In semiconductor physics, this equation is called the drift–diffusion equation. The word "drift" is related to drift current and drift velocity. The equation is normally written: J n − − q = − − D n ∇ ∇ n − − n μ μ n E J p q = − − D p ∇ ∇ p + p μ μ p E ∂ ∂ n ∂ ∂ t = − − ∇ ∇ ⋅ ⋅ J n − − q + R ∂ ∂ p ∂ ∂ t = − − ∇ ∇ ⋅ ⋅ J p q + R where