# PN junction: electrostatics

Thread Starter

#### Wikkez

Joined Jan 26, 2013
6
Hi,
First post here, I'm learning semiconductor physics&electronics on my own, but I'm a bit lost.
In the depletion regio,The total charge in the n-regio equals the total charge in the p-regio, so why is there actually an E-field.
Since u can calculate the strength of E-fields with Gauss' Law with the charge density and the charge density is the same but opposite in charge, why doesn't it cancel?
Why is there a field, if there's not net charge(p and n regio combined)?
ρp = ρn

Hopefully someone can sort this out for me

#### WBahn

Joined Mar 31, 2012
26,398
What is the basis for saying that the total charge in the N region is the same as the total charge in the P region?

Thread Starter

#### Wikkez

Joined Jan 26, 2013
6
What is the basis for saying that the total charge in the N region is the same as the total charge in the P region?
What do you mean with that question?

Charge neutrality?

#### WBahn

Joined Mar 31, 2012
26,398
What do you mean with that question?

Charge neutrality?
So is the total charge on one plate of a charged capacitor the same as the total charge on the other? If so, then how do you explain the presence of an electric field in between the plates?

Thread Starter

#### Wikkez

Joined Jan 26, 2013
6
I reviewed some electromagnetics and I understand it now, guess I was mistaking electric field for electric flux and charge density for charge.
My excuses.
But I have another question that's probably stupid:
One of the boundary conditions for solving the electric field on the pn-junction with poisson equation, is stating that @ x=0 the electric field must be continuous.
Well, if the field is not continuous, it means that there is a presence of charge just @ your junction right since field lines always end on a charge.
So why exactly is this impossible?
Also: why does the electric field reaches a maximum value @ x=0, what is the physical meaning of this.

Thanks in advance.