**4 Description of the Simulator**
**MINIMOS-NT**

Heterostructure devices, in particular HEMTs, consist of different
expitaxial layers with almost abrupt interfaces. To account for this fact
in the simulation the device is split into different geometric regions
called segments. In each segment different physical parameters and models
to describe the carrier transport in a volume can be used. As the gate
lengths of most HEMTs of interest are becoming well below a quarter micron
nonlocal behavior of the electron transport in the channel is getting extremely
important. Several attempts have been made to develop hydrodynamic (HD)
transport models suitable for device simulation [40,
41, 42,
43].
However, the calculations are very computational power consuming since
one more set of unknowns for each carrier type has to be solved compared
to the drift diffusion (DD) model. Additionally, convergence usually is
slower. Both effects can be minimized if the hydrodynamic transport model
is only applied to some semiconductor segments and DD to all others.

The discontinuous behavior of physical quantities at an interface between two different segments can be treated by interface models. This approach offers the possibility to include effects such as tunneling of electrons through energy barriers. Thus a thermionic field emission model for a heterojunction interface can be used [5].

In the following sections of this chapter the most important volume
and interface models of the simulator are described.

**Next:** 4.1 Volume Models **Up:**
Dissertation Helmut Brech** Previous:** 3.3
Determination of Capacitances by Quasi Static Approximation

Helmut Brech 1998-