## 3 Experimental Results and Discussion
Figure 8: Measured (blue symbols) and equivalent-circuit simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}=30\) V, \(V_{GS}=-3.1\) V, and \(T_{a}=145\)\({}^{\circ}\)C.  
Figure 6: Measured (blue symbols) and equivalent-circuit simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}=30\) V, \(V_{GS}=-3.1\) V, and \(T_{a}=35\)\({}^{\circ}\)C.  
Figure 7: Measured (blue symbols) and equivalent-circuit simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}=30\) V, \(V_{GS}=-3.1\) V, and \(T_{a}=90\)\({}^{\circ}\)C.  
Figure 11: Measured (blue symbols) and ANN simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}\) = 30 V, \(V_{GS}\) = \(-\)3.1 V, and \(T_{d}\) = 90 \({}^{\circ}\)C.  
Figure 10: Measured (blue symbols) and ANN simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}\) = 30 V, \(V_{GS}\) = \(-\)3.1 V, and \(T_{a}\) = 35 \({}^{\circ}\)C.  
Figure 9: Measured (blue symbols) and equivalent-circuit simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}\) = 30 V, \(V_{GS}\) = \(-\)3.1 V, and \(T_{a}\) = 200 \({}^{\circ}\)C.  
Figure 14: Measured (blue symbols) and GRU simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}=\) 30 V, \(V_{GS}=-\)3.1 V, and \(T_{a}=\) 35 \({}^{\circ}\)C.  
Figure 4:0 Measured (blue symbols) and ANN simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}=\) 30 V, \(V_{GS}=-\)3.1 V, and \(T_{a}=\) 145 \({}^{\circ}\)C.  
Figure 4:1 Measured (blue symbols) and ANN simulated (red lines) behavior of (**a**) \(S_{11}\), (**b**) \(S_{12}\), (**c**) \(S_{21}\), and (**d**) \(S_{22}\) with the frequency range spanning from 200 MHz to 65 GHz for the DUT at \(V_{DS}=\) 30 V, \(V_{GS}=-\)3.1 V, and \(T_{a}=\) 200 \({}^{\circ}\)C.  
As illustrated in Figures 6-9, a quite good agreement between measurements and simulations is achieved by using the equivalent-circuit model. As can be observed from Figures 10-13, a better agreement is obtained by using the ANN-based model, which has good learning ability and generalization capability, enabling the achievement of accurate predictions even for data different from those used in the training phase (i.e., at the ambient temperature of 145 \({}^{\circ}\)C). Figures 14-17 show that a very good agreement is also obtained by using the GRU-based model, which allows for achieving a faithful reproduction of the measured \(S\)-parameters.