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> Air presumably doesn't mean atmospheric air here, otherwise you're getting a random mix of gases and water vapour inside your nanoscale wizardry

Doesn't matter, as explained in the second paragraph of the article: the air gap (35 nm) is shorter than the mean free path [1] in air, so the electrons are effectively moving in a vacuum.

> Is this basically the return of the thermionic valve after fifty years in the wilderness?

Yes. The paper's abstract starts out saying just that: "Scattering-free transport in vacuum tubes has always been superior to solid-state transistors. It is the advanced fabrication with mass production capability at low cost which drove solid-state nanoelectronics. Here, we combine the best of vacuum tubes with advanced nanofabrication technology." [2]

[1] https://en.wikipedia.org/wiki/Mean_free_path

[2] https://pubs.acs.org/doi/10.1021/acs.nanolett.8b02849

Nanoscale vacuum tubes sounds like something out of steampunk/dieselpunk. I hope it happens for aesthetic considerations alone!

Ah yes.. the sweet sweet distortions of valve on my mp3 player!

Everything old is new again?

Took a bit longer than usual, or maybe I'm not old enough to notice cycles longer than 30 years ...

> so the electrons are effectively moving in a vacuum

That’s true enough for a single cycle of a single transistor, not so much a billion of them doing a billion operations per second. Which is why they are calling this “vacuum-like.”

Thank you for your excellent comments. I never understood what made vacuum tubes good transistors. How does the gate voltage control the commercial loan conductivity of the vacuum Gap?

> I never understood what made vacuum tubes good transistors.

They generally aren't. Vacuum as-in the medium may be technically superior, but vacuum tubes as practically implemented weren't just displaced because solid state was easier to manufacture, but also because solid state quickly outperformed valves in many areas. (There are areas where valves held out for much longer and are still used today in some cases, e.g. transmitter output stages).

That being said a great thing about valves is that they're very hard to kill. They can take huge momentary overloads (a bit like magnetics, but with much shorter time constants), which would literally detonate similarly specced solid state output stages. And because they can run much hotter than any known solid state tech they can have huge energy densities and can be cooled very efficiently (boiling water cooling): https://upload.wikimedia.org/wikipedia/commons/8/80/Rs2041_s... (Siemens RS2041V, 600 kW output tube, diameter of the copper anode dissipating about 240 kW is 239 mm, was still made in 1999)

Would it still be vulnerable to helium, like MEMS devices are?

> What does this mean for cooling, especially in the high-density 3D architectures mentioned?

Well, it's not even clear why they think it helps a 3D architecture. AFAIK, there is no mature layer deposition technique that allows gaps for the vacuum those transistors require, so I'd imagine it makes 3D harder.

For cooling, keep in mind that at those scales air cooling is much less efficient than conductive cooling.

> Does this mean that chips would have less resistive heating?

Complementary logic (the "C" in C-MOS) sets the heat dissipation into a fixed amount per clock cycle, whatever resistance you have. When you hear about "lower resistance transistors", you should hear "higher frequency transistors".

> is there is also a lattice-spacing constraint

There is also a lattice-spacing constraint. Manufacture constraints don't change (they used a well known, very expensive, and hard to scale technique for their proof of concept), insulation constraints don't go away, and they replaced the charge carrier orbital constraints with a constraint on the length of the electrode separation.

The air doesn't matter. The article says that the 35nm gap between electrodes is less than the mean-free path of electrons in air. In other words, from the electron's perspective, it's the same as a vacuum since it doesn't (usually) encounter any air molecules when passing between the electrodes.

The device being described amounts to N type device as electrons are the majority carriers, and that's the problem.

You can make electrons jump from emitter to collector over nanoscopic distances in "air," but how to you make a hole to do the same? With my level of understanding of physics, that's impossible.

And without a P device, you can't make a CMOS pair. Thus, any logic family using it will eat tons of current.

You can't use it for logic in a real world application, but if they can solve the endurance issue, that will be an enviable device for use in power gating. Active logic may also be of interest (it is still being used for ultra high frequency DACs in military radar tech)

Holes are virtual particles. They are spots with a missing electron. A hole "moves" when an electron moves in an opposite direction.

True, but not really an answer to the question.

Unlike semiconductor devices, there is only one "kind" of vacuum tube. A push-pull amplifier with vacuum tubes requires an anti phase generator before the high and low side drivers, while with both n- and p- type available you sorta get this for free.

