It will use almost no
power and take up almost no space. But it will store lots of data permanently,
even when power is interrupted. It's a smart transistor, and the first
version was built and demonstrated recently at ORNL.
ORNL researchers
Rodney McKee and Matt Chisholm and University of Tennessee researcher
Fred Walker are building an even better prototype of a smart transistor
by taking advantage of their recent materials breakthrough in depositing
a high-quality, crystalline film of barium titanate on germanium. This
powerful transistor is "smart" because barium titanate's crystal structure
gives it desirable ferroelectric properties, such that in certain regions
of the film, positive and negative ions separate, setting up a semi-permanent
internal field. As a result, the transistor "remembers" information
even when the power is turned off.
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Z-contrast
scanning transmission microscope image of the BaTiO3-Ge
interface structure that promotes the ferroelectric field effect
needed for transistor action.
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"Smart transistors
could be used in smart cards because they can cram in more information
and need much less power to get information in and out," McKee says.
"Because one smart transistor can retain as much information as two
silicon transistors and two power-hungry capacitors, a chip with germanium–barium
titanate transistors will hold one million bytes of data compared with
256,000 bytes for a silicon chip of the same size. A smart, low-power
chip could serve as the hard disk drive of a laptop computer and extend
the lifetime of laptop batteries."
The researchers built
a field-effect transistor (FET) by depositing barium titanate as a dielectric
film on a germanium substrate. Three electrodes were also placed on
the germanium transistor.
FETs, which are
used as common switching devices in modern electronic equipment, are
normally made of silicon. When a conventional FET is turned on, electrons
injected from a source electrode flow as a current through the silicon
base and are collected at a drain electrode. To turn the transistor
off, a gate electrode between the other electrodes applies an electrical
voltage to a silicon dioxide dielectric film, causing it to "pinch off"
the current by raising resistance in the silicon base. In this way,
a transistor can function as an on-and-off switch or as a repository
for a bit of information (e.g., an "on" transistor stores a 1 and an
"off" transistor stores a 0).
Depending on whether the
direction of the field of the barium titanate dielectric film is up
or down, it either pulls up or pushes away electrical charges in the
germanium substrate, facilitating or resisting the flow of electrical
current (and making an "on" or "off" transistor). Unlike the case with
a silicon transistor, the direction of the field on the new transistor
stays up or down all the time, so no external power is needed unless
the field must be flipped. All information in the "on" and "off" transistors
is retained despite loss of power.
To deposit a barium
titanate film on germanium, McKee and Walker used molecular beam epitaxy
(MBE), a precisely controlled process for growing thin films under an
ultrahigh vacuum. McKee knew that to make a smart transistor, the barium
titanate had to be put into the right oxidation state on germanium.
The correct state gives the film the insulating properties needed to
make it work effectively. The only way to get the proper oxidation state
is to use the most reactive form of oxygen—ozone. But the ozone
must be made quickly and released at the proper rate.
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|
A
translucent silica gel in a vessel turns deep purple as ozone
adsorption on the gel reaches full saturation. Ozone is released
at a controlled rate from this vessel to the MBE equipment used
to deposit a barium titanate (BaTiO3) film on a germanium
(Ge) substrate.
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To solve this
problem, Alex Gabbard and Charles Malone, both of ORNL's Metals and
Ceramics Division, developed an ozone-dispensing device. In this device,
a column of silica gel beads is placed in a small lab vessel cooled
to cryogenic temperatures. An oxygen-ozone mixture from a standard ozone
generator flows into the vessel and up through the gel. At the right
temperature, ozone, unlike oxygen, adsorbs onto the gel surface. After
20 minutes, as ozone adsorption on the gel reaches full saturation,
the translucent gel turns deep purple.
To retain the
ozone, the gel is cooled to a constant temperature of –100°C
using liquid nitrogen, which is at –192°C. The temperature
of the gel, which affects the rate at which ozone is collected or exhausted,
is controlled by the flow of nitrogen gas in a jacket surrounding the
gel. A vacuum chamber enclosing this jacket and the silica gel chamber
inside it isolates the jacket and inner column of gel from the extreme
cold of liquid nitrogen. When the gel is saturated with ozone, the ozone
is released at a controlled rate to the MBE equipment by flowing more
nitrogen gas into the chamber to heat the gel slightly.
The winning combination
of technologies perfected at ORNL to make a smart transistor is attracting
the attention of the electronics industry.
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