I am struggling with some approximation while deriving I-V characteristic of a pn junction.
Let's consider a quasi-neutral region on the n side of pn junction.
There are four current components as follows:
Jndiff: majority diffusion current
Jndrift: majority drift current
Jpdiff: minority diffusion current
Jpdrift: minority drift current
Because there is no field in the neutral region then there can be no net charge at any point in this region. Then the excess majority carrier concentration should follow the decay of the excess minority carrier concentration. This results to Jndiff = Dn/Dp* Jpdiff.
I can see that Jndiff and Jpdiff is on the same order of magnitude. Also Jndiff is larger than Jpdiff because Dn > Dp.
The total current density:
Jtotal = Jndiff + Jndift + Jpdiff + Jpdrift
The approximation is as follows: Jndiff << Jndift so we can ignore Jndiff.
(similarly Jpdrift << Jpdiff, so we can ignore Jpdrift)
This results in Jtotal = Jndrift + Jpdiff.
However, what I am confused is that we ignore Jndiff and keep Jpdiff while Jndiff and Jpdiff are on the same order of magnitude (Jndiff is even larger than Jpdiff). If we ignore Jndiff then we should also ignore Jpdiff because Jpdiff is smaller than Jndiff. Could anyone please explain it?