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As far as I understand in an PNP transistor the base voltage Vb is higher potential than the collector voltage. So since Vb>Vc the base collector junction is then reverse biased:

enter image description here

But in Ebers-Moll model one of the diode current is Icd as shown below:

enter image description here

And this Icd is formulated as follows:

Icd = Ics * (exp(Vcb/Vt) - 1)

But isn't such a formula for a diode valid only for forward biased diode?

pnatk
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    No, this formula is valid for reverse-biased diodes too. What you're seeing is the reverse leakage current of the diode; it's very small, but it's not zero. – Hearth Nov 13 '18 at 01:20
  • And that model will work correctly when the transistor is operated in reverse, with the collector and base interchanged. (I **cannot** remember the details, but TTL logic has one of the transistors running in reverse for one of the transitions, in a way that pulls current out of some node and speeds things up. Mind you, I'm remembering fragments of a lecture that I attended 35 years ago, so it is a bit foggy...). – TimWescott Nov 13 '18 at 02:00
  • @TimWescott I'm interested. Perhaps if you see [TTL AND](https://electronics.stackexchange.com/a/368467/38098), [TTL inverter/NAND](https://electronics.stackexchange.com/a/304722/38098), and [TTL output section](https://electronics.stackexchange.com/a/396739/38098) it might trigger recollection. If I missed something important I'd want to improve those answers with what you recall. – jonk Nov 13 '18 at 06:34
  • Here are the [three equivalent 1st level Ebers-Moll models](https://electronics.stackexchange.com/a/252199/38098) to examine. They are just three different ways of looking at the same thing. All equivalent. – jonk Nov 13 '18 at 06:36

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