* Air presumably doesn't mean atmospheric air here, otherwise you're getting a random mix of gases and water vapour inside your nanoscale wizardry - so will these be nitrogen-filled, like binoculars etc?
* What does this mean for cooling, especially in the high-density 3D architectures mentioned? Can you use air flow through the chip to cool it?
* "tungsten, gold, and platinum were evaluated as metals of choice" - compare their resistivities of 5.60e-8, 2.44e-8, and 1.06e-7 ohm-metres respectively to silicon's of 6.4e+2 [1] (i thought tungsten was much higher than that, TIL). Does this mean that chips would have less resistive heating? Is resistive heating a significant part of the TDP of a modern chip?
* "replacing silicon with metal means these ACT devices can be fabricated on any dielectric surface, provided the underlying substrate allows effective modulation of emission current from source to drain with a bottom-gate field" - is there is also a lattice-spacing constraint, where the substrate's spacing has to work with that of the conductor, as i believe there is with silicon chips?
* Is this basically the return of the thermionic valve after fifty years in the wilderness?
> 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]
> 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)
> 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)
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.
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.
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).
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.
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).
@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.
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.
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.
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.
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.)
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.
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.
> 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.
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?
As you might expect, this is more of an engineering press release and not a balanced technical discussion. Two things jump out at me:
> The nanoscale air gap is less than the mean-free path of electrons in air, hence electrons can travel through air under room temperature without scattering.
Electron transport without scattering is usually termed "ballistic transport", and as gate sizes shrink in traditional MOS devices, a larger fraction of channel electrons make it across the channel before scattering. So maybe air gaps might speed up the arrival of mass-production ballistic transistors.
However, there is a new problem: To get an electron out of the metal, you need to kick it up to the vacuum level (the electron work function of the material). Giving energy to the electron to get it out of the metal was the reason that tubes had heating elements back in the day. In more modern research, it's the reason why people have been exploring diamond thin-film emitters: It turns out that diamond has the interesting property that the conduction band is above the vacuum level (!), which means that a conduction-band electron will fall out of the material given the chance. (Surface physics gets in the way, which is why diamond engagement rings aren't positively charged rocks, but very cool nevertheless.)
Alternatively, you need to apply an electric field to have electrons tunnel out, but usually that corresponds to a very large field where other bad things start to happen (breakdown, electromigration, etc)
> Because the electrons flow between the electrodes just as well in a vacuum (think vacuum tube) as in air, radiation will not modulate channel properties, making ACT devices suitable for use in extreme radiation environments and space.
When radiation from space goes through an MOS-type device, there are two big problems:
1. Ionizing radiation in the channel creates electron-hole pairs, which creates excess current between source and drain temporarily,
2. Ionizing radiation in the dielectric creates electron-hole pairs in the gate, and often the hole gets trapped in the gate leading to a permanent threshold shift. This eventually causes failure, and is usually the bigger problem when designing a space-qualified process.
So while "radiation will not modulate channel properties" is maybe technically right, the press release is ignoring that the radiation will still modulate gate properties, which will in turn modulate channel properties.
This design also struck me as a non-tunneling version of a single electron transistor. Stick a 10nm metal island in the middle, make the spacing bigger, and it would look the same. Are the electric fields required really so big?
Can someone explain why the electrodes have to be sharp? Intuitively it feels right, but what is the actual reason?
As far as I can see, the challenge is to get the E-fields (aka voltage gradients) high enough that electrons can easily leave the metal. But for that you need the electrodes close to one another, but they don't have to be sharp.
I suppose if you have too big an area of metal at near-contact, then you will get unwanted capacitance. But is that the only reason?
They do have to be sharp. As an exercise, consider a metallic cone with a charge of 1 and look at the distribution of the charge that minimizes the electrostatic energy. You will find that the charge collects near the tip of the cone. In general, charges will collect where the curvature is high. It's easier to push electrons out of a pointy tip (or inside of a pointy tip) because they are denser there.
Error margins? There's a lot more room for error with a flat surface. I'm not sure if that level of precision is at all important, but it's my best guess.
In general, the dielectric strength of a gas varies with the gap length, so for small gaps it's not strictly correct to say that air has a dielectric strength of 3 kV/mm or any other single value. [0] But at this scale, the gap is so small that the idea of "dielectric strength" isn't really meaningful anyway. There are so few atoms between the electrodes that electrons can flow with negligible resistance at any reasonable field strength.
The first ionization energies of oxygen, nitrogen etc are all > 10 eV [1]. If you accelerate an electron over 35 nm with a 1 V electric field, it picks up 35e-9 eV. So it can't ionize anything, even if it happens to collide with a molecule, which is unlikely [2].
Correcting myself: if you accelerate an electron through a potential difference of 1 V, it picks up 1 eV and nothing else. While the conclusion stands (it can't ionize anything),
I plead temporary insanity for the way I arrived at it.
