Illinois ECE 440 Solid State Electronic Devices, Lectures 8 and 9: Drift Mobility

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Carrier Mobility and Drift

ECE 440: Lectures 8-9
Carrier Mobility and Drift

Let’s recap the 5-6 major concepts so far:

Memorize a few things, but recognize many.
(why? semiconductors require lots of approximations)

Why all the fuss about the abstract concept of EF?
Consider (for example) joining an n-doped piece of Si with a p-doped piece of Ge. How does the band diagram look?

So far, we’ve learned the effects of temperature and doping on carrier concentrations.

But no electric field = not useful = boring materials.

The secret life of C-band electrons (or V-band holes): They are essentially free to move around at finite temperature & doping. So what do they do?

Instantaneous velocity given by thermal energy:

Scattering time (with what?) is of the order ~ 0.1 ps.

So average distance travelled between scattering: L ~

But on average, this electron goes: _________________
So turn ON an electric field:

F = ± qE

F = m*a  a =

Between collisions, carriers accelerate along E field:

vn(t) = ant = for electrons
vp(t) = apt = for holes

In the energy band picture this looks like:

On average, velocity is randomized again every


University of Illinois at Urbana-Champaign ECE 440: Solid State Electronic Devices

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Researchers should cite this work as follows:

  • Eric Pop (2009), "Illinois ECE 440 Solid State Electronic Devices, Lectures 8 and 9: Drift Mobility,"

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