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.