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
If r = q(po - no + ND
- NA) = 0, then the sum of the charges associated with
the carriers must equal zero:
This equation is useful in many areas, including computing po
and no in equilibrium. We usually consider NA
and ND to be known, so it gives us one equation to relate two unknowns,
po and no. We also know that pono=ni2
and ni is considered known, so we can use the two equations to solve for
po and no in terms of ND, NA, and