Especially for W-band applications at 77 GHz, the understanding of the process variations is
desirable as a function of as many parameters as possible due to the gate length
required.
The absolute changes of transistors are relatively more important due to the scaled geometries and
frequencies than for larger gate-lengths necessary e.g. for 2 GHz applications. This is especially
true for the small- and large-signal performance. As was shown in Fig. 7.2, the output
characteristics including self-heating gives the necessary agreement to allow for such a study.
The ultimate verification of a HEMT simulation model assuming a specific equivalent circuit such
as given in Fig. 4.1, is the comparison with bias dependent S-parameter measurements. Except
from thermal and frequency dispersive effects, the bias dependence of the S-parameters describes
large-signal operation completely. Simulation allows to extract the physical sources of the bias
dependence of the extracted circuit elements. Fig. 7.7 and Fig. 7.8 show
simulated and measured S-parameters for two different biases and temperatures.
Fig. 7.7 presents S-parameters between 0.5 GHz and 50 GHz for
for
and
= 1.5 V. An overall good agreement is found between simulation and measurements
at
= 373 K. The agreement is similar to the agreement presented in Fig. 4.5 for
= 300 K. Fig. 7.8 shows the simulated and measured S-parameters for the transistor
at pinch-off for
= 2 V and
= -0.5 V, measured and simulated between 1 GHz and 120 GHz
for
= 300 K. Also the behavior of the pinched-off device is predicted correctly.
and
merge to the symmetrical passive behavior. For
and
an asymmetry is seen in the simulation, which is due to a slight deviation of the
simulated and measured currents for subthreshold bias.