Vacuum channel transistor combines best of semiconductors and vacuum tubes

April 4, 2017 by Lisa Zyga feature
Illustrations and scanning electron microscope image of the nanoscale vacuum channel transistor. Credit: Han et al. ©2017 American Chemical Society

(Phys.org)—Although vacuum tubes were the basic components of early electronic devices, by the 1970s they were almost entirely replaced by semiconductor transistors. But in the past few years, researchers have been developing "nanoscale vacuum channel transistors" (NVCTs) that combine the best of vacuum tubes and modern semiconductors into a single device.

Compared to conventional transistors, NVCTs are faster and more resistant to and radiation. These advantages make NVCTs ideal candidates for applications such as radiation-tolerant deep space communications, high-frequency devices, and THz electronics. They are also candidates for extending Moore's law—which states that the number of transistors on a computer chip doubles approximately every two years—which is expected to soon hit a roadblock due to the physical limitations of shrinking semiconductor transistors.

On the other hand, traditional tubes have certain disadvantages compared to semiconductor transistors, which caused them to become obsolete. Notably, vacuum tubes are very large and consume a lot of energy.

With the new NVCTs, size is no longer an issue because the new devices are produced using modern semiconductor fabrication techniques, and so can be made as small as a few nanometers across. Whereas traditional vacuum tubes look like light bulbs, NVCTs look more like typical and can only be seen under a scanning electron microscope.

To address the more pressing issue of energy consumption, in a new study researchers Jin-Woo Han, Dong-Il Moon, and M. Meyyappan at the NASA Ames Research Center in Moffett Field, California, have designed a silicon-based NVCT with an improved gate structure that reduces the drive voltage from tens of volts to less than five volts, resulting in a lower energy consumption. Their work is published in a recent issue of Nano Letters.

In an NVCT, the gate is the component that receives the drive voltage and, based on this voltage, it controls the flow of electrons between two electrodes. In contrast, in the old , electrons were released by heating the emitter of the device. Because the electrons traveled through a vacuum (the vacuum gap), they moved at very high speeds, which led to the fast operation.

In NVCTs, there is not actually a vacuum, but instead the electrons travel across a space filled with an inert gas such as helium at atmospheric pressure. Since the distance between electrodes is so small (as little as 50 nm), the probability of an electron colliding with a gas molecule is very low, and so the electrons move just as quickly through this "quasi-vacuum" as they do in an actual vacuum. Even with some collisions occurring, the gas molecules are not ionized due to the lower operating voltage.

Perhaps the greatest advantage of the new vacuum transistors is their ability to tolerate high temperatures and ionizing radiation, which makes them promising candidates for the harsh environments often experienced by military and space applications. In the new study, the researchers experimentally demonstrated that the NVCTs continue to operate at the same level of performance at temperatures of up to 200 °C, whereas conventional would cease to function at this temperature. Tests also showed that the new NVCTs are robust against gamma and proton radiation.

In the future, the researchers plan to further improve the performance of this "new old" technology.

"Future research plans include device modeling work at the nanoscale, including structure and material properties," Han told Phys.org. "Also we plan to study aging mechanisms to improve reliability and lifetime."

Explore further: Return of the vacuum tube

More information: Jin-Woo Han, Dong-Il Moon, and M. Meyyappan. "Nanoscale Vacuum Channel Transistor." Nano Letters. DOI: 10.1021/acs.nanolett.6b04363

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20 comments

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dirk_bruere
3.7 / 5 (3) Apr 04, 2017
The irony for me being that I invented this when I was around 14 years old - 50 years ago! Of course, as it turned out I was not alone. It's an old idea.
gkam
2.5 / 5 (8) Apr 04, 2017
Actually doing it is what makes the news.

And with those low voltages they will not need a suppressor grid.
gkam
2.1 / 5 (7) Apr 04, 2017
Oh, . . I got a one on that one.

Someone want to argue about the need for a suppressor grid?
Whydening Gyre
5 / 5 (1) Apr 05, 2017
FINALLY....
Analogue takes back it's crown...
Particulizing only clouds the picture...
Eikka
not rated yet Apr 05, 2017
Oh, . . I got a one on that one.

Someone want to argue about the need for a suppressor grid?


Secondary emissions are an issue in a tetrode tube which have an extra screen electrode to compensate the Miller capacitance of the tube and prevent oscillation in certain circuits. The electrons striking the anode have sufficient energy to knock off extra electrons which are then captured by the screen electrode, interfering with its operation. The suppressor grid was placed between the anode and the screen, making a pentode tube.

In simpler triode tubes this is not an issue because there is no screen electrode, so the secondary electrons are simply recaptured by the anode. The anode can also be treated/coated/shaped/surfaced to reduce and redirect secondary emissions to the point that it's not a problem.

The NVCT is structurally a triode tube, so it doesn't need a suppressor grid in the first place.
TheGhostofOtto1923
3.7 / 5 (3) Apr 05, 2017
"which caused them to become obsolete"

-Except in the music industry where marshall stacks supposedly sound better with glowing valves that give off a pleasant warmth.

