Diffusion Current :
The carrier currents are also due to concentration gradients in the doped material which lead to diffusion of carriers from high concentration region to low concentration region. A concentration gradient exists if either the number of holes or electrons is greater in one region as compared to the rest of the region. When a concentration gradient of carriers exists in a material, the charge carriers tend to move from the region of high concentration to the region of lower concentration. The process is called the diffusion and the current due to this process is called the diffusion current. Let us assume that at reference plane x, the density of holes is p per m3. Let the carriers have a mean free path of Ax and mean free time of at between collisions.
Total Current in a semiconductor
It is possible that a potential gradient and a concentration gradient may exist within semiconductor. In such a case, the total current is the sum of drift current due to potential gradient and the diffusion current due to charge carrier concentration gradient.