For
decades, one material has so dominated the production of computer chips and
transistors that the tech capital of the world — Silicon Valley — bears its
name. But silicon’s reign may not last forever.
MIT
researchers have found that an alloy called InGaAs (indium gallium arsenide)
could hold the potential for smaller and more energy efficient transistors.
Previously, researchers thought that the performance of InGaAs transistors
deteriorated at small scales. But the new study shows this apparent
deterioration is not an intrinsic property of the material itself.
The
finding could one day help push computing power and efficiency beyond what’s
possible with silicon. “We’re really excited,†said Xiaowei Cai, the study’s
lead author. “We hope this result will encourage the community to continue
exploring the use of InGaAs as a channel material for transistors.â€
Cai, now
with Analog Devices, completed the research as a PhD student in the MIT
Microsystems Technology Laboratories and Department of Electrical Engineering
and Computer Science (EECS), with Donner Professor Jesús del Alamo. Her
co-authors include Jesús Grajal of Polytechnic University of Madrid, as well as
MIT’s Alon Vardi and del Alamo. The paper will be presented this month at the
virtual IEEE International Electron Devices Meeting.
Transistors
are the building blocks of a computer. Their role as switches, either halting
electric current or letting it flow, gives rise to a staggering array of computations
— from simulating the global climate to playing cat videos on Youtube. A single
laptop could contain billions of transistors. For computing power to improve in
the future, as it has for decades, electrical engineers will have to develop
smaller, more tightly packed transistors. To date, silicon has been the
semiconducting material of choice for transistors. But InGaAs has shown hints
of becoming a potential competitor.
Electrons
can zip through InGaAs with ease, even at low voltage. The material is “known
to have great [electron] transport properties,†says Cai. InGaAs transistors
can process signals quickly, potentially resulting in speedier calculations.
Plus, InGaAs transistors can operate at relatively low voltage, meaning they
could enhance a computer’s energy efficiency. So InGaAs might seem like a
promising material for computer transistors. But there’s a catch.
InGaAs’
favorable electron transport properties seem to deteriorate at small scales —
the scales needed to build faster and denser computer processors. The problem
has led some researchers to conclude that nanoscale InGaAs transistors simply
aren’t suited for the task. But, says Cai, “we have found that that’s a
misconception.â€
The team
discovered that InGaAs’ small-scale performance issues are due in part to oxide
trapping. This phenomenon causes electrons to get stuck while trying to flow
through a transistor. “A transistor is supposed to work as a switch. You want
to be able to turn a voltage on and have a lot of current,†says Cai. “But if
you have electrons trapped, what happens is you turn a voltage on, but you only
have a very limited amount of current in the channel. So the switching
capability is a lot lower when you have that oxide trapping.â€
Cai’s team
pinpointed oxide trapping as the culprit by studying the transistor’s frequency
dependence — the rate at which electric pulses are sent through the transistor.
At low frequencies, the performance of nanoscale InGaAs transistors appeared
degraded. But at frequencies of 1 gigahertz or greater, they worked just fine —
oxide trapping was no longer a hindrance. “When we operate these devices at
really high frequency, we noticed that the performance is really good,†she
says. “They’re competitive with silicon technology.â€
Cai hopes her
team’s discovery will give researchers new reason to pursue InGaAs-based
computer transistors. The work shows that “the problem to solve is not really
the InGaAs transistor itself. It’s this oxide trapping issue,†she says. “We
believe this is a problem that can be solved or engineered out of.†She adds
that InGaAs has shown promise in both classical and quantum computing
applications.
“This
[research] area remains very, very exciting,†says del Alamo. “We thrive on
pushing transistors to the extreme of performance.†One day, that extreme
performance could come courtesy of InGaAs.
This
research was supported in part by the Defense Threat Reduction Agency and the
National Science Foundation.