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Would it be possible to make a solid state vacuum tube?

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T3sl4co1l:
They're the same thing, on a low enough level.

In semiconductors, electrons are promoted from a bound (valence) state, to a conduction state.  The energy change is the band gap, a few eV.

Unique side effect: the vacancies (holes) also move, with somewhat poorer mobility (in silicon, about 2.5x slower -- which is why PMOS have to be bigger than NMOS to make a complementary (CMOS) circuit; most semiconductors have a much greater ratio, making CMOS impractical in anything else).

Semiconductors have a bulk charge balance, so the charge carrier density can be extremely high.  Downside, the bulk also obstructs flow, causing thermalized diffusion motion.  Since the junctions can be made very small (indeed, must be), this relatively low velocity only has a limiting effect at high frequencies.

Also, the energy distribution of those electrons, given by Fermi-Dirac statistics, has an exponential cutoff.  Which is essentially why semiconductors have an exponential response.  (Diodes and BJTs, and MOSFETs in the subthreshold region; FETs above subthreshold are another matter, which I forget why at the moment.)


In vacuum tubes, electrons are promoted from a bound (valence) state, to a free (vacuum) state.  The energy change is the work function of the cathode, a few to tens of eV.

Electron emission can be stimulated by simply heating the cathode, or by irradiating it with light of adequate energy (photoemission) or other particles (secondary emission).

Only electrons can be emitted in this way.  Probably, with some work, a "P type" tube could be made, using a hydrogen fill gas and a hollow cathode to emit ions (protons).  Unfortunately it would have utterly terrible performance (proton mass ~2000 times the electron!).

Vacuum tubes do not have a bulk charge balance, so the electron density is low, and this is forced by the space charge of those electrons in the beam.  (That is, electrons repel, so a cloud or beam of them tends to push back on itself.)  With no obstructions, the beam can reach high velocity -- ballistic transport.  Unfortunately, though high voltages can be used, practical reasons limit how closely electrodes can be spaced, so the effective transit time is worse than most semiconductors.

Electron energies are given by the Maxwell-Boltzmann distribution, which has an exponential tail.  Vacuum tubes are also exponential in the cutoff region; this is a relatively small part of their operating range however, and dynamic range isn't that great to begin with (due to the high temperatures, and construction and some chemistry details, leakage currents are relatively high).  In the normal operating range, a 3/2 power law (give or take exact electrode geometry) occurs.  (While this is more linear than an exponential transfer function, it's also much lower gain -- on top of the already low gain due to the low charge density in the vacuum.)

The peculiar characteristic of the triode (and to a lesser extent, multigrid tubes as well) is a low plate resistance; this is not fundamental to the mechanics, so much as a consequence of the electrode arrangement.  Some plate voltage leaks through the grid.  The grid acts as an electrostatic shield, but a poor one, so there is a ratio of influence between grid and plate -- the amplification factor, µ.  This applies for all electrodes, so that in a pentode, the suppressor grid partially shields the plate, then the screen and grid in turn as well.  So the µ from plate to grid can be quite high, but it's just the product of each grid's µ to the next.  (This is different from FETs, which have a mechanism that effectively shields the gate and drain.  A different, much weaker effect dominates: channel length modulation.)


So to a certain extent, vacuum tubes and semiconductors already are the same things. :)

To a more meaningful extent -- if you want a semiconductor that acts like a triode specifically, there's this:
https://en.wikipedia.org/wiki/Static_induction_transistor
Like the triode, the electric field inside the junction happens to influence current flow, thus giving a low output impedance.  (They aren't very available, if they're in production at all?)

Others have covered the more traditional interpretation (substituting semiconductors into a vacuum tube circuit) so I thought I would talk about something more fundamental.

Tim

KE5FX:

--- Quote from: T3sl4co1l on August 14, 2020, 02:38:27 pm --- Probably, with some work, a "P type" tube could be made, using a hydrogen fill gas and a hollow cathode to emit ions (protons).  Unfortunately it would have utterly terrible performance (proton mass ~2000 times the electron!).

--- End quote ---

But it impresses visitors, which is what really counts.

TimFox:
Note that most gas-filled tubes (except for VR tubes) are not gas-discharge devices.  In a Hg-vapor rectifier, for example, the very heavy Hg+ ions move very slowly, but neutralize the space charge from the faster electrons.  This allows high electron current at very low plate-cathode voltages, compared with a vacuum rectifier.  (If the peak current is too high, ion bombardment can damage the hot cathode.  See data sheet.)

T3sl4co1l:
Yup, gas serves the purpose of neutralizing charge, just as the semiconductor bulk does; the difference is that the gas atoms have much lower ionization energy, which is easily met with applied voltages.  (Whereas in semiconductors, quite a lot of current can flow before ballistic transport is encountered*, and impact ionization -- avalanche breakdown -- usually occurs in the same region.)  This means the operating range where controllable, neutralized current flow can occur, is rather narrow, and tricky to use.  (Deforest's original tubes were supposed to operate this way; turns out hard vacuum simply works better for amplification.  He wasn't actually all that clever when it came to science.)  This is fine for uncontrolled devices (rectifiers) and latching devices (thyratrons, etc.), where the increasing ionization simply makes the device more efficient.

*This is actually possible in some materials, GaAs for one.  A Gunn "diode" is actually a monode, a single lightly doped hunk of semiconductor -- no PN junction.  It's able to oscillate at such high frequencies (10s GHz) because the transition from drift to ballistic transport manifests as a negative resistance V(I) characteristic, and has almost no time constant -- it's an intrinsic property of the semiconductor, no recombination or anything involved.  The applied voltage must be limited (including reflected power, say if the coupling network is tuned badly), else avalanche breakdown soon ensues and destruction may occur!

Tim

David Hess:

--- Quote from: TimFox on August 14, 2020, 01:23:41 pm ---It is somewhat similar to a cascode series connection of two triodes, where the plate voltage on the lower triode (emulating a screen grid in the tetrode) is constant and its current flows through the upper triode (without much effect of the upper triode's voltage).
--- End quote ---

FETRONS almost always used two JFETs in the cascode configuration to replace triodes, tetrodes, and pentodes but they were application specific.  So a different FETRON was used to replace a pentode amplifier and a pentode oscillator because the operating conditions were different.

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