensions, extreme confinement, and low-deficiency materials and interfaces in fabricated gap-broken TFET. These features were achieved through a well-developed process controlled at the nanometer scale,” says Aryan Afzalian, a senior technical staff member at the nanoelectronics research organization imec, who was not involved in the work.
This research is funded in part by Intel Corporation.