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Vacuum tubes were a mainstay of digital computer technology until around the 1960s, where transistors (and later integrated circuits) replaced them for most applications. In real life history, one of the last commercial tubes to be developed and widely manufactured was the nuvistor, a small, relatively reliable tube used in radio equipment and televisions. The linked Wikipedia article has photographs of the tube, which has a metal casing and is about 11 mm in diameter.

In a hypothetical setting where the invention of transistors and integrated circuits were delayed indefinitely, how much smaller and more reliable could 'traditional' vacuum tubes operating on the principles of thermionic emission been made, given sufficient commercial demand for further innovation? What limitations (heat dissipation, fundamental physical effects) might come into play that would prevent tubes from shrinking beyond, say, the size of a grain of rice, or a grain of sand?

(I'm aware of the existence of nanoscale vacuum tubes, but these devices are fabricated using the same photolithography technology invented for manufacturing integrated circuits, and for the sake of this question I'd like to exclude them, or anything else that relies on semiconductor technology or technology developed as a direct result of semiconductors.)

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    $\begingroup$ Tiny little subminiature valves were a thing. The major problem with thermionic tubes is that they work only at relatively low frequencies, because the electrons have to travel along a large distance, about 2 millimeters in the smallest tube I have seen. In digital applications, they have a second major disadvantage in that they use insane amounts of power: tubes consume power all the time whereas transistors use power only when switching state. (And please, while all the world has seen American notes, American coins are pretty much unknown outside the U.S.A.) $\endgroup$ – AlexP Mar 25 '18 at 5:17
  • $\begingroup$ Thank you, that's good to know! (Upon a second glance the linked article has actual measurements, so I will edit my question to reference those instead.) $\endgroup$ – AustinZ Mar 25 '18 at 5:22
  • $\begingroup$ And I believe you have no idea how small transistors are. Consider a primitive microprocessor such as the Intel 8080, introduced in 1974. A grain of sand is, let's say, 0.2 mm across. On the 8080, a transistor was 0.006 mm across, about 1000 times smaller. $\endgroup$ – AlexP Mar 25 '18 at 5:22
  • $\begingroup$ I'm aware even early transistors were very small and the vast majority of their visible size was the packaging and leads. My question isn't concerned with hypothetical tubes in comparison to transistors, but simply tubes on their own merits; if I worded it unclearly I would like to improve it. $\endgroup$ – AustinZ Mar 25 '18 at 5:25
  • $\begingroup$ @AlexP I was mighty puzzled at your comment about coins until I looked at the original question... :) $\endgroup$ – RonJohn Mar 25 '18 at 6:01
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The only similar technology which has survived until today is incandescent light bulbs.

I say they are similar in the sense that they require to put some electrodes in a vacuum and have current flowing between them.

A google search shows this article, allegedly reporting the smallest light bulb in the world.

smallest light bulb in the world

Though, as the article says

The lamp's filament is just 100 atoms wide. It is made from a single carbon nanotube.

it is pretty evident from the photo that the bulb in its whole is not that small.

Let's scale it down by a factor 2 to compensate for the different usage, we still are in the mm length realm. And I don't think you can go much lower, as the real issue is handling a small bubble of molten glass and let it stay a bubble while it becomes solid.

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