Diffusion current

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Above the pn junction before or during the diffusion process, below it in equilibrium (compensation of the diffusion current by drift current)

The diffusion current is an electrical current that arises - even without the presence of a force field - when free charge carriers move due to their heat movement in such a way that any differences in concentration are compensated ( diffusion ). Such a non-constant density of free charge carriers occurs, for example, at pn junctions of semiconductors.

The diffusion current density

is proportional

  • to the charge  q of the individual charge carriers and
  • to the “ gradient vector ” of the (location-dependent) charge carrier concentration

With

Since, in general, the charge carrier concentration does not only change linearly in one direction, the diffusion current is also practically not constant.

The diffusion current is balanced by a field current , as described by the continuity equation. For example, a space charge builds up on semiconductor boundary layers due to the diffusion current , which in turn creates an electric field ( diffusion voltage ). This creates a drift current (also called field current) which “flows against” the diffusion current.

The transport equation thus applies to the total current density as the sum of field current density and diffusion current density :

literature

Individual evidence

  1. Reinhold Paul: Transistors . Verlag Technik, Berlin 1969.