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I recently built a transistor curve tracer on a breadboard with my Oscilloscope and Arbitrary Waveform Generator. Most BJT Si transistors (NPN and PNP) show typical/normal traces but one device under test - a 2N3906 - produces abnormal trace when collector-emitter voltage (Vce) goes higher than about 6 Volts. My other 2N3906 parts don't show this "abnormal" effect.

Does anyone know what is the cause of this abnormal trace (transistor or test setup for examples)?

I present a few images below showing my test setup, a good trace, and the weird trace.

image 1: LTSpice showing my simple circuit

image 1: LTSpice showing my simple circuit

image 2: breadboard showing the circuit I am using with scope probes and AWG waveform inputs

image 2: breadboard showing the circuit I am using with scope probes and AWG waveform inputs

image 3: AWG setup

image 3: AWG setup

image 4: a 2N3906 transistor showing typical results

image 4: a 2N3906 transistor showing typical results

image 5: the problem result - a 2N3906 under test where Vce seems to break down after 6 volts DC

image 5: the problem result - a 2N3906 under test where Vce seems to break down after 6 volts DC

Link to my report on this subject

http://www.biophysicslab.com/2021/04/27/testing_transistor_hfe

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    \$\begingroup\$ I couldn't quickly read all those details. But since it is very common for BJTs to have reverse-voltage breakdown of their BE junctions at around 6 volts or so, is it possible you've got a pin-identification problem with your device? \$\endgroup\$ – jonk May 4 at 17:11
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    \$\begingroup\$ Looks like the CE junction got zapped and acts as a SCR with a negative incremental resistance . Might be good for a tunnel diode oscillator at RF if biased at -7V. ;) This is a breakdown arc failure with say 40K/mm might be 40V/um so 7V is about a 175 nm gap that the E-Field bridged This PNP should be good to -40V and act like a Zener near -50V. In the 70’s I needed a HV Zener for uA ref. So I choose a transistor breakdown voltage to make the old TV work. \$\endgroup\$ – Tony Stewart EE75 May 4 at 17:14
  • \$\begingroup\$ 6 V breakdown is indeed to low for Base-Collector but the expected value for Base-Emitter. Note that BJTs do work "upside down" (so swapping collector and emitter) so that's why you still get the transistor curves but the you will see breakdown at a much lower voltage and also a lower beta. \$\endgroup\$ – Bimpelrekkie May 4 at 17:14
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    \$\begingroup\$ Looking at your photo it does look like you have connectied things up properly. Do realize that the 2N3906 is a PNP so the collector voltage needs to be negative with respect to ground. So in your LTSpice schematic, both V_stair and V_triangle would need to have a negative value. \$\endgroup\$ – Bimpelrekkie May 4 at 17:20
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    \$\begingroup\$ Is everyone agreed , it’s a defective part? \$\endgroup\$ – Tony Stewart EE75 May 4 at 17:44

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