where formula_2 is the electric field, "e" is the elementary charge (1.6×10 coulomb), and "p" is the hole density (number per unit volume). the electric field make holes drift along the field direction, and for diffusion holes move in the direction of decreasing concentration, so for holes a negative current results for a positive density gradient. (if the carriers are electrons, the hole density "p" is replaced by the electron density "n" with negative sign; in some cases, both electrons and holes must be included.) when the two current components balance, as in the p–n junction depletion region at dynamic equilibrium, the current is zero due to the einstein relation, which relates "d" to "σ".