Diffusion Current

Diffusion is a process of movement of carriers from a region of high concentration to a region of low concentration.
In addition to drift current, there exists a Current component in a semiconductor due to the concentration gradient (non-uniform concentration) of holes or electrons.
Since there exists a concentration gradient, the charge carriers have the tendency to move from a region of higher concentration to a region of lower concentration.
Thus a movement of charge carriers takes place resulting in a current known as diffusion current

Drift Current

When a small electric field is applied across a semiconductor bar, the holes move in the direction of the applied field whereas the electrons move in a direction opposite to that of the applied field.
This combined effect of the movement of holes and electrons constitutes an electric current, which is known as drift current.