Electrical resistivity and conductivity results in the important concept of "quasineutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma ("n" = <Z>"n"), but on the scale of the Debye length there can be charge imbalance. In the special case that "double layers" are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation: Differentiating this
Depletion region the bulk semiconductor, then a spontaneous depletion region forms if the gate is electrically shorted to the substrate, in much the same manner as described for the p–n junction above. The principle of charge neutrality says the sum of positive charges must equal the sum of negative charges: where "n" and "p" are the number of free electrons and holes, and formula_28 and formula_29 are the number of ionized donors and acceptors "per unit of length", respectively. In this way, both formula_28 and formula_29 can be viewed as doping spacial densities. If we assume full ionization and that formula_32, then: