# 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

## Diffusion Current

- 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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