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The drift current, by contrast, is due to the motion of charge carriers due to the force exerted on them by an electric field. Diffusion current can be in the same or opposite direction of a drift current. The diffusion current and drift current together are described by the drift–diffusion equation. [1]
The drift velocity is the average velocity of the charge carriers in the drift current. The drift velocity, and resulting current, is characterized by the mobility; for details, see electron mobility (for solids) or electrical mobility (for a more general discussion). See drift–diffusion equation for the way that the drift current, diffusion ...
In a region where n and p vary with distance, a diffusion current is superimposed on that due to conductivity. This diffusion current is governed by Fick's law: = where: F is flux. D e is the diffusion coefficient or diffusivity; is the concentration gradient of electrons
The convection–diffusion equation can be derived in a straightforward way [4] from the continuity equation, which states that the rate of change for a scalar quantity in a differential control volume is given by flow and diffusion into and out of that part of the system along with any generation or consumption inside the control volume: + =, where j is the total flux and R is a net ...
drift of carriers, driven by the electric field, with electrons being pushed one way and holes the other way; diffusion of carriers from zones of higher carrier concentration to zones of lower carrier concentration (following a gradient of chemical potential). These two "forces" may work one against the other at any given point in the cell.
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This process generates current, referred to as diffusion current. Diffusion current can also be described by Fick's first law = /, where J is the diffusion current density (amount of substance) per unit area per unit time, n (for ideal mixtures) is the electron density, x is the position [length].
A current (due to the combined effects of drift and diffusion) will only appear if there is a variation in the Fermi or quasi Fermi level. The current density for electron flow can be shown to be proportional to the gradient in the electron quasi Fermi level.