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Why is the collector current smaller in value when a transistor is in saturation mode of operation? My guess is that although both the junctions are forward biased, the (forward) currents are 'opposing' each other and hence the net current is a small value. Is this explanation true? And if not, why is it so?

(The comparison of current in saturation mode is being made with the current in active mode for an NPN transistor in CE configuration)

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In an NPN in active mode Vcb must be so large (with collector positive) that the maximum number of electrons from the emitter, coming through the base, are sucked into the collector. Increasing Vcb hardly changes the current as the electron flow is at maximum already. The electron flow is determined by the BE junction!

In an NPN in saturation mode Vcb is smaller, so small that the flow of electrons is influenced by Vcb. This is the red part of the graph in Andy's answer, a small change in Vce (which is just Vcb + Vbe) will cause a large change in Ic. In saturation many electrons make it to the base instead of being pulled into the collector. That makes the base current larger and the collector current smaller. When the base current is kept constant that would result in a smaller collector current.

Note that there is no "opposing" current, the BC junction does not need to be fully forward biased to already influence Ic. It is more the fact that the BC junction isn't so far into reverse mode for it to be able to "pull out" all the electrons it possibly can out of the base region. So less electrons reach the collector resulting in a smaller Ic.

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Study this picture for a minute or two: -

enter image description here

Then ask your self; does the current get bigger or smaller when you increase VCE and, when you do increase VCE do you go from saturation mode to active mode or from active mode to saturation mode?

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  • \$\begingroup\$ On increasing VCE, the current is increasing. Also, increasing VCE makes us transit from saturation to active mode. But how does that answer my doubt? Please correct me if I have understood something wrong. \$\endgroup\$ – Pikachu May 24 '18 at 11:13
  • \$\begingroup\$ Your question appears to be: "Why collector current is small in value in saturation mode?". What is your doubt about this? \$\endgroup\$ – Andy aka May 24 '18 at 11:15
  • \$\begingroup\$ I am comparing the current magnitudes in saturation and active modes of operation. I want to know "Why collector current is small in value in saturation mode?" (compared to active mode). \$\endgroup\$ – Pikachu May 24 '18 at 11:18
  • \$\begingroup\$ The problem is that I just don't understand why it's not obvious from the picture in my answer. \$\endgroup\$ – Andy aka May 24 '18 at 11:21
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    \$\begingroup\$ As the reverse voltage between collector and base gets smaller the current gain of the device is reduced because the ability to drag electrons from the emitter (via the base) to the collector (normal BJT action) is reduced. \$\endgroup\$ – Andy aka May 24 '18 at 11:31

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