For MESFETs and HEMTs the well known intrinsic eight-element small-signal equivalent circuit is shown in Fig. 4.1, as published in . The extraction of parasitic elements from measurements is performed according to standard procedures [36,230] which are described below. Contrary to the classical separation of the intrinsic device and the parasitic environment, Fig. 4.1 additionally shows a simulator shell, which includes the intrinsic shell and the parasitic resistances. This is useful as the semiconductor contributions to the resistances are a genuine part of the simulation domain. However, parasitic capacitances and inductances can better be obtained from gate width dependent measurements, so that they are considered extrinsic for the physical simulation. Extended equivalent circuits were suggested, e.g. by  to include leakage and impact ionization phenomena in InAlAs/InGaAs based HEMTs.
Fig. 4.2 shows an extended equivalent circuit for a mm-wave HEMT. In the input
circuit two resistances
are added to account for the gate leakage. In the
output circuit three additional elements are added:
due to impact ionization and the combination of
additional time constant explained below. Genetic algorithms were suggested in  to
determine small-signal equivalent elements, and are also used in this work. This circuit, usually
applied for noise modeling, is used to extract breakdown information from measured S-parameters
bias in combination with standard procedures.
In a one-dimensional linear charge control model, as given in , the
is described as:
As can be seen from (4.1), both a relatively higher sheet density
higher effective carrier
velocity lead to higher
. This explains the differences of
the material systems AlGaAs/InGaAs and InAlAs/InGaAs. As an example, InAlAs/InGaAs transistors have
both a higher sheet density and higher effective carrier velocities than AlGaAs/InGaAs HEMTs.
Furthermore, the difference of DC- and RF-
can be explained due carrier
generation/recombination resulting in changes of
. Neither velocity saturation nor parasitic
charge modulation are included in this model. Describing velocity saturation a term called
modulation efficiency (ME) can be derived writing an intrinsic delay time