I think that is the general idea, the field below regulates that arc.
edit - This design is so absurdly straight forward, I am genuinely impressed. The damn thing is even reversible. Is cool as hell that this works. I wonder what the pitfalls are?
edit2 for magnat as it is not letting me reply to the comment below - The way I understand it (given it is described as a FET), those two electrodes sit directly on top of a gate. If you flood the gate with electrons, then the gate's negative electric field will keep the electrons in the electrodes away from the gap, reducing or completely halting the flow of current.
For anyone who doesn't feel like reading the wiki, these things are normally switches you use when all other types of switch would be destroyed by the amount of current you're trying to pass (the pictured device has a mesh cage to catch fragments if it blows up).
If you've ever seen one of those demos where someone 'shrinks' a quarter or crushes cans with magnetic fields, there's a good chance that's what they're using to switch the power on.
Seeing the same concept used in a cpu is bizarre, but really exciting if it works.
It does not seem to use arcs, so it does not use a trigger. It's more suitable to think about this as a triode valve or a FET where the the channel is made of vacuum instead of silicon. (And only comes on N variety, what is not great.)
There is mention of one issue - melting of the tips. They are working on modifying the collector to reduce the stress on the emitter, which suggests that a practical device will not be reversible. (from a sufficiently abstract point of view, transistors appear to be reversible too, but in practice they are not.)
> The nanoscale air gap is less than the mean-free path of electrons in air, hence electrons can travel through air under room temperature without scattering.
The device is so small that air needs to be thought of like ping-pong balls bouncing around inside a lottery machine, not a homogeneous bulk. Does that mean things like arc formation work differently? I honestly don't know!
I get the impression that the gas molecules are not being ionized, the electrons merely pass between them.
Thinking some more about this, does not dielectric breakdown in gases depend on electrons gaining sufficient energy to cause ionization in collisions with the molecules [1]? With the gap well below the mean free path, perhaps they cannot reach that speed at the voltages being used?
I'd also wonder at the thermal properties and with a gap of 35nm, these would make chips large again. However, I suspect they would need to do vertical placement to allow the gap to be large enough without stepping back in silicon real estate. However, the question then becomes more important about the thermal properties of such an approach.
Pretty much exactly that. Only... the environment is hot enough that (a) you don't need a heater, and even more interesting, (b) 1 atmosphere is an effective vacuum for features of that size.
Amusingly, it sounds like these devices will be susceptible to helium poisoning.
This all makes me wanna stop playing with superconductors and start playing with tubes again...
You know this is a pretty new idea when googling "metal-air transistors" links to the posted article and this HN thread, and searching for the same term on YouTube reveals no videos (that I could recognize as being about this topic).
Hey SciShow and/or Computerphile, we need videos to give us the TL;DR on metal-air transistors!
What happened to graphene? A couple of years ago, that's all anyone talked about? I thought graphene was going to get us back on the exponential incline that is moore's law?
I'm guessing the answer is no ( betteridge's law ) and we are going to stay in the multi-core environment for a while. 128, 256, 512, 1024, ... cores. Though I suspect that is going to run into problems very quickly.
Progress is being made. The price of graphene has come down a lot in recent years. There's a new process by which you can coat a cd with graphite oxide and selectively burn paths of graphene with a LightScribe laser. At this point its pretty trivial to create flakes of graphene at home with some basic chemistry equipment. (enough to make your own 1 farad supercapacitor!)
(I took the Stanford nanoscale materials science class this summer, and we all had a good laugh. Graphene is very interesting, but it’s subject to the hype cycle.)
The person who picks a headline like this is generally trying to create the most attention with it without making a false statement.
Instead of "Can New Metal-Air Transistors Replace Semiconductors and Continue Moore's Law?", the headline writer would love to write "New Metal-Air Transistors Replace Semiconductors and Continue Moore's Law". That would be more effective. The fact that the headline writer could not say this tells us something.
The same goes for weasel words like "may".
> I find heuristics like this to be anti-intellectual.
It is absolutely appropriate to detect what might have been said but was not, consider the motivations of the speaker, and infer non-importance appropriately.
I did write the original headline. I did not write the article though.
In reading the article the question arises naturally but I wasn't able to answer it because I am not an expert of the domain. I thought that maybe others could be interested in the question so I posted it in hn. The conversation ended up being knowledgeable and interesting.
And yes, I would love these guys (or at least this solution) to succeed in continuing Moore's Law, why I wouldn't ?
> The fact that the headline writer could not say this tells us something.
Yes, it tells us that this is ongoing research. (And yes, you shouldn't have any high expectation that any specific preliminary research result will happen to change the world - yet, you can be sure that some will.)
Right. For the same reason, submissions on HN or elsewhere with a title that starts with a "Why ..." carry some sort of red flag telling me that I probably don't care to know.
I would say it is probably yes to the first and no to the second with a caveat - just because you can doesn't necessarily mean that you should. You can power your cellphone on an integral kerosene fuel cell but it doesn't really make sense to do so except maybe in very narrow circumstances. Not that you shouldn't but a lot of potential advanced techniques fall in and out of favor like how CMOS went from power efficent electronics to the way as power efficiency dominated over gate efficiency.
I wonder what happened to cold cathode display technology. It’s similar in that electrons are coaxed off a sharp tip. This would have the benefit of a controlling gate.
