So I've read quite a few books on how BJT's work and all of them seem to gloss over how a BJT in the linear mode with the
base/emitter junction forward biased and the
base/collector junction reversed biased is still able to conduct current from emitter to collector.
In this situation, the
base/emitter PN junction is biased in such a way that the depletion region is very small or negligible. However, the PN junction of the
base/collector region has its depletion region widened by the reverse biasing; which makes you think at first glance that no current should be allowed to conduct.
Here is my take on the device theory of why we still see (large) current from emitter to collector for an NPN BJT in the Active/Linear mode. Please correct me if my theory is wrong or has some misconceptions:
Since the base is very thin in relation to the collector and emitter, and it is lightly doped to a P-type material, there are not many holes available to be recombined with emitter electrons. The emitter on the other hand is a heavily doped N+ material with many,many electrons in the conduction band. When the mass amounts of electrons in the emitter get pushed into the base by the emf of the
base/emitter battery, the vast majority of them have no holes to combine with. Now we have a situation where we have a high entropy of electrons in the base which begin to defuse into the
base/collector depletion region; effectively doping and shrinking the
base/collector depletion region into N-type material. When enough electrons diffuse, the depletion region all but disappears into n-type material, allowing the positively biased n-type collector region to sweep those diffused electrons out of the collector terminal. Since the collector is moderately doped to N-type, the diffused electrons are able to pass right out the positive-polarity collector terminal.
So, am I right about entropy playing a role in this by diffusing the
base/collector depletion region with electrons to effectively dope it into n-type material?