Definition of Diffusion

It is defined as the flow of charge from high density region to low density region without any external applied field.
The flow of charge is due to non - uniform distribution of charge carriers in the semiconductor crystal.
This will result in flow of current without any external applied field.

Diffusion Constant

The flow of charge is directly proportional to carrier density gradient, the constant of proportionality is called as diffusion constant or diffusion coefficient D.

Current density due to hole diffusion

Jh = – eDh ( dp / dx )

Current density due to electron diffusion

Je = – eDe ( dn / dx )

Where

D_h = Hole diffusion constant

D_e = Electron diffusion constant

dp / dx = Density gradient of holes

de / dx = Density gradient of electrons

The diffusion depends upon the presence of space gradient of charge density whereas the drift current is a function of both electric field and charge density.
The diffusion leads to redistribution of charges which develop potential difference in the semiconductor.
The electrical field due to this potential difference sets up drift current which opposes diffusion current.
The drift current is equal to diffusion current resulting net current becomes zero after final equilibrium condition.

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