damaged BJT or diodes? with fine diode test

[coppercone2]

I read about this a little bit, but, what are the main ways a transistor can fail, and still read OK on the diode test?


I read the gain of transistors can go down while still reading OK on the diode test. Can anything else go wrong?

[David Aurora]

I've definitely been stumped by some odd failures in both things before. And not just passing diode tests, but also passing on basic multifunction testers at low voltage.

I haven't seen much in the way of simple gain changes, but I've seen other stuff. I've seen quite a few noisy/crackling diodes and transistors test perfectly fine on basic test gear but play up in circuit. Or breaking down well below the specified limits but still well above your standard low voltage tester (e.g. 50V rated parts that eat shit at 20V). Leakage can also be a thing that doesn't show up on basic tests (though sometimes can depending on meter specifics and severity of fault).

Hell, last week I had a REALLY weird one- I tested a suspect transistor in circuit and it was OK on diode tests. Wasn't convinced so I pulled it out and checked it on one of those little eBay transistor testers, it passed as a PNP transistor. Only it WASN'T a PNP transistor, it was a (bad) NPN transistor 

I usually chuck these things aside to look at properly later/keep as references of odd failures. In most cases it becomes extremely obvious that these parts are faulty when I check them on my Tektronix 576 at healthy test voltages/currents, but yeah, often that's a last resort test after something has passed basic inspection but presents symptoms pointing to a bad device.

[Zero999]

I've heard prolonged reverse bias above the breakdown voltage will cause this, but it isn't something I've experienced myself.

[m k]

So nicely and not so nicely partially faulty.
Maybe overheat treats it more gently than over current.

Partially exploded traces inside ICs is a regular thing, maybe bubbles also.

I guess all kind of diminished values are possible.

If tin whisker level connection is a power route, it probably burns away.
But adding over time and it's just an add-on.

[Laval]

The voltage used by the meter for the test can also have something to do with it. If you use a voltage around a few volts, the junction may test bad while it test good with a voltage below 1 volt like many meters use.

[David Hess]

Reverse voltage breakdown of the base-emitter junction will lower the hfe, especially at low collector current.  This does not matter at high collector current or for power transistors, but it will ruin the performance of differential amplifiers by causing high input bias current.

Transistors sometimes become "soft" with high saturation voltage.  This will show up as a very rounded transition at the knee shown on a curve tracer test.

[Kim Christensen]

Not really a failure of diode test itself, but another way is operator error:

User checks B-->E and gets OK
User checks B-->C and gets OK
User assumes transistor is OK, but C-->E is actually shorted.

In circuit:
Transistor has a resistor across B-->E or B-->C that makes it look like it's OK in the forward direction, but junction is open. (User doesn't check reverse polarity)

Out of circuit:
The act of desoldering the transistor temporarily "fixes" it. User puts it back in circuit and it fails again right away or a bit later.

I've seen diodes check OK, but once you put a bit of current through them, they start dropping more than 0.7V... Maybe 2-4V or so... (Silicon diode example)