Shown is a rendition of an experimental stack made with a layer of lead zirconate titanate, a ferroelectric material. UC Berkeley researchers showed that this configuration could amplify the charge in the layer of strontium titanate, an electrical insulator, for a given voltage, a phenomenon known as negative capacitance. Image: Asif Khan
Engineers at the University of California, Berkeley,
have shown that it is possible to reduce the minimum voltage necessary to store
charge in a capacitor, an achievement that could reduce the power draw and heat
generation of today’s electronics.
“Just like a Formula One car, the faster you run your computer, the hotter
it gets. So the key to having a fast microprocessor is to make its building
block, the transistor, more energy efficient,” says Asif Khan, UC Berkeley
graduate student in electrical engineering and computer sciences. “Unfortunately, a transistor’s power supply voltage, analogous to a car’s fuel,
has been stuck at 1 V for about 10 years due to the fundamental physics of its
operation. Transistors have not become as ‘fuel-efficient’ as they need to be
to keep up with the market’s thirst for more computing speed, resulting in a
cumulative and unsustainable increase in the power draw of microprocessors. We
think we can change that.”
Khan, working in the lab of Sayeef Salahuddin, UC Berkeley assistant
professor of electrical engineering and computer sciences, has been leading a
project since 2008 to improve the efficiency of transistors.
The researchers took advantage of the exotic characteristics of
ferroelectrics, a class of material that holds both positive and negative
electrical charges. Ferroelectrics hold electrical charge even when you don’t
apply a voltage to it. What’s more, the electrical polarization in
ferroelectrics can be reversed with the application of an external electrical
Getting more bang for the buck
The engineers demonstrated for the first time that in a capacitor made with
a ferroelectric material paired with a dielectric—an electrical insulator—the
charge accumulated for a given voltage can, in effect, be amplified, a
phenomenon called negative capacitance.
The team report their results in Applied Physics Letters. The
experiment sets the stage for a major upgrade to transistors, the on-off switch
that generate the zeros and ones of a computer’s binary language.
“This work is the proof-of-principle we have needed to pursue negative
capacitance as a viable strategy to overcome the power draw of today’s
transistors,” says Salahuddin, who first theorized the existence of negative
capacitance in ferroelectric materials as a graduate student with engineering
professor Supriyo Datta at Purdue
University. “If we can
use this to create low-power transistors without compromising performance and
the speed at which they work, it could change the whole computing industry.”
The researchers paired a ferroelectric material, lead zirconate titanate
(PZT), with an insulating dielectric, strontium titanate (STO), to create a
bilayer stack. They applied voltage to this PZT-STO structure, as well as to a
layer of STO alone, and compared the amount of charge stored in both devices.
“There was an expected voltage drop to obtain a specific charge with the
dielectric material,” says Salahuddin. “But with the ferroelectric structure,
we demonstrated a two-fold voltage enhancement in the charge for the same
voltage, and that increase could potentially go significantly higher.”
Computer clock speed hits a bottleneck
Since the first commercial microprocessors came onto the scene in the early
1970s, the number of transistors squeezed onto a computer chip has doubled
every two years, a progression predicted by Intel co-founder Gordon Moore and
popularly known as Moore’s
Law. Integrated circuits that once held thousands of transistors decades ago
now boast billions of the components.
But the reduced size has not led to a proportional decrease in the overall
power required to operate a computer chip. At room temperature, a minimum of 60
mV is required to increase by tenfold the amount of electrical current flowing
through a transistor. Since the difference between a transistor’s on and off
states must be significant, it can take at least 1 V to operate a transistor,
the researchers say.
“We’ve hit a bottleneck,” says Khan. “The clock speed of microprocessors has
plateaued since 2005, and shrinking transistors further has become difficult.”
The researchers noted that it becomes increasingly difficult to dissipate
heat efficiently from smaller spaces, so reducing transistor size much more
would come at the risk of frying the circuit board.
The solution proposed by Salahuddin and his team is to modify current
transistors so that they incorporate ferroelectric materials in their design, a
change that could potentially generate a larger charge from a smaller voltage.
This would allow engineers to make a transistor that dissipates less heat, and
the shrinking of this key computer component could continue.
Notably, the material system the UC Berkeley researchers reported shows this
effect at above 200 C, much hotter than the 85 C (185 F) at which a current day
The researchers are now exploring new ferroelectric materials for room
temperature negative capacitance in addition to incorporating the materials
into a new transistor.
Until then, Salahuddin noted that there are other potential applications for
ferroelectrics in electronics. “This is a good system for dynamic random access
memories, energy storage devices, super-capacitors that charge electric cars,
and power capacitors for use in radio frequency communications,” he says.