Dear Syed,

The drift velocity depends on the mean free path l. Between 2 scattering events the electron is accelerated in the field E with a = e*E/m (e electron charge, m electron mass). Therefore, you can achieve a velocity v = (2*a*l)0,5 ( direction and start velocity not considered, nonrelativistic consideration).

• ideal case - undisturbed crystal: If the electron moves in the undisturbed periodic potential of the solid, the mean free path is infinitely. The electron is accelerated like an electron within a vaccuum field. The velocity is limited by the extension of the microcrystal
• the electron has interactions with phonons and charges, other elementary excitations: The mean free path can be determined by the mobility of the electron and depends essentially from temperature, doping and crystal quality.
• amorphes material: There is no periodicity of the potential. Every lattice point represents a scattering center. The mean free path is nearly defined by the atomic distance.

Here only velocities are considered which can be due to the effect of the electric field. In metals you must add the fermi velocity, in semiconductors the thermal velocity. The drift velocity may not be confused with the velocity in the ground state of the electron.

With Regard

R. Mitdank

Dear Syed,

I think the drift velocity defines the response of the mobile charge carriers to the electric field provided that they are free to move and wander in the material because of the thermal energy. It is said the particles are free. This happens if the material is crystalline.

If the material is amorphous the charge carriers are lying in shallow potential wells meaning they are not free to move in long ranges. At first one applies the field till they come out of the wells and become free. Therefore appreciable mobility is measured first after crossing the mobility gap.

After the mobility gap the amorphous material behave like the crystalline materials. Under this case bone can speak from saturation velocity.

As for the micrcrystalline materials they have their conduction mechanism which is drift in the bulk of the grains and themionic emission or tunneling across grain boundaries.

In summary you can not speak for a saturation velocity for all materials.

Best wishes