*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.
- 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 = – eD_{h} ( 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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