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Question: Blocking and safe use of NPN BJT in reverse active region.

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Really

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Hello,

And thank you to anyone who wants to take a stab at this one.

I would like to use an npn bjt to conduct in forward mode and then block or approximately block current in reverse active mode when the base will be switched to the emitter. My circuit will be switching the base terminal to the emitter terminal so that they are almost shorted(except for the base resistor), then applying the negative voltage across the collector emitter. The voltages present will exceed 25Volts, well above the emitter base reverse breakdown voltage, and forward currents will be less than 2 amps for very short pulses with a low duty cycle.

My questions are:
A.) First, how to protect the emitter-to-base junction voltage from exceeding maximum voltage when that diode is reverse biased and the bjt is in the reverse active region. My concern is that since the base collector diode will be forward biased, that almost the full emitter to collector voltage will be seen across the emitter base junction, and there is nothing I can do about it since these connections are all internal. I.e., I can't add resistors to limit current/voltages. Do the datasheet max Vebo values apply here, or is there something I can do about it?

B.) Second, how to obtain sufficiently low current and gain from emitter to collector when in the reverse active mode. Yes, I said LOW. Ideally it would be completely off, but my hope is that the bjt asymmetry would be great enough that I might be able to get away with the resulting "low" beta hence low reverse current.

To clarify a bit, here are the basic steps that the circuit will be performing and to which the above questions pertain. In order:
1.) NPN is conducting in forward active or saturation.
2.) Base is "shorted" to emitter terminal. The external base resistor exists between the base and emitter, so not a
true short.
3.) A negative voltage is applied across the collector and emitter. It is in this phase that I am worried about
exceeding the Vebo voltage, and it is also the phase in which I would like to have no current or small
currents(positive) flowing from emitter to collector. Placing a diode in series with the bjt is not an option for me.


Thank you in advance,

Mark
 

Take a look at these curves of a typical NPN transistor.

The breakdown of the C-E when reversed biased is quite low. This is a transistor with a normal 100 Vce rating. When reversed connected, it breaks down at about 8 V.

When the base is shorted obviously you have one diode drop.
 

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I believe the problem (what you want to achieve) could be better explained by showing a schematic. Some terms in your post don't really match the usual analog circuit design terminology, so I'm not sure about the meaning.

For the time being, I'd like to emphasize that transistor maximum ratings are describing real transistor properties and can't be cheated somehow. The actual base-emitter breakdown voltage Veb0 is in the range of 7 to 10 V for most silicon transistors. You can consider the BE junction as a zener diode with some pulse handling capability. Stressing the junction in reverse breakdown will however deteriorate transistor noise performance and current gain in the long run.

- - - Updated - - -

In other words, a silicon transistor can be used in inverse active operation up to 5 or 6 V without conflicting with Vbe0. Main disadvantage is the low current gain of about unity. I have seen yet two meaningful applications of reverse transistor operation
- AC switching (utilizes both forward and reverse operation)
- analog switching with low DC offset (only reverse operation)
 

Only a lateral BJT would have any symmetric blocking
ability (and your base drive would have to flip its return
to capitalize on this). Very few if any non-integrated BJTs
of this type remain. Their attributes are generally inferior
to designed vertical (E inside B inside C) BJTs, other than
cheapness (only one implant) and symmetry.

Perhaps the blocking is best done by a device suited to
the role (like, say, a series Schottky - if you can tolerate
the added voltage drop).
 

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