We are learning about the different modes of BJT transistors currently and something that is a bit odd to me is that our textbook states that

"... an npn transistor whose EBJ is forward biased (usually, VBE ≃ 0.7 V) will operate in the active mode as long as the collector voltage does not fall below that of the base by more than approximately 0.4 V. Otherwise, the transistor leaves the active mode and enters the saturation region of operation."

My question is why the Emitter-Base Junction needs the 0.7 V we're all familiar with to become forward biased, but the Base-Collector Junction (\$V_{BC}\$) needs to only be 0.4 V to become forward biased? My line of thinking is that the collector is less doped than the emitter, so there is a smaller barrier voltage to overcome.

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    \$\begingroup\$ ah! the thing is, the EBJ is already biased to at least 0.7 V, when the BCJ needs to be 0.4 V. Think about it: that PN junction can't be quite as the other, since due to the EB-bias you've already got a different charge carrier concentration in the base. \$\endgroup\$ – Marcus Müller Apr 1 '20 at 17:05
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    \$\begingroup\$ There is nothing in the quoted passage that says that the collector-base junction is "forward biased" at 0.4V. The passage only says that the transistor will enter saturation. \$\endgroup\$ – Elliot Alderson Apr 1 '20 at 17:24
  • \$\begingroup\$ Elliot, it is implied that the BCJ will become forward biased in saturation mode. I figured that didn't need to be explained. Marcus, I believe you've answered my question. If I understand correctly, the barrier voltage across the BCJ is decreased because of the increase in minority charge carriers in the base? \$\endgroup\$ – ModularMan Apr 1 '20 at 18:43

When transistor is in active state the Base-Collector junction needs to be zero or reverse biased. When \$V_{BE}=0.7V\$ and \$V_{CE}=0.3V\$ you get \$V_{BE}-V_{CE}=V_{BC}=0.4V<0.7V\$.

It's not important for Base-Collector junction to be forward biased hence your confusion.\$V_{CE}\$ can be as low as \$0.2V\$ (\$V_{BC}=0.5V\$), and transistor would still be in forward active region. You just need to make sure Base-Collector voltage doesn't go over \$0.6-0.7V\$. That would mean you have entered saturation region, or in other words both of your PN juntions are forward biased.

  • \$\begingroup\$ I am not concerned about the modes, I already understand that if both are forward biased then it is in saturation mode. My question is why the BCJ will be forward biased with a smaller forward voltage than the EBJ. I believe Marcus answered my question above. Thank you for trying though, I appreciate it! \$\endgroup\$ – ModularMan Apr 1 '20 at 18:44

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