Charge Neutrality

Charge neutrality occurs when all the charge in a volume adds to zero; it is neutral, neither positive or negative.  The equation for charge density (Coulombs/cm³) is:
r = q(whatever has charge)where q = electronic charge.
In a semiconductor, the most common and most prominent sources of charge are electrons holes, and ionized acceptors and ionized donors.
The zero net charge does not mean that the electrons, holes, ionized donors, and ionized acceptors are not present in the semiconductor.  It simply states that in a uniformly doped semiconductor the negative charge associated with an electron or ionized acceptor would be canceled by the positive charge associated with a hole or ionized donor.  This does not mean that the actual electrons, holes, and ionized impurities have ceased to exist in the semiconductor, it means that  r = 0.
If r = q(p_o - n_o + N_D - N_A) = 0, then the sum of the charges associated with the carriers must equal zero:

po - no + ND - NA = 0

This equation is useful in many areas, including computing p_o and n_o in equilibrium.  We usually consider N_A and N_D to be known, so it gives us one equation to relate two unknowns, p_o and n_o.  We also know that p_on_o=n_i² and n_i is considered known, so we can use the two equations to solve for p_o and n_o in terms of N_D, N_A, and n_i

Anderson Jose Mariño Ortega

C.I. 17.456.750

E.E.S.

http://www.ece.utep.edu/courses/ee3329/ee3329/Studyguide/ToC/Fundamentals/Carriers/neutrality.html