> Does this mean that chips would have less resistive heating?

No. The parts of the chip that conduct electricity to the active areas in current chips are already made from metal. Basic chips are made from layers of metal / oxide / semiconductor (MOS).

Current flows through silicon only in small areas, and the way chips work is that you can "turn off" the high resistivity in those areas and make them conducting (by doping the material + applying electric currents & fields)

In the presented work, an air gap is used for these "active areas" instead of a semiconductor. An air gap does have resistance, so there will be resistive heating just like in normal chips.

> An air gap does have resistance, so there will be resistive heating just like in normal chips.

They also mentioned putting these in a vacuum. Presumably that won't have as much resistance as an air gap.

A perfect vacuum is a perfect insulator. There are capacitors that use vacuum.

Electrons can still pass through perfect vacuums.

I think that’s implied by his pointing out that capacitors use vacuum. A cap that doesn’t allow electron flow wouldn’t be a cap.

A capacitor shouldn't allow electron flow between its (dielectric separated) plates. The charges should build up on the plates, then exit through the terminals when discharging. If charges cross the dielectric, then you have leakage.

Hm. Apparently my Phys101-level understanding of electronics has caught up to me.

Caps don’t allow electrons through, do they? They resist DC, they only conduct AC?

That is correct. The dielectric ‘conducts’ an electric field, which attracts and repels electrons on the opposing plates.

Vacuum is a special case of an insulator. Others will conduct beyond some threshold voltage; vacuum will not. On the other hand, vacuum will conduct at any voltage as soon as free charges are injected into it.

But electrons can still pass through perfect vacuums. Whether or not a vacuum conducts (obviously it doesn’t) is irrelevant.

> A perfect vacuum is a perfect insulator.

If that were true, vacuum tubes (used for radio and early computers) wouldn't work.

It is better to call vacuum "the perfect semiconductor," then - it only conducts when free charges are injected into it.

An insulator by definition is matter that doesn't have freely flowing electrons so vacuums aren't an insulator. However, all insulators have a "breakdown voltage" at which point the electrons start "ripping" through the material so a vacuum has all of the properties of an insulator except the mass: the breakdown voltage is the the amount of energy required to eject electrons from the materials surrounding the vacuum and until you reach that point, no current will flow through the vacuum.

Vacuum tubes (thermionic valves) work by heating a tungsten filament-- the cathode-- until the filament is hot enough to emit electrons, which are then attracted to the positively-charged plate (anode), traveling through the vacuum (and thus flowing current). Current only flows in one direction in this setup, which has two electrodes, leading to the moniker "diode." Adding a third electrode in between the cathode and anode-- called the "grid" based on its physical shape-- allowed a vacuum tube (now a 'triode') to use a small signal to control a large one. An amplifier!

The key to the function is the heating of the cathode filament. Vacuum tube designs that use a separate heating current (which is, today, most of them) do not conduct if the heating current is not applied. "Cold cathode" type tubes are not vacuum tubes-- they are typically filled with a low-pressure gas.

a vacum diode have characteristics more similar to a ideal diode that a semiconductor diode.

Valves work because the heating filament fills the vacuum with free electrons.

Vacuum: isolator.

Vacuum filled with electrons: not so good isolator.

Vacuum dielectric strength is 20 megavolts per metre.

Diamond is almost perfect though, at 20 gigavolts per metre, and that only given if the diamond has at least some microscopic imperfections.

I think those might be reversed...

IIRC diamonds are good isolators because their crystal structure is both very close to perfect (few defects) and because their crystal lattice has all electrons tied up.

You are correct. Except, this quality of the vacuum is very fragile. "What's perfect is imperfect, it is the imperfect that is perfect." (Example: a perfectly rigid system will collapse under stress while an imperfect one might withstand it.)

The vacuum has a finite impedance of ~375ohms so it is not a perfect insulator. This comes from the fundamental magnetic permeability constant mu naught divided by the speed of light. Also interesting is the fact that the vacuum impedance is an exact number because it’s solely based on fundamental constants.

Given a small enough distance, arcing is possible between 2 surfaces in vacuum as charges are ejected from the surface (that’s how vacuum tubes and these transistors work).

