[SEDXC] Return of the Vacuum Tube

[Neil Foster]

  Return of the Vacuum Tube

by Jon Cartwrighton 23 May 2012, 1:44 PM

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sn-vacuum.jpg 
<http://news.sciencemag.org/sciencenow/assets_c/2012/05/sn-vacuum-thumb-800xauto-13338.jpg>

*Red hot again. *Vacuum tubes fizzled out in the 1960s thanks to the 
invention of the transistor, but new research could fire-up the 
technology once more.

Peer inside an antique radio and you'll find what look like small light 
bulbs. They're actually vacuum tubes---the predecessors of the silicon 
transistor. Vacuum tubes went the way of the dinosaurs in the 1960s, but 
researchers have now brought them back to life, creating a nano-sized 
version that's faster and hardier than the transistor. It's even able to 
survive the harsh radiation of outer space.

Developed early last century, vacuum tubes offered the first easy way to 
amplify electric signals. Like light bulbs, they are glass bulbs 
containing a heated filament. But above the filament are two additional 
electrodes: a metal grid and, at the top of the bulb, a positively 
charged plate. The heated filament emits a steady flow of electrons, 
which are attracted to the plate's positive charge. The rate of electron 
flow can be controlled by the charge on the intervening grid, which 
means a small electric signal applied to the grid---say, the tiny output 
of a gramophone---is reproduced in the much stronger electron flow from 
filament to plate. As a result, the signal is amplified and can be sent 
to a loudspeaker.

Vacuum tubes suffered a slow death during the 1950s and '60s thanks to 
the invention of the transistor---specifically, the ability to 
mass-produce transistors by chemically engraving, or etching, pieces of 
silicon. Transistors were smaller, cheaper, and longer lasting. They 
could also be packed into microchips to switch on and off according to 
different, complex inputs, paving the way for smaller, more powerful 
computers.

But transistors weren't better in all respects. Electrons move more 
slowly in a solid than in a vacuum, which means transistors are 
generally slower than vacuum tubes; as a result, computing isn't as 
quick as it could be. What's more, semiconductors are susceptible to 
strong radiation, which can disrupt the atomic structure of the silicon 
such that the charges no longer move properly. That's a big problem for 
the military and NASA, which need their technology to work in 
radiation-harsh environments such as outer space.

"The computer you and I buy is what NASA buys, but they won't want it 
exactly the same way," says Meyya Meyyappan, an engineer at NASA Ames 
Research Center at Moffett Field in California. "It takes them a few 
years to radiation-proof it. Otherwise the computer you put in the space 
shuttle or the space station basically will get zapped and stop working."

The new device is a cross between today's transistors and the vacuum 
tubes of yesteryear. It's small and easily manufactured, but also fast 
and radiation-proof. Meyyappan, who co-developed the "nano vacuum tube," 
says it is created by etching a tiny cavity in phosphorous-doped 
silicon. The cavity is bordered by three electrodes: a source, a gate, 
and a drain. The source and drain are separated by just 150 nanometers, 
while the gate sits on top. Electrons are emitted from the source thanks 
to a voltage applied across it and the drain, while the gate controls 
the electron flow across the cavity. In their paper published online 
today in /Applied Physics Letters/, Meyyappan and colleagues estimate 
that their nano vacuum tube operates at frequencies up to 0.46 terahertz 
<http://apl.aip.org/resource/1/applab/v100/i21/p213505_s1?isAuthorized=no>---some 
10 times faster than the best silicon transistors.

The team's device isn't the first attempt at miniaturizing the vacuum 
tube. Contrary to previous work, however, the researchers do not need to 
create a "proper" vacuum: The separation of the source and drain is so 
small that the electrons stand very little chance of colliding with 
atoms in the air. This is a huge benefit, says Meyyappan, because it 
opens the door to mass production.

Electronics engineer Kristel Fobelets at Imperial College London agrees. 
"Vacuum technology within a semiconductor fabrication line would make 
fabrication costs very high," she says. Still, she cautions, the nano 
vacuum tube is more of a "proof of concept" than a working device, since 
its operational requirements do not yet match modern transistors. As one 
example, about 10 volts is needed to switch the device on, whereas 
modern transistors operate at about 1 volt; in this respect, the nano 
vacuum tube isn't compatible with modern circuits.

Even so, the potential is great, says Meyyappan. The new vacuum tube's 
inherent immunity to radiation could save the military and NASA a lot of 
time and money, while its faster operation makes it a rare candidate for 
so-called terahertz technology 
<http://news.sciencemag.org/sciencenow/2010/05/x-ray-vision-without-the-radiati.html?ref=hp>. 
Sitting between the microwave and infrared regions of the 
electromagnetic spectrum, the terahertz region can pick out the 
"fingerprints" of certain molecules. The technology could therefore be 
used at airports to safely scan for illicit drugs, for instance.

So are vacuum tubes poised to make a comeback? Meyyappan thinks so. "We 
are combining the best of the vacuum," he says, "and the best of what we 
have learned in the past 50 years about integrated-circuit manufacture."


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