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Vacuum Nanoelectronics: Interview with M. Meyyappan and Jin Woo Han
In the past, vacuum tubes were used for amplifying electrical signals, but have long been replaced by other devices such as MOSFETs, which require lower power and less cost to fabricate. Researchers Jin-Woo Han, M. Meyyappan, and Jae Sub Oh however, last year completed research in which they were able to use silicon technology to create a gate insulated vacuum channel transistor, which offers higher frequency and power than other commonly used devices. You can read about their research in vacuum nanoelectronics in their article, “Vacuum Nanoelectronics: Back to the Future?” by clicking the link below:
I interviewed M. Meyyappan and Jin-Woo Han from NASA’s Ames Research Center about their research, and we discussed the superiority of vacuum tubes, possible applications of their nanoscale vacuum transistor, and the challenges that stand between the technology and widespread market availability.
Superior Technology: Higher Speeds, Power, and Frequency
There are several ways in which vacuum tubes could be used to create superior devices. Jin Woo Han explained that in widely-used silicon technology, electrons have to travel through a silicon lattice, which puts a limitation on how fast electrons can travel, as there are obstacles in the path of the electrons that cause them to bounce back and forth. In a vacuum however, such obstacles do not exist, and higher speeds, power, and frequencies can be reached. By making everything out of silicon except for the silicon-conducting channel, which would be left as a vacuum, the researchers were able to have the best of both worlds — they eliminated the limitation the speed of electrons, but kept costs down with silicon technology.
Another way in which vacuum tubes are superior to other technologies, M. Meyyappan explained, is that they are unaffected by radiation. This means that vacuum tube transistors could be very beneficial to both space and military industries. “It’s no secret,” he said, “that anything that is used in space is a few generations — two, three, or even four generations — behind the electronics that you and I buy. That is because it takes many years for the electronics to be space-qualified; they have to be packaged in such a way that they won’t get slaughtered by radiation… and the same thing goes for the military. The distance the military goes is smaller, but they also have different types of radiation, some in common with NASA, to worry about.”
“Spending a lot of time packaging and preparing the device that you and I have access to already and putting it in space is not only time-consuming,” he went on, “it is also an expensive proposition. That’s why military electronics and space electronics are far more expensive than the electronics that everybody else buys.”
However, he continued, because there is no semiconductor in a vacuum transistor, and no charge transport (which would be detrimental to device operation under conditions in space) the vacuum transistor is “inherently immune to space radiation.” Essentially that means that, with a vacuum transistor, NASA and the military would be able to use chips and new technology when everyone else does, rather than having to “wait 5 more years to package it, and then send it up.”
The vacuum transistor would also be immune to high temperatures, and could be put close to things like a jet engine, which is very hard to do with current technology. Because of the higher speeds and frequencies that can be achieved, the vacuum transistor could also have telecommunications applications, Jin Woo Han explained.
Applications in Telecommunications
“Electrons can go at pretty much an unlimited speed in a vacuum compared to silicon,” M. Meyyappan continued. “In silicon there is a lattice like a matrix, and so it’s like watching a pinball machine. The electron gets bounced around because it hits a few obstacles — like in a pinball machine. For that reason it’s slower than a vacuum, which is not like a pinball machine. [In a vacuum] an electron shoots from one place to another, and the electrons travel at a much, much higher speed. What that means for telecommunications is higher frequency and higher bandwidth.”
In a silicon-conducting channel, electrons get bounced around, much like they would in a pinball machine. Photo Courtesy of Wikipedia user The Consumerist
I asked what challenges stand in the way of market-scale production of nanoscale vacuum tubes, and M. Meyyappan explained that lifetime testing of the devices still is yet to be done, and also a 95% production yield must be met (which means only 5% of wafers would be discarded) before anyone would consider large-scale production. “It’s not like we have demonstrated industrial scale,” he said, “We have fabricated on a wafer… but the expectation these days is a 95% yield, [but ] nobody has even tried on that scale.”
However, he pointed out, because the technology is silicon-based, industrial scale manufacturing is very doable. “There is not a whole lot to invent,” he said. “We are not talking about fancy new materials like carbon nanotubes, where you have to learn something — everything is still made in a silicon fab, using a silicon-fab line, and using silicon processing. In that sense it is not anything very hard.”
Unconventional Uses & the Forbidden Gap
Not only could vacuum tubes revolutionize military and space electronics, but there could be other, less conventional, uses, Jin Woo Han pointed out. If you could utilize a vacuum nanoelectronic array, for instance, you could analyze the performance of electrons and “utilize [the device] as a gas sensor or a gas detector.”
What that means, is the possibility of completely unconventional applications. Because the device operates at a terahertz frequency, between 1 terahertz and 30 terahertz, at what is called the forbidden frequency, and because there are no commercial devices that operate at that frequency, M Meyyappan explained, the device could have many unconventional applications such as detecting gas, detecting drugs, and other applications. “Since the vacuum nanoelectronics and vacuum nanotransistors can perform at a terahertz frequency,” he said, “[they]could be used for things like spectroscopy and imaging, and could be a replacement for the kind of x-ray-based scanning (such as that at airports ) that has a negative connotations in terms of health and so forth. You wouldn’t have to worry about that with the terahertz frequency, and it could also be used for identifying drugs, contrabands, and those kinds of things. There are lots of unconventional applications for this type of device.”