Transistor Ic vs Vce Curve Trace for one 2N3906 Shows Weird Scope Display Not Sure Why

Question

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

image 3: AWG setup

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

Link to my report on this subject

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

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? — jonk, May 04 '21 at 17:11
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. — Tony Stewart EE75, May 04 '21 at 17:14
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**. — Bimpelrekkie, May 04 '21 at 17:14
Are we assuming this PNP is actually an NPN with a negative trace? — Tony Stewart EE75, May 04 '21 at 17:19
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**. — Bimpelrekkie, May 04 '21 at 17:20
@Bimpelrekkie: "So in your LTSpice schematic, both V_stair and V_triangle would need to have a negative value. " Sort of, maybe. Only the difference matters. [This IV tracer](https://josepheoff.github.io/posts/iv-1-toc) does PNP transistors with just the normal 5V power supply of an Arduino Nano. — JRE, May 04 '21 at 19:04
@JRE Sure, a negative supply isn't needed **if you connect the emitter of a PNP to the + 5 V**. But if you connect that emitter to ground (0 V), which is what is done here, then you would need - 5 V. — Bimpelrekkie, May 04 '21 at 19:09
The AWG image shows both waveforms going from -.7 to -5 for staircase, and 0 to -5 triangle volts respectively. I think I meet the negative supply issues on physical board. LTSpice simulation also uses negative voltage waveforms as well although the signal images are a little hard to see. — Ron at BiophysicsLab, May 04 '21 at 22:40