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I read a question about cooling a Venus lander, and got to thinking that the problem was in building a probe that has no problem with Earthlike temperatures and pressures, then trying to insulate it so that it could last long enough to send back useful information before dying from the heat.

So why not build probes that aren't affected by the prevailing conditions? I recently read this article about microelectronics using vacuum tube technology.

There are other potential uses for probes that operate at high temperatures and pressures: Sun divers and probes into the gas giants.

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  • $\begingroup$ What would be the up side of doing something like that? We're good at making refrigerators, air conditioners as well as and all kinds of electronics and sensors that work at lower temperature. What would be a possible benefit of starting over? Can it ever off the cost of doing everything from scratch? $\endgroup$ – uhoh Jul 25 '16 at 23:48
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    $\begingroup$ @uhoh You remove a point of failure, and possibly enhance the mission duration if the insulation cannot be sustained. Possibly reduce the weight too. $\endgroup$ – Antzi Jul 26 '16 at 9:40
  • $\begingroup$ @Antzi great! You know of a vacuum tube computer that is lighter than one made from silicon? I'm half-joking, there has been work in small scale vacuum electronics, but for logic circuits, maybe a GaN related or some very high band-gap semiconductor is the better option. I up-voted the question as soon as I saw it - my comments were meant to suggest ways to improve the question - should have been labeled as such. Some questions of the form "Why don't they just use X?" where X does not exist have a hard time. $\endgroup$ – uhoh Jul 26 '16 at 9:59
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    $\begingroup$ @Antzi I haven't read the details yet (next trip to the library) but it sounds like they are using tunneling in a vacuum but without a heated emitter (no filament, cold cathode). I'm pretty sure what is described there could be adjusted (different cathode, larger tunnel gap) to work at higher temperature, and of course that means it would probably only work at higher temperature. A semiconductor solution would be the most straightforward. Searched "high temperature electronics" found this for example. $\endgroup$ – uhoh Jul 26 '16 at 10:32
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    $\begingroup$ @uhoh Thank you for the link to the high temperature electronics page. I can see that more development is going on than I realized. Also, isn't it interesting that the Solar Probe Plus looks like a coffee maker? $\endgroup$ – Howard Miller Jul 26 '16 at 12:55
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@DavidVomLehn is right. I've just read news today of a the recent paper Prolonged silicon carbide integrated circuit operation in Venus surface atmospheric conditions. See also Ars Technica's We finally have a computer that can survive the surface of Venus. Small circuits were fabricated on semiconductor wafers made from silicon carbide rather than silicon, and there were tested for extended periods of time while they were subjected to a Venus-like temperature of 460C and Venus-like pressure of 9.4 Mbar (about 94 standard atmospheres).

The circuits were ring oscillators, a standard test structure used in IC testing. SiC JFETs were used to build rings of NOT gates (inverters) containing an odd number of gates; read more here. This configuration doesn't have naturally stable state, so it naturally oscillates. Changes in the oscillator's waveform shape, amplitude and frequency provide information on changes in the circuit, which caused by several different effects involving both the silicon carbide crystal itself, and the implanted/diffused dopants, metallizations and insulators.

SiC can crystallize into several different structures, and 4H-SiC was probably chosen for it's very high band gap. For comparison, silicon is only 1.1 eV. When we say "room temperature is 1/40th of an eV (or 0.025 eV) that's the product of the Boltzmann constant $k_B$ (about 8.62E-05 eV/K) and the temperature (20C or 293K). While the high temperature and pressure and naughty chemistry presents several mechanical and chemical insults on the chip, the temperature has a profound effect on the electronic properties of the semiconductor.

While the math for doped semiconductor devices is more complex, the simple exponential term from the carrier concentration of an intrinsic semiconductor is enough to show the importance of high band gap;

$$n_c \approx e^{-\frac{E_{bg}}{k_B T}}$$

For silicon with a band gap of about 1.1 eV, that value starts at about $10^{-19}$ at 293K but rises to the order of $10^{-8}$ at 733K! It would be so highly ionized (so many of the silicon atoms would have produced carrier electrons) that it would be far too conductive to function as a semiconductor or near-insulator when needed.

On the other hand, in the case of 4H-SiC with a bandgap of 3.2 eV, those numbers go from an amazingly low $10^{-55}$ at 293K (you'd think of it as a ceramic or crystaline insulator, not a semiconductor) and rises only to the order of $10^{-22}$ at 733K! SiC at Venus temperatures would be a suitable starting semiconductor material in the same way that silicon would be at room temperature. Of course the carrier concentration of intrinsic SiC could be raised by doping, but there's no way to recover from a too-high value for silicon on Venus.

enter image description here

above: Figure 1 from the paper. Before and after being tested under Venusian atmosphere parameters.

Silicon Carbide Ring Oscillators

above: Figure 2 from the paper. The drop in ring oscillator frequency shows the JFETs switch faster at elevated temperature, which is an effect that can be tolerated with proper circuit design. The loss of signal from the 11 stage oscillator was found to be a connection problem - the device continued to work once better connections could be made after the conclusion of testing.

Band Gaps various forms of Silicon Carbide

above: Band gap energy of various crystal forms of silicon carbide, vs temperature. From here.

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In addition to vacuum tube technology, NASA is looking into high temperature semiconductors for use with Venus probes.

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  • $\begingroup$ I read the article. I guess it's not surprising to find myself a few beats behind the people actually working on stuff like this. Thank you for the information. $\endgroup$ – Howard Miller Jul 29 '16 at 3:28

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