Electronic – the precise definition of forward and reverse bias for a P-N junction

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I have always thought that for a P-N junction, forward-biased simply means that \$ V_{PN} > 0 \$ and reverse biased means that \$ V_{PN} < 0 \$.

However, in this article about NPN BJTs, there is the following sentence (emphasis by me):

Vbe < 0.7V(emitter-base junction is not forward biased)

So it defines forward bias of a P-N junction as \$ V_{PN} > V_{th} \$, where \$ V_{th} \$ is the threshold voltage for the junction to conduct.

So which one is the correct definition of forward-bias? When I see an expression like: "Base-emitter is forward biased" should I understand:

  • \$ V_{BE} > 0 \$

Or

  • \$ V_{BE} > V_{th} \$

Best Answer

It's a bit of handwaving. What they really mean is that the B-E junction is not carrying any significant forward current. If VBE is between 0.0 V and 0.65 V, it is technically forward biased, but the bias is not enough to overcome the internal barrier voltage of the junction, so it does not conduct.

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