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what is the difference btwwn diffusion current and drift current?

A P type semiconductor of large concentration of holes and N type has large concentration of electrons. When they are in contact with with each other (as in a PN junction diode) there will be diffusion of holes towards the N block and electrons towards the P block. This happens because of the concentration difference. The diffused electrons make the P type negatively charged and the diffused holes make the N type positively charged. As a result an electric field is created near the junction (directed from N type to P type). This electric field prohibits the further diffusion of electrons and holes. Thus, no charge carrier can remain in a small region near the junction also the diffused holes and electrons have combined to make the region charge free. This region is called depletion region.

 

The current that is observed when the holes diffuse to N type and the electrons diffuse to P type is called diffusion current. Diffusion current is directed from P type to N type material.

 

Even after the depletion layer is formed, minority carriers (holes in N type and electrons in P type) enter the depletion region and drifts along the electric field. The minority carrier, electrons from P type move to the N type material and holes from N type move to P type material. This drift of minority carriers gives us a current from N type to P type which is called drift current. The drift current is also constituted by the electron-hole pairs that are created in the depletion region due to thermal collisions of atoms in the region. The electron and hole so appeared drifts on the influence of the electric field of the depletion region and gives us diffusion current.

 

Thus, diffusion current is the constituted by the diffusion of majority carriers across the junction. Diffusion current is from P type to N type material. Whereas, drift current is constituted by the minority carriers and is directed from N type to P type material.

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 Specifically, drift relates the movement of a carrier (e.g., an electron or hole) to an applied electric field (i.e., the velocity of the carrier is proportional to the electric field, where the proportionality constant, mobility, is a quantity derived in solid-state physics). Diffusion relates the movement of carriers due to random (i.e., thermal) behavior and non-uniform distribution (i.e., the velocity of the carrier is proportional to the logarithmic derivative of the density of carriers, where the proportionality constant, the diffusion constant, is a quantity derived in solid-state physics).

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Diffusion current is a current in a semiconductor caused by the diffusion of charge carriers while drift current is the electric current, or movement of charge carriers, which is due to the applied electric field, often stated as the electromotive force over a given distance.

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but drift current is also observed when no external source is provided..!
i m not getting.. it!
pls be a bit more precise..

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