IEEE Spectrum December, 2015 - 50

The Quest for the
ultimate vacuum Tube
co nti n u e d f ro m pag e 3 3

and Shoulders described nearly 50 years ago is still the most
promising: field emission. All you need to make it work is a
strong electric field at the surface between the emitter and
the vacuum. Ordinarily, free electrons in a metal at room temperature encounter an energy barrier near the metal's surface
that keeps them confined. But when you heat the metal-as in
a thermionic emitter-you're giving some of those electrons
enough energy to overcome the barrier.
Instead, let's apply an inward-directed electric field at the
surface, which exerts an outward force on the electrons
and thins the barrier; as the field increases, the barrier is
thinned still more. To achieve high field emission at the level
you would need in a traveling-wave tube, the barrier must be
so thin that the electrons leak right through it. In quantum
mechanical terms, the electrons have "tunneled" through
the barrier. To get significant numbers of electrons to escape,
you'll need to apply a very high electric field, of more than
a billion volts per meter. For comparison, during a thunderstorm a local electric field of around 2,000 volts per meter
is considered dangerous.

So how do you get a high field without having to apply excessively high voltage? The answer lies in nanofabrication. We
know that the electric field between two conducting electrodes
gets stronger as the electrodes are brought closer together.
So if we bring the positively charged gate electrode (that is,
the electrode that modulates the emission from the cathode)
and the negatively charged emitter very close together-as in
less than a micrometer apart-a strong inward electric field
results at the surface of the emitter. We also shrink the emitter features to the nanometer scale and take advantage of the
field enhancement that occurs naturally at sharp edges and
points, sort of like a nano lightning rod. Now you have a fieldemitter cathode that operates at less than 100 V.
To date, the best-performing cold-cathode device, as measured by total emission current and current density, was
demonstrated by Spindt, Christopher Holland, and Paul
Schwoebel at SRI. Their cathode consists of tens of thousands
of micrometer-size molybdenum cones deposited on a circular
silicon substrate with an area of about one square millimeter.
Each cone acts as an electron emitter and sits in its own well,
carved into a 2.5-micrometer-thick silicon-dioxide insulating
layer. A thin metal coating over the insulating layer serves as
the gate electrode for the array of field emitters.
For more than a decade, my colleague David Whaley and
I, working with the SRI team, have been trying to turn this

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Table of Contents for the Digital Edition of IEEE Spectrum December, 2015