By taking only the first two moments, eqns. (2.99) and
(2.102), into account
and closing the equation system at
the drift-diffusion transport model is obtained. Since no information about the carrier
temperature is available, the carrier temperature is set equal to the lattice temperature
, assuming the thermal equilibrium approximation .
This transport model takes only local quantities into account. As such, it completely
neglects non-static transport effects which occur in response to a sudden variation of the
electric field, either in time or in space.
In order to enhance the validity range of the drift current expression, a field dependent
mobility2.10 is generally used, accounting for
hot-carrier effects. Diffusivity, however, is largely underestimated with the
EINSTEIN equations, if the lattice temperature rather than the carrier temperature
is being used [7, p.145].
If needed, the average energy can be estimated via the homogeneous energy balance
However, for rapid increasing electric fields, for instance, the average energy lags behind
the electric field. As a consequence the average energy can be considerably smaller than the
one predicted by the homogeneous energy balance equation
(2.146). Another important consequence is that the lag
of the average energy gives rise to an overshoot in the carrier velocity. The reason for this
velocity overshoot is that the mobility depends to first order on the average energy
rather than on the electric field. As the mobility has not yet been reduced by the increasing
energy but the electric field is already high, an overshoot in the velocity
is observed until the carrier energy comes into equilibrium with the electric field
again. One of the first works dealing with this effect is . Non-local effects
like this one cannot be modeled using the drift-diffusion transport model.
M. Gritsch: Numerical Modeling of Silicon-on-Insulator MOSFETs PDF