The drift current density resulting from an electric field can be calculated from the drift velocity. Consider a sample with cross-sectional area A, length l and an electron concentration of n. The current carried by each electron must be − − e v d , so that the total current density due to electrons is given by: J e = I n A = − − e n v d Using the expression for v d gives J e = e n μ μ e E A similar set of equations applies to the holes, (noting that the charge on a hole is positive). Therefore the current density due to holes is given by J h = e p μ μ h E where p is the hole concentration and μ μ h the hole mobility.