First superconducting transistor promises PC revolution
The breakthrough promises to put today's gigahertz processors in
the shade - if practical hurdles can be overcome
by Paul Marks
THE world's first superconducting transistor, a long-standing goal for
applied physicists, could lead to dramatically faster microchips.
Last year Andrea Caviglia and his colleagues at the University of
Geneva in Switzerland grew a single crystal containing two metal
oxides, strontium titanate and lanthanum aluminate, as separate
segments. At the interface of these materials, the team found a layer
of free electrons called an electron gas (Science, vol 317, p
1196). At 0.3 kelvin - just above absolute zero - these electrons flow
without resistance and so create a superconductor.
Now the same group says it can switch this superconductivity on and
off by applying a voltage to the interface. The result is a
superconducting version of the field effect transistor (FET) - a
mainstay of digital electronics.
The team can switch the superconductivity on and off by applying a
voltage
A conventional FET contains a sliver of a semiconducting material with
a so-called "source" electrode at one end and a "drain" electrode at
the other. Above this source-drain channel is an electrode called the
gate, which acts like a tap: when a "switch-on" voltage is applied to
the gate, a current flows through the semiconductor channel. That
current's state - either off or on - can act as a digital 0 or 1.
The speed at which a FET can switch is limited by the resistance of
the channel, which creates heat. Higher speeds create more heat until
eventually the device burns out. That's why a superconducting FET
could run much faster.
Caviglia's team made such a transistor by using the lanthanum
aluminate side of its crystal as a source-drain channel and the
strontium titanate layer as the gate (Nature, vol 456, p 624). "With
no electric field, there is zero resistance between the source and
drain as the device is superconducting," says Caviglia. But with an
electric field applied to the strontium titanate, the dense electron
gas gets shifted away from the interface and the lanthanum aluminate
stops conducting current.
Caviglia said that computers using such transistors would be "much
faster than the gigahertz speeds currently available".
David Cardwell, a superconductor specialist at the University of
Cambridge, thinks the work is an important breakthrough: "This is an
exciting effect and has clear potential for a new generation of
high-speed transistors."
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