Current flow in a PN junction is generally expressed in terms of the excess minority carrier concentrations in a PN junction, i.e. the excess holes on the N side and the excess electrons on the P side. As an aside, for example when discussing high-level injection, some texts will mention that when one applies a forward bias to a PN junction, actually the majority carrier concentration increases as well, and furthermore that excess holes on the P side ($p_p$) follow the same distribution in space as the excess electrons on the P side ($n_p$). The same situation is found on the N side. A diagram of all the excess carrier densities can be seen on page 10 of this lecture.
I haven't been able to find an explanation for why this happens other than an appeal to conservation of charge, that $\Delta p = \Delta n$. This would make sense to me e.g. in a photogeneration process where a new electron-hole pair are simultaneously created, but it's harder for me to understand here since the excess charge profiles are a result of an applied bias alone. That is, the very high equilibrium concentrations of free electrons/holes in the N/P regions respectively are because of doping, not light.
I understand that if the N side contains some total number of excess holes injected from the P side and the P side likewise contains a number of excess electrons from the N side, and that these numbers are not necessarily the same due to different doping on each side, that somehow charge neutrality must be maintained in the majority carrier numbers.
Specifically, why does it make sense that excess majority carriers follow the exact same distribution in space as the minority carriers on each side of the junction, as shown in slide 10 above? The excess minority carriers are injected there due to the barrier lowering of the forward bias, but what's bringing the majority carriers there?
