Mobility parameters

Specifically for Si,

N = dopant concentration i.e at room temperature

are determined experimentally

Parameters for Si:

Parameters for GaAs:

Temperature Dependence

Again from experiment we find  all have a temperature dependence of the form                                                                     

For Si we have:

GaAs behavior slightly different

High field effect

We know that drift velocity

But linear proportionality is valid only at low temperatures.

For E field intensity and E are no longer directly proportional. Nonlinear behavior is seen.

Now geometry is often small (submicron) so even with 1 V across dimension,

So mobility concept may fail in such high field zones.

For very high E field saturates.

For Si at 300 K, for both electrons and holes.

Model,

For GaAs initially decreases with E after a critical field is 2 10 3 v/cm and then very slowly increases, In a region of high which can be considered as saturated velocity .

However, in GaAs for holes the velocity saturates and Independent of E field. The variation of the drift velocity with electric field for holes and electrons are shown in Fig 5.8 and 5.9