IEEE Spectrum May, 2011 - 44

A

morphous silicon has long
been the king of flat-panel
displays. It began its reign in PC
monitors and high-definition
TV, then conquered netbooks,
e-readers, and smartphones. No other substance
was as suitable for the thin-film transistors that
sit behind a display's hundreds of thousands
of pixels, turning each one on or off.
But soon the dominion of amorphous silicon
will pass, because it can't provide what
coming generations of electronic products
will require. For one thing, it isn't fast enough.
Next-generation LCD TVs will be refreshed at
least 240 times a second, which is two to four
times as quick as today's versions; that way,
they'll provide sharper fast-action sports and
movies. Three-dimensional displays will need
refresh rates twice again as high, to provide all
that fast-motion goodness to each eye.
44

NA * iEEE SpEctrum * mAy 2011

Nor are today's thin-film transistors
stable enough for displays that use the
organic light-emitting diode (OLED), a
thin, efficient, high-contrast technology.
Stability matters because the "threshold
voltage" that an amorphous silicon transistor needs to turn on tends to drift as
the transistor works. And both the problems of slow switching and drift get
worse when amorphous silicon devices
are made on flexible plastic, which is a
critical design requirement for tomorrow's roll-up displays. Such displays
will enable a laptop-size screen to fold
away for storage inside a smartphone.
For these reasons, researchers and
display manufacturers need a replacement in hand when the day comes for
amorphous silicon to step down from its
throne. And they already have their eyes
set on a promising heir-in fact, a whole
family of materials, known as amorphous oxide semiconductors. They're
amorphous because like today's silicon
standby, they lack a regular crystalline
structure, and they're oxides because
they're made of oxygen compounded
with two or three metals, most commonly selected from zinc, indium, gallium, and tin.
Amorphous oxides can form thin
films that are transparent and electrically conductive, which is why they
already serve as the see-through electrode layer in displays and solar cells. It
was this quality that led to the surge in
research that began in 1996, when Hideo
Hosono and his colleagues at the Tokyo
Institute of Technology first noted the
merits of amorphous transparent conducting oxides.
We believe that amorphous oxides
could do more than simply serve as
a passive electrode. They could also
replace amorphous silicon as the active
semiconducting material that does the
heavy lifting as the channel in thin-film
transistors.

here's what's great about amorphous oxides: First and foremost, charge
can zip through them 20 to 40 times as
fast as in amorphous silicon. This speed
is defined by a material's charge-carrier
mobility, and the Continued on page 51
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Table of Contents for the Digital Edition of IEEE Spectrum May, 2011

IEEE Spectrum May, 2011 - Cover1
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