Electronic – Do symmetric BJTs exist


Fundamentally, a BJT is a very thin layer of P (or N) type semiconductor sandwiched between two thicker layers of N (respectively P) type semiconductor. There's nothing intrinsic to that design that would indicate the collector and the emitter have to be physically distinct. Is it possible to make a symmetric BJT, that can operate identically in forward and reverse mode? If so, are any manufactured on a commercial scale?

I understand that making a transistor symmetric would necessarily sacrifice some desirable traits. I don't understand why, or which traits (probably β at least), though; I just know that if there weren't advantages to making them asymmetric, they wouldn't be made asymmetric.

Best Answer

Check out "muting transistors" in consumer audio gear:


The purpose of these is to short the outputs of a device to ground, for example while it powers up and down, to avoid producing a THUMP. Since the signal to short out is AC and has a DC component of 0V, a bidirectional switch is needed. MOSFETs would not work because the body diode would chop off half the signal when the switch is open, so the usual suspect here is a BJT, 2SC2878. Q801/Q802 here:

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These are optimized for high reverse hFe and high Vebo (so the base junction doesn't act as a zener when they're off).

Apart from these special perks which make them pretty much the only BJT able to fulfill this role, the rest of their characteristics is very unremarkable.

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