I am currently reading through a semiconductor physics book and I am currently in a section discussing the I-V characteristics of an abrupt P-N junction. A few things have come to my attention that I am having a difficult time understanding (that the book does not seem to elaborate on):
When finding the diffusion current density of the minority holes in the N region, the diffusion current density exponentially decreases as a function of x away from the end of the depletion region xn (with a similar analogue for the minority electrons in the P region). However, the diode current equation is then written by using the sum of the diffusion current densities at the boundaries of the depletion region and multiplying that expression by the area of the junction. If the current density decreases exponentially away from the depletion region, why can the total current through the diode be represented by the current density through the edges of the depletion region?
What of the drift current density? I assume that it's negligible, but what justification do we have for saying that it's negligible?
Why is the current governed by the minority charge carriers rather than the majority charge carriers?
Any help would be appreciated! Thanks!