Diffusion current

Diffusion current is the net flow of charge due to the diffusion of charge carriers in a semiconductor. This happens when the density of charge carriers isn’t uniform within the material, so holes will diffuse from a region of high to low concentration. The resulting hole concentration profile is given below. Holes will diffuse in the positive $x$ direction. The magnitude of the current at any point is proportional to the concentration gradient at that point:

$$J_{D,p}=-q{D}_p\frac{dp(x)}{dx}$$

where $J_{D,p}$ is the hole-current density, $D_p$ is the diffusion constant (or diffusivity) of holes, and $p(x)$ is the hole concentration at point $x$.**