A project log for Bipolar dudes

A group for those who want to build their own bipolar transistors

Yann Guidon / YGDESYann Guidon / YGDES 06/22/2021 at 23:268 Comments

One reason we love old equipments/parts/technologies is they are more "graspable", easier to understand. They are not a magic black box and we can "see" them working. That's why we love relays, flip-dots, and other electromechanical devices so much, despite their ridiculously poor performance or efficiency. This is a bit less so for vacuum tubes, though "microphonics" (?) effect can be observed in some conditions, but for transistors, which are usually metal-canned or epoxy-potted, forget it. You can't "see" a circuit working, unless you add LEDs for examples (extra kudos for Tim with his #LCPU - A CPU in LED-Transistor-Logic (LTL)). But still, this is far from a completely direct observation of the circuit's working.

Then I got a batch of OCP71 on eBay.

They are a variation of the vintage OC71, Germanium PNP in glass housing, with 2 modifications to make them useful as photo-sensitive transistors, or even phototransistors : no black paint, and the filler silicon is transparent (unlike the diffusing and opaque sludge of the standard OC71). This makes any circuit using them sensitive to light, so expect a lot of drift and weird bugs in "normal conditions". Already the OC71s' characteristics are altered when exposed under intense light (the black paint can't block everything, particularly infrared).

But the photo-electric effect is reversible ! This means that with the right equipment, one could observe the junctions' emissions when the circuit is working. The new problem is that it is not visible by the naked eye.

For example, says that Ge is an indirect bandgap semiconductor with an energy of about 0.67eV at ambient temperature, translated to a 1.84 μm wavelength. To visualise the circuit, one would need a camera that is sensitive to the 2µm-1.8µm band. This is in the medium infrared, larger than 1µm (the near infrared that your TV remove uses) but smaller than the far IR, or thermal infrared, that is used by thermal cameras for example. Germanium is in theory active around 95THz or an equivalent black-body of 1600K. So it will not be enough to remove the IR filter of a CMOS camera, or use a temperature imager...

The cameras for 1.8µ are much less easy to source cheaply. Worse, mid-IR is a band with military interests : PbS at 3.34µm was used as the first "heat seeking" sensor in Sidewinder missiles, for example. And the exotic chemistries make such cameras expensive AND probably suspiciously considered.

I would love to have a small, cheap camera to record images of a circuit made of the transparent OCP71 transistors and make videos, or even live demos (to kids and old kids) but sourcing the old glass-germanium PNPs was only the easiest part.

I need your help to find, source and get such a camera, if it exists. I have found some but they are either "out of band" or very expensive ("industrial grade"). And I don't see the point in buying expensive gear that I'm not going to use a lot or regularly.


Yann Guidon / YGDES wrote 06/24/2021 at 12:24 point

There is also the "infrared viewer" device that transforms the IR to visible light with a mechanism similar to photomultipliers. One example is

The abris-m 2000 seems to cover the 1500-2000 nm range well :

Then it's a matter of fitting a camera behind the eyepiece... But it's in the 1500€ range :(

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Yann Guidon / YGDES wrote 06/24/2021 at 09:38 point

If the emission of Ge "under normal operating conditions" is low (as expected), would it be possible to add a photomultiplier as alluded to in your 1960 article ?

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Tim wrote 06/24/2021 at 16:58 point

Well, that would be a way to detect very low level of light. But you will not get spatial resolution.

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Tim wrote 06/24/2021 at 07:12 point

Light emission from silicon is used for quality control in some areas, for example solar cells. Google for "electroluminscence solar cell" for some images. The amount of light emission is quite low, since silicon is an indirect semiconductor. So long exposure times are needed. No idea if it is possible to observe in operation.

I have no ieda about germanium. I would expect that cameras that can record direct light emission from germanium fall under us export law, so even if you find some, you may have a hard time buying it.

You can also get visible light emission from avalanche breakdown:

some pictures (bottom of page)

No idea how well that works with germanium. But obviously, the avalanche breakdown is not a normal operating condition.

EDIT: Ok, found a reference for germanium

Visible light emission from avalanche breakdown seems to work as well, but the entire emission spectrum is shifted towards the infrared, which will significantly reduce the intensity of the visible part.

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Yann Guidon / YGDES wrote 06/24/2021 at 08:53 point

Good finds !

yes, "export restrictions" are a thing but in this context, I'm looking for a cheap lab equipment with very close distance (10cm ?) and very high aperture (about 2), which can't be considered of military interest (no funky optics for example). Interface would be USB (bleh, drivers...) or Ethernet, or direct recording on Flash media on the cam.

The question of the emission bandwidth of Ge in "normal operating conditions" is still not settled but we have some estimates. So maybe we can simply try with "what's available" and see how well it fits ?

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Tim wrote 06/24/2021 at 09:11 point

Under normal operating conditions it should be close to thebandgap of Ge. So something like 0.7-0.8eV.

Actually already the focal plane arrays (the sensing) elements are export restricted when they are 2D. It's a huge nuisance and impediment to innovation. But there should be some labs with cooled InGaAs cameras they may be able to see something. Maybe in a university or a semicondcutor failure analysis lab.

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Yann Guidon / YGDES wrote 06/24/2021 at 09:37 point

So my hope of a quick fun cheap hack to amuse the kids and make 10 views on YT is back to the dream shelf...

Now, "export restriction" is for exporting from a zone. Maybe we can find a manufacturer within the zone.
=> looks eager to "export" :-D

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Yann Guidon / YGDES wrote 06/23/2021 at 02:05 point

I found this : but sensitivity at 1700nm is low and might not extend down to 1800-2000nm ...

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