I know that in a transistor, the base-emitter (B-E) junction has to be forward biased while collector-base (C-B) junction needs to be in reverse bias.

Question: What would happen if we alter this biasing?

Case 1: Both B-E and C-B junctions are reverse biased.

Guess: Nothing will happen as electrons are being pulled away from the base region leading to no flow of current.

Case 2: C-B junction is forward biased and B-E junction is reverse biased

Guess: Transistor can be operated in normal fashion. But I know that collector is purposefully designed to be larger for heat dissipation. In our case, as emitter (which is not as large as collector) serves the purpose of a collector. It will cause heating in our circuit which can damage the transistor.

Case 3: Both B-E and C-B junctions are forward biased.

Guess: For this one, I suppose one out of two things should happen (I am not sure what).

Possibility 1: As both junctions are forward biased, electrons accumulate at the thinly occupied thinly doped base region. This causes overheating thus damaging the transistor.

Possibility 2: As both junctions are forward biased, electrons accumulate at the thinly occupied thinly doped base region. This constitutes to rise in base current. Thus transistor conducts like two independent separate diodes.

Are these above guesses right or wrong? Detailed explanation would be helpful


1 Answer 1


Case 1: Correct, nothing will happen. Only leakage currents will go through.

Case 2: Main disadvantage is that there is much less current amplification and lower breakdown voltage in this mode of operation.

In some cases amplification could be <1, which would make it rarely useful configuration. Heat dissipation is also valid point, but in transistors that I've seen under microscope - areas of collector and emitter are more or less similar, not 10x different.

Main difference between collector and emitter is doping profile which lead to this amplification difference, as well as breakdown voltage difference. In reverse breakdown voltage could be as small as 5V.

Case 3: Transistor conducts like two independent separate diodes. Will it burn or not will depend on power dissipation. Base typically has high resistance (as it is thin), so these diodes would be quite terrible.

I.e. base resistance will severely limit current. If you increase voltage until you get significant current and power dissipation - it will surely burn.

  • $\begingroup$ The further confusion here is that even with V_BC being positive and forward-biased, the collector current still seems to be flowing in the reverse direction due to the usual mechanism of minority charge carriers being conducted in reverse-bias. $\endgroup$
    – Milind R
    Apr 12, 2021 at 8:02

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