Electronic – A confusion on the current gain of an NPN transistor

Bipolar junction transistors (BJT) have 3 primary modes of operation:

1. Off: no significant current flows from the collector to the emitter
2. Saturated: the transistor behaves like a diode (which is why there's a fixed voltage drop across the collector/emitter pair under saturation)
3. Linear: this is where the transistor behaves like a current amplifier. The current flowing from the collector to the emitter is proportional to the current flowing from the base to the emitter. Note that in this region the voltage drop across the collector/emitter pair is NOT roughly constant, but linear.

In cases 2 and 3 there is current flowing from the collector to the emitter.

You can saturate the BJT by attaching a smaller load (i.e. higher resistance) than the linear limit allows. Think of a pipe with a valve. If there's something clogging the pipe upstream, at a certain point no matter how much you open the valve you're not going to get any more fluid flowing through the valve.

Yes, the electrons do literally move to generate the current, though this isn't directly used for most electrical engineering analysis.

For most EE analysis we don't consider the motions of charged particles directly, instead relying completely on the abstract concept of current. This is an approximation of reality, but it turns out to be remarkably good in most cases.

Ironically, in my day-to-day work with plasma physics we do consider the motion of charged particles instead of relying on abstract currents because the analysis does differ depending on what species are moving and where/how fast they are moving.

The simulation result is correct. What you are seeing is Ibc: Current flowing from base to collector. Remember that the base collector junction is forward-biased.

Normally we don't think about the current flowing in that direction. But it can/does.

In saturation, some base current flows to the collector. The net flow may be from outside of the transistor into the collector, but that does not mean that Ibc = 0. This is something I did not fully grasp until quite recently, and I even down-voted someone over it. I mean, I am sure I knew it at one time when I was in school. But I must have forgotten it long ago.