Diffusion current

Diffusion current is a current in a semiconductor caused by the diffusion of charge carriers (electrons and/or electron holes). This is the current which is due to the transport of charges occurring because of non-uniform concentration of charged particles in a semiconductor. 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.

It is necessary to consider the part of diffusion current when describing many semiconductor devices. For example, the current near the depletion region of a p–n junction is dominated by the diffusion current. Inside the depletion region, both diffusion current and drift current are present. At equilibrium in a p–n junction, the forward diffusion current in the depletion region is balanced with a reverse drift current, so that the net current is zero.

The diffusion constant for a doped material can be determined with the Haynes–Shockley experiment. Alternatively, if the carrier mobility is known, the diffusion coefficient may be determined from the Einstein relation on electrical mobility.

Overview

Diffusion current versus drift current

Further information: Drift current

The following table compares the two types of current:

Diffusion current	Drift current
Diffusion current = the movement caused by variation in the carrier concentration.	Drift current = the movement caused by electric fields.
Direction of the diffusion current depends on the slope of the carrier concentration.	Direction of the drift current is always in the direction of the electric field.
Obeys Fick's law: $J=-qD{\frac {d\rho }{dx}}$	Obeys Ohm's law: $J=q\rho \mu E$

Carrier actions

No external electric field across the semiconductor is required for a diffusion current to take place. This is because diffusion takes place due to the change in concentration of the carrier particles and not the concentrations themselves. The carrier particles, namely the holes and electrons of a semiconductor, move from a place of higher concentration to a place of lower concentration. Hence, due to the flow of holes and electrons there is a current. This current is called the diffusion current. The drift current and the diffusion current make up the total current in the conductor. The change in the concentration of the carrier particles develops a gradient. Due to this gradient, an electric field is produced in the semiconductor.

See also

• Alternating current
• Conduction band
• Convection–diffusion equation
• Direct current
• Drift current
• Free electron model
• Random walk
• Maximal Entropy Random Walk – diffusion in agreement with quantum predictions

References

• Encyclopaedia of Physics (2nd Edition), R.G. Lerner, G.L. Trigg, VHC publishers, 1991, ISBN (Verlagsgesellschaft) 3-527-26954-1, ISBN (VHC Inc.) 0-89573-752-3
• Concepts of Modern Physics (4th Edition), A. Beiser, Physics, McGraw-Hill (International), 1987, ISBN 0-07-100144-1
• Solid State Physics (2nd Edition), J.R. Hook, H.E. Hall, Manchester Physics Series, John Wiley & Sons, 2010, ISBN 978 0 471 92804 1
• Ben G. Streetman, Santay Kumar Banerjee; Solid State Electronic Devices (6th Edition), Pearson International Edition; pp. 126–135.
• "Differences between diffusion current". Diffusion. Archived from the original on 13 August 2017. Retrieved 10 September 2011.
• "Carrier Actions of Diffusion Current". Diffusion. Archived from the original on 10 August 2011. Retrieved 11 October 2011.
• "derivation of difusion current". Archived from the original on 14 December 2011. Retrieved 15 October 2011.