Strange electronic components

10.12.2003 18:46

I bought 20 of these strange components on a sale in a local electronic shop for around 200 SIT (that is cca. 75 cents). They were in a bag together with some other old and not so old electronic components like LED displays, static RAM chips, resistors, etc.

They are similar to phototransistors or multicolor LEDs. Each component has three terminals (they are now slightly oxidized, apparently because they were stored somewhere for a long time). The casing is made of a tough and smooth transparent material which is lined at the bottom with a band of metal. One of the terminals is marked by a small tab on the casing. Diameter at the widest point is 9 mm. The metallic lining is identical to the one on the standard TO-5 casing. I found no product codes or other marks.

A more careful look will reveal that there is no semiconductor die connected to the terminals. It looks like the three terminals were simply inserted in a drop of glass. A simple multimeter measurement revealed no connections between the terminals. I could measure no current when I applied voltages up to 12 V between any terminals.

The casing appears to be made of glass, so I rubbed one of the components with a piece of sand paper to see how easy it is to make a scratch on it. From the results of this experiment I ruled out plastic materials.

Glass is known to have a very high resistance at room temperatures, but it is also known that if you heat it enough its resistance will gradually fall. So to really prove that the casing is made of glass I heated one of the components with a blowtorch while I measured resistance between two terminals with a multimeter. Surely enough when the metal lining and the terminals began to glow orange the resistance dropped.

Now I knew what the components were made of, but I still didn't know what they are good for. Since nobody knew what they were for, I asked at the Laboratory of Semiconductor Devices if I could use their equipment to make some more precise measurement. This is the result:

This is a low frequency high voltage I(U) characteristic. All other characteristics (all together 6) were similar. There is some non-linearity at low voltages. You can see from the picture that around 10 pA flowed through the component at 100 V between terminals (the resolution of the analyser was 10 fA). This current was a bit different for each pair of terminals. I believe this difference is the result of slightly unsymmetrical placing of the terminals. The resistance (100V/10pA is around 10 TOhms) is comparable with glass insulators, so I don't believe these components can be used for anything in electronics. More current probably flows through the printed circuit board material than through one of these things.

The only idea left that I haven't dismissed is that these components are insulators, used for insulating the printed circuit board from the conductive casing. I imagine they could be soldered to the side of the PCB that is facing the casing of a device. That would prevent the uninsulated traces on the PCB coming into the contact with the casing and causing a short circuit. But this doesn't explain why these components have three terminals and why one of them is marked like a emitter on a transistor. Perhaps they were produced on the same production line as the other semiconductor elements.

Michał Miszewski writes about another possibility:

Components that you have described on your blog probably are some kind of samples. Series like that are made while testing the production process of component casings. They can also be a result of some malfunction during the fabrication, resulting in no semiconductor inside.

Frank Olejnik sent the following comment:

The components could be fake transistors.

I know todays fakes leave these ones for dead, but back in the days of early transistor radio's, manufacurers would compete on the number of transistors they could put in a radio. The thinking being that the prospective purchaser - on looking at the available range of radios - would pick one with the highest transistor count on the case thinking that more was better.

So the manufacurer would take a 5 transistor circuit, and add a few fake ones on the PCB just so he could etch "8 transistor" on the front of the case.

If you have any more ideas or if you know what these things are good for, drop me a mail.