BJT base recombination current

[antonantal]

I thought that the base recombination current (say in a pnp bjt) was the current caused by the electrons that come up the base wire to compensate the loss of free electrons from the recombination process in the base. But recently I read that this are actually two different currents that sum up in the formula of the base total current.
So what is the base recombination current?

 

[es]

The first models of the P-N junction are developed with some assumptions which include:
1. No electron-hole recombination occurs in the depletion region.
2. The electric field in the neutral regions is negligible.

A slightly more complicated model, which better fits the experimental data, does not make assumption 1. The new term which results is called the base-emitter depletion layer recombination current.

This link, specifically equation 5.2.13, shows the two currents
http://ece-www.colorado.edu/~bart/book/book/chapter5/ch5_2.htm

 

[antonantal]

Thank you es. I still am not sure what the base recombination current is (meaning the second term in the right-hand side of the equation 5.2.13). Is it the current caused by the electrons that come up the base wire to compensate the loss of free electrons from the recombination process in the base?

 

[es]

Recombination current in general is defined as current that occurs
when an electron and a hole with a small amount of kinetic energy meet and, as you mentioned, these electrons contribute to the overall
current flow.

However, it is important to remember that electrons and holes are everywhere inside the transistor. There is, of course, a bunch of holes in the p-type base region, but there are also some in the depleation region as well. However there are so few that one can get reasonable accuracy out of their model even if they ignore it. If you want better accuracy out of the model then you don't get to ignore it anymore.

So for this model, equation 5.2.13 defines the base current as:
I_b = I_e,p + I_r,b + I_r,d
Where
I_e,p : The hole diffusion current
I_r,b : The base recombination (the mechanism you describe but occurring only in the base)
I_r,d : Recombination current in the depletion region (same mechanism different spot of the transistor but the electrons ultimately come from the same place)