PNP / NPN Reverse current and voltage (Vce/Vec and Ice/Iec)

Question

I discovered that these two circuits behave in an identical way from a loose perspective (only the following schematics tested):

^{simulate this circuit – Schematic created using CircuitLab}

R1 and R3 both had 4.3v across them, and R2 and R4 both had 5v across them.

Similarly, when R1 and R3 were connected to +5v: R2 and R4 had zero volts across them.

Is this expected behaviour? From generic diagrams of the junctions of a PNP transistor, the junctions appear symmetrical, so from that perspective it makes sense.

What's frustrating: the BD438 datasheet gives absolutely no information on characteristics for reverse operation; Somewhere I read that reverse voltage for BJTs should generally be limited to 5v. Googling for me rarely uncovers anything about reverse operation of BJTs.

It would be great if someone could demystify reverse Vce behaviours and limitations, including interpolating from datasheets absolute ratings and typical characteristics for reverse operation given the forward operation information, (if it's possible).

Answer 1

Most jellybean silicon BJTs have much worse characteristics in reverse. Beta is much less and breakdown is more like 8 or 9V. The characteristics are not guaranteed or even given as typical on the datasheet because most people do not use them that way. Since they makers won't sell any more parts by specifying those parameters they don't bother.

You can sometimes find the reverse beta estimated in the SPICE model, and a (usually conservative) limit for reverse Vbe is almost always given on the datasheet (5V is common, but lower and higher voltages are seen).

There are (or were) transistors made with better characteristics in reverse, or even made to be symmetrical. Some are aimed at the analog mute function, and in the old days some were used for analog switches. They are not as good for most applications as the more common asymmetrical BJTs.