Perhaps these little thingies can replace them as well. But I doubt it.
gkam
1 / 5 (4) Apr 05, 2017
Eikka looked it up.

Some of us already knew it.

But those are beam power tetrodes such as the 8121 used in VHF communications, and push large amounts of current. Thy are fan-cooled.
Eikka
3 / 5 (2) Apr 05, 2017
-Except in the music industry where marshall stacks supposedly sound better with glowing valves that give off a pleasant warmth.


Vacuum tubes can be overdriven without causing unpleasant odd harmonics, so you don't need to pull down the drums etc. in mixing. The overdriven tube amp acts as a sort of natural compressor, which increases the percieved loudness without increasing amplitude, and loudness sounds better.

It also adds even harmonics and the tubes ring and buzz and have all sorts of "problems" in reaction to the signal, which is why they're used for guitar effects. They play like instruments in their own right.

Some of us already knew it.


Don't pretend you did. You obviously thought the operating voltage was the main reason, and were just looking to drop some technical jargon you memorized to look smart.

But those are beam power tetrodes such as the 8121


What "those"? It seems like you're having an imaginary conversation.
gkam
1 / 5 (4) Apr 05, 2017
Sorry, Eikka, but once again experience beats wiki.

We used the 8121 in the VHF 101. A Beam Power Tetrode. Look it up.

gkam
1 / 5 (4) Apr 05, 2017
"The overdriven tube amp acts as a sort of natural compressor, which increases the percieved loudness without increasing amplitude, and loudness sounds better."
--------------------------------

An overdriven amplifier creates odd harmonics by clipping the peak.
Eikka
3 / 5 (1) Apr 06, 2017
Sorry, Eikka, but once again experience beats wiki.

We used the 8121 in the VHF 101. A Beam Power Tetrode. Look it up.


Again, you're having some sort of imaginary conversation with yourself. Nobody brought up 8121 tetrodes anywhere, and they're not relevant to the article.

An overdriven amplifier creates odd harmonics by clipping the peak.


Tube amps are "soft clipping", meaning they exhibit non-linear behaviour at the limits instead of a hard cut-off, which creates even harmonics.

http://www.gmarts...lip1.jpg

Tubes are also tolerant of overloading, so a small tube amp can drive a big speaker loudly. If you do that with a transistor amp, the sound starts to "break", whereas the tube amp adds a kind of effect to the sound that is percieved as pleasant.

To understand even harmonics, it's kinda like playing a chord on the piano. Instead of just pressing the middle C, you also lightly tap every other C up the scale to add to the sound.
Eikka
not rated yet Apr 06, 2017
Example of soft and hard clipping effect on sound

https://www.youtu...XnW2Z7UI
gkam
1 / 5 (4) Apr 06, 2017
Clipping the tops creates odd harmonics.

The extreme example is the square wave, which is an infinite number of odd harmonics.
gkam
1 / 5 (4) Apr 06, 2017
Eikka, if you have a harmonics generator (mine only works on XP), youcan start adding harmonics and see. Starting with the thirds, see the peak which causes all the problem with Neutrals melting. Add fifths, and see the shoulders come up. As you add the seventh ninth and eleventh, you will see the shape.
Eikka
5 / 5 (1) Apr 06, 2017
Eikka, if you have a harmonics generator (mine only works on XP), youcan start adding harmonics and see.


Yes, it's all in Fourier series. I have a harmonics generator on my table built out of op-amps on a breadboard right now, and I can observe the waveforms on my oscilloscope.

Clipping the tops creates odd harmonics.


Again, depends on how you do it. See this circuit:

http://www.aaronc...fuzz.gif

What happens when you input a triangle wave of sufficient amplitude that the circuit starts to clip? It turns into a close approximation of a sine wave because the triangle wave consists of all the even harmonics and the logarithmic negative response of the feedback diodes add opposing even harmonics until only the fundamental frequency remains.

That is roughly how a vacuum tube behaves at the clipping point. It does not square the peaks of the wave, making odd harmonics, but rounds it off, creating even harmonics.
OhmMcG
not rated yet Apr 08, 2017
Very interesting. Would love to see it evolve into a usable practical technology, looks very promising.
OhmMcG
not rated yet Apr 08, 2017
I would have liked to comment on some of the comments above but unfortunately I seemed to be unable to write short comments, so the system does not allow me to post and to say it all properly squires more than 1000 characters. Bit disappointing, considering this is a science based page.
Da Schneib
not rated yet Apr 08, 2017
One thing this won't do is replace vacuum tubes in guitar amplifiers.

It might, however, replace klystrons in radio and television broadcast equipment, and in microwave/GHz applications, and if so it will have higher efficiency.
OhmMcG
not rated yet Apr 09, 2017
We will have to wait until the technology matures before we know that for sure. Just because it is capable of working at some lightningbspeed is no guarantee that it can't be used at low frequencies of audio.
IronhorseA
not rated yet Apr 18, 2017
The one thing I wonder is if there is there an equivalent of n-type and p-type parts so that you can create the equivalent of cmos logic gates for use in computers?

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