IEEE Spectrum July, 2014 - 33

The drawbacks of tubes weren't so vexing when
you just needed a handful of them to run your radio
or television set. But they proved really troublesome
grid
with more complicated circuits. For example, the 1946
ENIAC computer, which used 17,468 vacuum tubes,
consumed 150 kilowatts of power, weighed more than
27 metric tons, and took up almost 200 square meters
of floor space. And it kept breaking down all the time,
anode (plate)
with a tube failing every day or two.
The transistor revolution put an end to such frustracathode
tions. But the ensuing sea change in electronics came
about not so much because of the intrinsic advantages
of semiconductors but because engineers gained the
ability to mass-produce and combine transistors in integrated circuits by chemically engraving, or etching,
a silicon wafer with the appropriate pattern. As the
technology of integrated-circuit fabrication progressed,
more and more transistors could be squeezed onto
microchips, allowing the circuitry to become more
elaborate from one generation to the next. The electronics also became faster without costing any more.
That speed benefit stemmed from the fact that as
the transistors became smaller, electrons moving
through them had to travel increasingly shorter distances between the source and the drain, allowing
each transistor to be turned on and off more quickly.
Vacuum tubes, on the other hand, were big and bulky
and had to be fabricated individually by mechanical
machining. While they were improved over the years,
tubes never benefited from anything remotely resembling Moore's Law.
the simplest vacuum tube capable of amplification is the triode, so named
But after four decades of shrinking transistor dimenbecause it contains three electrodes: a cathode, an anode, and a grid. typically,
the structure is cylindrically symmetrical, with the cathode surrounded by the
sions, the oxide layer that insulates the gate electrode
grid and the grid surrounded by the anode. operation is similar to that of a fieldof a typical MOSFET is now only a few nanometers
effect transistor, here with the voltage applied to the grid controlling the current
thick, and just a few tens of nanometers separate its
flow between the other two electrodes. (triode tubes often have five pins to
accommodate two additional electrical connections for the heated filament.)
source and drain. Conventional transistors really can't
get much smaller. Still, the quest for faster and more
energy-efficient chips continues. What will the next
transistor technology be? Nanowires, carbon nanotubes, and graphene are all being developed intensively. Perhaps one of these approaches will revamp
the electronics industry. Or maybe they'll all fizzle.
source and the drain through the solid semiconducting material that separates them.
Why did vacuum tubes give way to solid-state electronics so
e've been working to develop yet another
many decades ago? The advantages of semiconductors include
candidate to replace the MOSFET, one that relower costs, much smaller size, superior lifetimes, efficiency,
searchers have been dabbling with off and on
ruggedness, reliability, and consistency. Notwithstanding
for many years: the vacuum-channel transistor.
these advantages, when considered purely as a medium for
It's the result of a marriage between traditional
transporting charge, vacuum wins over semiconductors. Elecvacuum-tube technology and modern semiconductor-fabrication
techniques. This curious hybrid combines the best aspects of
trons propagate freely through the nothingness of a vacuum,
vacuum tubes and transistors and can be made as small and as
whereas they suffer from collisions with the atoms in a solid
cheap as any solid-state device. Indeed, making them small is
(a process called crystal-lattice scattering). What's more, a vacwhat eliminates the well-known drawbacks of vacuum tubes.
uum isn't prone to the kind of radiation damage that plagues
In a vacuum tube, an electric filament, similar to the filament
semiconductors, and it produces less noise and distortion
in an incandescent lightbulb, is used to heat the cathode suffithan solid-state materials.

cHip iN a bOTTle

IllustratIon by

James Provost

SPectrum.ieee.orG

north american

jul 2014

http://SPectrum.ieee.orG

Table of Contents for the Digital Edition of IEEE Spectrum July, 2014