The diffusion coefficient and mobility are related by the Einstein relation as above: D n = μ μ n k B T q , D p = μ μ p k B T q , where k B is the Boltzmann constant and T is absolute temperature. The drift current and diffusion current refer separately to the two terms in the expressions for J, namely: J n , drift − − q = − − n μ μ n E , J p , drift q = p μ μ p E , J n , diff − − q = − − D n ∇ ∇ n , J p , diff q = − − D p ∇ ∇ p .