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Below on the left side of the illustration there is a point-contact transistor which is the first transistor invented in 1947:

enter image description here

It seems the symbol of BJT actually comes from this point-contact transistor even though they operate very different way.

But what is the purpose of using a "spring" for establishing contacts?

In this source it is written:

The whole triangle was then held over a crystal of germanium on a spring, so that the contacts lightly touched the surface.

Is that the reason for using the spring? To ensure a weak touch to the germanium surface? But if so, why is a weak/lightly touch is needed?

Edit:

Another source from page 201 of the book called "Conquering the Electron":

enter image description here

user16307
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  • looks like a female emitter (lol) , maybe they thought base resistance must be higher for more better current gain and not disturb charge flow between CE – Tony Stewart EE75 Mar 30 '18 at 16:13

3 Answers3

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Here are some experimental results I have obtained with a galena detector.

Preamble

Galena detectors were the predecessor of diodes. It was composed of a galene cristal and a needle of copper in contact with it. So far I know galena detectors always have a compression spring.

Here is such a detector similar to the one I have tested. One can see the spring.

enter image description here

The tests were done with a curve tracer.

Results

To obtain a detector effect, the pressure must not be too much nor too light.

Thanks to the spring the operator is able to master this pressure. For this reason, the spring is clearly essential

The best results (ie non-linearity) are obtained when one first apply a little bit to much pressure then release progressively the pressure until a good non-linear curve appears on the screen of the curve tracer.

I don't have the theory behing this. Maybe the first pressure flattens or cleans the surface (oxyded ?), but I seem to recall that when one apply once again more pressure after having obtained a good result, things are worse.

So the modelisation of this kind of problem seems to be complicated. There are some effects with memory that can improve the results.

andre314
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    For those of us that grew up in the 50s, the "crystal radio" was a "thing" -- just about anyone with a technical bent would build one (often from a kit). The "crystal" took many forms, often (in the "foxhole radio") being the oxidized coating on a razor blade. Pressure for the point contact was typically supplied by a bent piece of modestly stiff wire. The point contact itself was often a piece of carbon-clay pencil "lead". – Hot Licks Mar 30 '18 at 23:57
  • I don’t know if it applies to the (manmade?) crystal in the transistor case, but part of the reason for the adjustable whisker contact on galena diodes was so that you could make contact with just on crystal face. That got the best rectification. – Bob Jacobsen Mar 30 '18 at 23:59
  • [related](https://electronics.stackexchange.com/a/252328/143764) – andre314 Nov 29 '18 at 18:04
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You need enough force to maintain contact, but not so much that you crack the Ge crystal.

If you just directly connected the contact to the structure shown holding the spring, with no flexibility, it would be very easily to apply excess pressure and damage the crystal.

The Photon
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  • Yet Germanium has a shear modulus (GPa) slightly greater or same as Titanium and same Young's modulus for elasticity. – Tony Stewart EE75 Mar 30 '18 at 16:27
  • So being extremely rigid and probably if used as very thin slice makes it too sensitive for applied force? – user16307 Mar 30 '18 at 16:29
  • Makes sense but do we have source for this or it is your own opinion? – user16307 Mar 30 '18 at 16:30
  • @user16307, I've done bare die assemblies, and I know you do not want to use an over-constrained mechanical system to mount them. Even if you use an over-constrained mechanical system to push a heat sink up against a BGA packaged part you can have problems (cracked solder balls, cracked die, ...). – The Photon Mar 30 '18 at 16:34
  • The Shear and Young's modulus come from periodic tables, perhaps ideal purified, but I suspect Bardeen predicted the gap required to create the bandgap potential and excessive force could make the gold sliver contact buckle. – Tony Stewart EE75 Mar 30 '18 at 16:35
  • @ThePhoton I see, I asked just to be sure to select as the answer, in case someone else stumbles upon this one day. Thanks. – user16307 Mar 30 '18 at 16:35
  • Bardeen was the Physicist who actually made it work and knew the quantum mechanics required. Shockley was peeved and blocked his support therafter. – Tony Stewart EE75 Mar 30 '18 at 16:39
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    @TonyStewart.EEsince'75, 1. Silicon has even higher shear modulus and is still easily cracked when in millimeter-thick crystals. 2. Shear modulus doesn't tell you anything about shear strength. – The Photon Mar 30 '18 at 16:39
  • AS I read this crystal was huge and no so thin. – Tony Stewart EE75 Mar 30 '18 at 16:40
  • I attempted to add clarity to your assertion with the photo of the actual demonstration – Tony Stewart EE75 Mar 30 '18 at 16:49
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    @TonyStewart.EEsince'75 That triangle is the plastic(if you mean huge for that), I think he is talking about the germanium at the bottom. – user16307 Mar 30 '18 at 16:51
  • On the other hand there is this photo: https://i.pinimg.com/736x/6b/c3/63/6bc363c780492e4af0eb01d36095425b--inventions-timeline.jpg (with no scaling hard to say the thickness) – user16307 Mar 30 '18 at 16:53
  • I think the triangle is the phosphor bronze point contact that collects surface charges from by polarity of negative Vbe bias on the polished surface of Ge with some gradient of impurities so get more gain in one direction than the other for Vce bias. Or is the triangle sputtered phospor bronze on another crystal? – Tony Stewart EE75 Mar 30 '18 at 16:56
  • (time for me to spring sheer cut my Schnoodle) – Tony Stewart EE75 Mar 30 '18 at 17:01
  • I added another source see my edit. – user16307 Mar 30 '18 at 17:09
  • The triangle is a piece of plastic. Two of the contacts are on its edges. The third is the piece of metal that the slice of germanium is laying on. – JRE Mar 30 '18 at 17:46
  • That close up picture of the first transistor sort of mangles my mental image of the apparatus a scientist uses. I always have this image of precision hand crafted, intricate gadgets. That looks like the mess some body made while learning to use a soldering iron. – JRE Mar 30 '18 at 17:48
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    As I understand the cat's whisker method of maintaining a junction, too much pressure pushes the assembly into full conduction. I'm having trouble finding a reference that doesn't feel anecdotal, though. Was this not the case? –  Mar 30 '18 at 23:24
  • @TonyStewart.EEsince'75, "high shear modulus" doesn't mean "very strong", it means "very stiff". – Mark Mar 31 '18 at 00:06
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The junction had to be formed by pulsing a current through it. It needed a light pressure or the junction would be to deep. In forming a tunnel diode the pressure is moderate and the well is more profound. Anyway the two wells can not touch or the transistor is destroyed. The playoff is two wells very close together but not touching. You might find this interesting: http://radio.radiotrician.org/2018/04/the-mystery-of-how-point-contact.html

greg
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