[sdiy] Ge transistor tester weird results

Mike Bryant mbryant at futurehorizons.com
Mon Feb 7 19:02:02 CET 2022

Nobody is going to program a tester for Ge devices.

My guess is the tester sees a lot of leakage and assumes this is due to the current being fed into the base.  So it calculates some almost random number for the hFE.  I'd be very surprised if you have any of those devices more that about 30.

Why are you using them anyway ?  Best use for them was for strengthening tarmac which we used to do in the 1960s with all the dead ones :-)

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of cheater cheater via Synth-diy
Sent: 07 February 2022 17:46
To: synth-diy
Subject: [sdiy] Ge transistor tester weird results

Hi all,
I recently bought and tested a few Germanium transistors in my small semiconductor tester and I got weird behaviors.

I have this transistor tester:


As you can see, it's a simple device that has a ZIF socket. It will auto-find the device's pinout and can figure out what sort of device you put in it. For a transistor it will calculate the Vf and the hFE.

When I test a new Si transistor, say a BC558, it will auto-find the BCE pins and show a schematic diagram of a PNP transistor. The hFE might drift between tests a little, up to 1%.

I also tested the 20 AF306's I got, which are Ge transistors, with hFE min 10, and they have the same TO-92 package as the BC558. First of all, the schematic shown always shows the transistor in parallel with a diode. I assume that's because the Vf is ~0.3V, and the tester assumes Si, so it guesses there's a diode there.

Second of all, the hFE on those drifts a LOT between tests. As I keep on repeatedly testing, it will drift up or down by maybe 10-20%, first quickly and then slowly. The direction of drift is always the same for the same transistor, but it will be different between transistors. As the hFE drifts, the Vf drifts with it, too. If I take a small pause of several seconds, the hFE will drift back close to its initial value, but if I resume rapid test runs, it will drift away to its "final"
value more readily.

In addition to that, out of the AF306's I tested, there are some in the hFE 10-40 range, some (fewer) in the 40-100 range, and then there was one that's hFE 100, one that's around 250, and a couple that's 400-500. I don't know how to explain the ones that are hFE > 100.
What's going on here? Are they really that strong? Are they broken?
Also worth mentioning at hFE 500 the tester gets confused and assumes the device is a P-JFET.

My guess is that once the tester figures out this is a transistor it will just test hFE with a DC voltage and a very small current. So given that is the case, and that a lot of the Germanium transistors are in the right range, I don't think that the very high hFE measurement has to be off... but I don't know what might be happening.

My main worry is that those transistors are broken and if I put them in a reasonable circuit they'll try to output so much power they'll go up in smoke. Or maybe it's something more subtle, like a lot of noise being read by the ADC as high output current, or something like that.

How does one explain the weird drift and the super high hFE?
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