Carrier Mobility and DriftECE 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