The output conductance
, respectively the channel
resistance
are subject to a number of influences that must
be carefully separated. Special care has to be taken when
comparing DC-
and RF-
. DC-
values obtained
from the output characteristics can appear to be negative. A
first effect to be separated is the effect of self-heating and the
temperature dependence of the parasitic resistances, especially
the gate resistance
. Self-heating leads to a rise of
and thus to a reduction of the internally applied
bias, so self-heating causes an internal bias shift
[169]. A second effect is the influence of carrier
generation/recombination. As reported especially for GaN
HEMTs [81], but also for pseudomorphic
HEMTs [211], the occurrence of traps leads to a
hysteresis in the DC-output characteristics, an effect, which is
not seen in RF-parameters. The RF-value of
extracted of
measured data is therefore always positive.
In a compact current model extending one-dimensional charge models, a saturated velocity
model leads to a constant DC-output conductance as a function of
in the so-called saturation
region [95]. Introducing first order compact approaches [20] for impact
ionization leads to an exponential increase of the drain current
as a function of
bias.