World-record speed for thin-film transistors could revolutionize flexible electronics
pair of UW-Madison researchers have developed
a method of making flexible, thin-film transistors (TFTs) that are not
only inexpensive to produce, but also capable of high speeds—even
microwave frequency, impossible before now. Assistant Professor Zhenqiang
(Jack) Ma and graduate student Hao-Chih Yuan recently demonstrated
flexible TFTs capable of operating at a world-record speed of 7.8 GHz.
TFTs are transistors that are currently widely used
in electronics such as LCD displays and electronic and radio-frequency
tags. For example, in an LCD screen, TFTs control individual pixels
for high-quality images. TFTs made on flexible substrates could have
a variety of applications, says Ma, including flexible and wearable
electronics, flexible sensors, large-area surveillance radar, embedded
signatures and more.
Until now, flexible TFTs have been relatively slow,
operating in the 0.5 GHz range, says Yuan. This is fine for applications
such as LCD displays, but not for applications such as military surveillance
antennas that require high-performance but flexible circuitry for easy
storage. “The application of current low-speed TFTs is very limited,”
says Ma. “Fast TFTs offer significant advantages in terms of power
consumption and operation frequency, beside their flexibility and robustness
against breakage.”
Flexible TFTs are usually made on organic materials
or amorphous or poly silicon, but the research team instead uses nanoscale-thin
membranes of single-crystal silicon, which has greater electron mobility
and therefore greater speed. The membranes can be peeled off from the
bulk silicon used for fabrication with an inexpensive, patent-pending
method. But mobility is not enough to bring the TFTs up to speed, Ma
says. Low-resistance electrode contacts are also important.
However, achieving this is challenging because the
high temperatures needed to activate low-resistance contact connections
melt the polymer substrates the transistors are fabricated on. “That
is the major obstacle to realizing the high speed operation of TFTs,
regardless of the fact that high mobility has been already demonstrated
in single-crystal silicon on flexible substrate,” says Ma.
Ma and Yuan overcame this obstacle with an innovative
technique. They made the transistors in “hot” and “cold”
steps. First, they made the contact connectors on a bulk silicon substrate
to achieve low resistance, and then transferred the single-crystal nanomembranes
to the flexible substrate to continue fabrication. Ma and Yuan published
a paper detailing this novel method in a recent issue of Applied
Physics Letters.
Another factor in the new TFT’s speed is that
instead of the usual silicon dioxide, they made the gates of silicon
monoxide, which carries the advantage of lower processing temperatures.
“In addition, silicon monoxide has higher electric capacity and
can be made thinner than the dioxide. As a result, the device speed
becomes even faster,” says Yuan.
The next step, says Ma, is developing even more advanced
processing technologies and materials for even higher speed TFTs. He
also hopes for the realization of potential applications, including
an entire flexible radio-frequency system. “We opened numerous
possibilities with this breakthrough,” he says.
