Diffusion current

If the density of free charge carriers is not constant, then will these move due to the thermal motion of nature so that these concentration differences are compensated. By this diffusion occurs even without the presence of a force field, a charge carrier stream (electrical) diffusion current. This phenomenon occurs, for example in pn junctions and interferes with resulting space charges with the "normal " drift current.

The diffusion current density is proportional to the charge q of the individual charge carriers and the " slope " of the ( position-dependent ) charge carrier concentration n The proportionality factor is the diffusion coefficient D. This gives the vector form

Is the diffusion coefficient in semiconductors is dependent on the temperature and is proportional to the mobility of the charge carriers, and b so that depending on the material and the type of charge carriers. For electrons in silicon it is for example 35 cm ² / s

Since, in general, the carrier concentration does not change only in a linear direction, is virtually constant and the diffusion current. Because of the validity of the continuity equation this must be compensated by a field current. For example, a space charge built up of semiconductor junctions by the diffusion current, which in turn causes an electric field ( diffusion potential ) to form a drift current ( field current ) is produced, the " counter flow" of the diffusion current.

Therefore it applies the sum of current density drift and diffusion current density for the total current density as so-called transport equation