Recently I was wondering why UCE can be smaller than UBE. I've drawn a small diagram with the typical values of the BC547 NPN transistor.

Schematic diagram

This amazes me all the more as UBE shares its PN-regions with UCE.

NPN regions of transistor

Can someone shed some light on this "miracle"?

  • 1
    \$\begingroup\$ The operating mode of the transistor is called "saturation". There is plenty of information about that to be found if you search for "BJT saturation" so explaining that here serves no purpose. Realize that you might want to fully understand the "active mode" of the transistor first before attempting to understand saturation mode. \$\endgroup\$ – Bimpelrekkie Jul 5 '19 at 20:27
  • \$\begingroup\$ @Bimpelrekkie All pages I have found on this topic mention only the fact, but do not explain how this effect is possible. At Stackexchange I couldn't find an explanation either and I could imagine that others would be interested as well. \$\endgroup\$ – Kitana Jul 5 '19 at 20:55
  • \$\begingroup\$ Have you watched this video: youtube.com/watch?v=4MkPYKEIJ2Y ? I suggest that you watch similar videos and presentations about how a BJT works. \$\endgroup\$ – Bimpelrekkie Jul 6 '19 at 11:55

You are missing one value in your picture. Voltage between base and collector (Vbc, which is actually opposite to Vce).
enter image description here

Do you already see that there is no miracle?
Even without deep knowledge of PN junctions, just elementary circuit laws (like Kirchhoff's Laws).

  • \$\begingroup\$ @Ch Why do you draw the 0.61V arrow from P to N while the (technical) current flows from N to P? \$\endgroup\$ – Kitana Jul 5 '19 at 21:07
  • \$\begingroup\$ @Kitana Because the direction of the voltage is opposite the direction of the current. \$\endgroup\$ – Hearth Jul 5 '19 at 21:17
  • \$\begingroup\$ @Kitana You can draw that arrow other way round and use value -0.61V instead. Your question was not about mechanics in PN junctions, but about how is it possible that Vce is lower than Vbe. So, two different leagues. And I'm not able to give you answers about detailed physics behind PN junctions. Apparently there can be a negative voltage across PN junction. \$\endgroup\$ – Chupacabras Jul 5 '19 at 21:18
  • \$\begingroup\$ @Chupacabras (My edit took too long) The collector current is flowing into the transistor, then how can Ucb be negative? In active mode Ucb is positive. Why and how does that change at the saturation point? And my question is exactly about the mechanism behind this behaviour. \$\endgroup\$ – Kitana Jul 5 '19 at 21:25
  • \$\begingroup\$ Apparently there can be a negative voltage across PN junction Yes and we call that reverse mode. The trick with a BJT in saturation is that the BE junction is in forward mode and it is so close to the CB junction (the base is thin) that it "swaps" the (reverse biased) CB junction with so many carriers that is can conduct current. \$\endgroup\$ – Bimpelrekkie Jul 6 '19 at 12:00

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