It is essential to distinguish between characteristic impedances (of free space or other) and resistances. Although both are real values measured in ohms, characteristic impedances are non-dissipative.

Just because EM waves can propagate in vacuum does not mean you can induce a current density.

First part is correct. Second part: ...that's the basis of all RF systems, and exactly how antennas work, isn't it?

Obviously you can't induce a current in vacuum, but you can definitely convert EM waves to current flux in a receiving structure.

Right. I see now that I was unclear in that second part - I should have written something like "just because EM waves can propagate in vacuum does not mean you can induce a current density in vacuum." Of course EM waves can induce current density in antennas and other structures.

> Obviously you can't induce a current in vacuum ...

Yes. I wrote that second part because 'mmmBacon seemed to be arguing that one could induce a current in vacuum; that because vacuum has a characteristic impedance, it isn't an insulator (i.e. can support current because it has characteristic impedance).

Ah, gotcha, appreciate the clarification.

@twtw was not arguing that one can induce a current in the vacuum. However, I did say that a surface can arc in vacuum. This is because a surface can emit charges either thermally or in presence of a large electric field. This is correct and didstinct from current induction in media. Please note this phenomenon should not be confused with arc discharge in gas.

See: https://en.m.wikipedia.org/wiki/Vacuum_arc

Right, but that effect is pretty much unrelated to the characteristic impedance of free space and doesn't mean that vacuum is not an insulator.

I’m sorry but I fundamentally disagree. GP said vacuum is a perfect insulator. Comment was in response to that demonstrating that it is not.

The current does not flow through a CMOS device in a conventional sense. Single nanoamperes, at most, but when you have billions of transistors that stacks up quickly.

In CMOS, only one device of a PMOS/NMOS pair is open at the time. In simplest terms: electric field gets through a CMOS device, but not so much of current.

And what happens during the switching transition then? Answer: both conduct for a very short period, hence the dependency of power consumption on frequency.

Yep, just enough to flip the device state.

There are two factors to that. Cross conduction is one, the other is the current pulse needed to charge the capacitance of the switched signal (the trace capacitance but also the gate capacitance of the next stage(s); of course this simple model is only an idealized concept with current process nodes).

> Is this basically the return of the thermionic valve after fifty years in the wilderness?

Yes, but with a twist of quantum.

> The experimental data shows that influential operation mechanism is Fowler–Nordheim tunnelling in tungsten and gold devices, while Schottky emission in platinum device.

The bad grammar makes this hard to understand; is FN-tunneling the influential mechanism or an influential mechanism? With the help of Wikipedia I can parse it as an explanation of how the electrons get out of the metal.

When you apply an electric field, there can be a potential barrier: the electron must "climb" up out of the metal but can then "fall" down the externally applied field [1]. With Schottky emission, electrons jump the barrier using their thermal energy; increasing the field lowers the barrier, enhancing the emission. All this comes from the days of thermionic diodes.

But as the field increases, the barrier also gets narrower. And when it is narrow enough, even low-energy electrons can get out via quantum tunnelling[2]. Apparently, this is called Fowler–Nordheim tunneling.

Now these named effects both have equations attached to them, which predict the amount of electrons emitted under different circumstances. But real devices are complicated, and involve both of the above physical effects, plus others. So what researches have to deal with are measured curves that kind-a-sort-a fit the equations if you squint.

The mangled sentence in the abstract makes me think that they have measured something (perhaps current vs.temperature curves) look a bit like the Schottky prediction in platinum, and a bit like the Fowler-Nordheim prediction in gold and tungsten.

[1]: https://en.wikipedia.org/wiki/Schottky_effect [2]: https://en.wikipedia.org/wiki/Quantum_tunnelling

The link in a sibling comment https://spectrum.ieee.org/semiconductors/devices/introducing... suggests that helium could be used, and that things could be designed such that "the electrons will never acquire enough energy to ionize helium."

Do you know why it makes sense to construct these devices on dielectric surfaces as opposed to other non-conductive surfaces?

All non-conductive materials are dielectric to some degree. Strictly speaking, so are conductive materials.

Do they use the term dielectric to mean non-conductive — I’m aware that they’re not equivalent — or does the dielectric nature of the material have any relevance to the device? Maybe it’s relevant to the working of the gate?

It's possible that the dielectric is indeed necessary to isolate the gate from the source/drain, but I can't say for sure.