I have two oppositely oriented diodes connected at the same polarity terminal. Is this a BJT?

Question

The connection is p-n-n+-n-p. Can it be considered a pnp bipolar junction transistor?

Answer 1

Without any connection to n+, the current is limited by the reverse-biased pn junction. The voltages will distribute between the two pn junctions to provide a consistent (very low, almost negligible) current. You might idealize this as a BJT with zero base current.

If there is a third terminal (normally the base) connected to n+, most of the forward-biased current may leave through the base (depending on attached resistances), and hardly any will remain to inject into the reverse biased pn junction, unless the n-n+-n region is very thin - thin enough that there is no significant recombination in the n-n+-n region. So this is like a BJT with a wide base.

Answer 2

It is strictly speaking a bipolar junction transistor, but the emitter-collector current is likely very small. There is some current even with reverse bias. The forward-biased diode injects some current into the reverse-biased one, but there can be significant recombination. As the width of the N type material is reduced, the BJT behavior becomes more apparent. Otherwise it is just considered a parasitic BJT.

Answer 3

The middle portion of the two connected diodes would be considered the Base of the BJT, but unfortunately it doesn't work by just putting 2 diodes back-to-back. There are things that go on in a transistor Base that cannot happen in an external connection.

In a back-to-back diode connection, you get your Base current flowing down through the Base-to-Emitter diode, but there is no current amplification because that Base current cannot effect the majority carriers in the Collector diode. It has to be one piece of silicon sandwiched in between the Collector and Emitter.

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