A problem in Semiconductor Devices when derivating Shokley Equation

[alexyangfox]

sorry for reposting this here.
I have just posted this in " Mathematics & physics"
But I think guys here may be more familiar with this.

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Hey guys.
I met a problem when derivating the Shockley equation.
a few steps before I finally get the Shockley equation , I met the Equation below:

the book said , because there's no electric field in the neutral region.
So the E is 0,that means the second term disappears.

But why?
When there's a voltage say 5v Across the Pn junction(say,forward bias) ,how could it be possible that there's no electric field in the neutral region?

Or the Ideal diode model computes the "voltage" in the depletion layer,say twist the bands?


thanks,
Best regards.

 

[timof]

In a p-n junction (at least at low current level), all applied voltage is dropped on the depletion region. Voltage drop on quasineutral regions is zero (or almost zero), and thus the electric field is zero.