Silicon Vacuum Tubes

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Researchers at the University of
Pittsburgh have developed a
semiconductor device with a vacuum
channel etched in silicon for electron
transport, instead of a conventional
solid-state channel. This represents a
return to vacuum tube technology,
but on a much smaller scale.
Fast electronic devices need on short
carrier transport times, which are
usually achieved by decreasing the
channel length and/or increasing the
carrier velocity. In an ideal device,
carrier motion is ballistic with no
collisions, but it is difficult to achieve
ballistic transport in a solid-state
medium because the high electric
field used to increase the carrier
velocity also increases scattering.
Vacuum is an ideal medium for
ballistic transport, but vacuum devices
typically have low emission currents
and high operating voltages.
The researchers decided to combine
the strengths of vacuum and solid-
state technologies. They fabricated a
low-voltage field effect transistor with
a vertical vacuum channel etched into
a metal-oxide semiconductor
substrate. With a channel length of
approximately 20 nm, they measured
a transconductance of 20 nS/µm, an
on/off ratio of 500 and a turn-on gate
voltage of 0.5 V under ambient
conditions. Coulombic repulsion in
the electron sheet at the interface
between the oxide and the metal or
the semiconductor reduces the
energy barrier for electron emission,
leading to a high emission current
density of approximately 10 μA/cm²
with a bias of just 1 V.
More info
Nature Nanotechnology article