# A Thorough Evaluation of GaN HEMT Degradation under Realistic Power Amplifier Operation
## 5 Experimental Results
### Tuned-Load Class-AB Stress at 100 \({}^{\circ}\)C  
After the class-AB stress measurement at 30 \({}^{\circ}\)C was completed, it was repeated on the same device while raising the backside temperature to 100 \({}^{\circ}\)C. By doing this, we expected an acceleration in the degradation phenomena. The results of this characterization are reported in Figures 12 and 13. It is immediately visible that the drain current was lower as compared to the previous cases, which is expected because of the reduction of the carrier mobility due to the higher temperature. During the stress, the load line showed a bigger variation, leading to a reduction in the output power and drain efficiency, respectively from 4.80 W/mm to 4.55 W/mm (\(-\)5%) and from 63.6% to 62.0%, as illustrated in Figure 12j.  
Figure 11: Complete I-V characteristics measured before (black dashed lines) and after (red and blue continuous lines) the 60 h stress. Data collected for the (**a,c**) class-AB stress and (**b,d**) class-E stress.  
Figure 10: Comparison between the degradation achieved from the class-AB (red continuous lines) and the class-E (blue dashed lines) stress conditions. Variations, along 60 h of stress, of (**a**) output power, (**b**) average drain current, (**c**) drain efficiency, (**d**) threshold voltage at \(I_{D}\) = 100 mA/mm, (**e**) saturated drain current at \(V_{G}\) = 2 V and \(V_{D}\) = 4 V, and (**f**) gate leakage current at \(V_{G}\) = \(-\)4 V and \(V_{D}\) = 30 V.  
Interesting results can be seen in the behavior of the gate port. First, the maximum value of the gate current waveform was significantly reduced with respect to the 30 \({}^{\circ}\)C case, which was, again, expected because of the higher temperature. The peak on the current reduced with stress time from 3.4 mA/mm to 2.3 mA/mm (\(-\)15%), although it seemed to settle after 10 h (Figure 12f), which is confirmed by the data related to the average gate current (Figure 12g). We also note that, during the stress, the shape of the current waveform changed, and a small current appeared when the Schottky junction operated below its threshold. As the gate current is a good indicator of the degradation of the transistor, this clearly confirms that the state of the device changed. We also point out that this kind of variation would be unknown if the measurements were carried out at microwaves frequencies.  
Figure 12: Stress measurement results for the 2 MHz tuned-load class-AB condition at 100 \({}^{\circ}\)C: (**a**) load lines vs. initial DC I/V characteristics (grey lines), (**b**) drain voltage waveforms, (**c**) drain current waveforms, (**d**) input loci, (**e**) gate voltage waveforms, (**f**) gate current waveforms with the evolution of its maximum instantaneous value in the inset, (**g**) average gate current, (**h**) average drain current, (**i**) output power, and (**j**) drain efficiency. From (**a**–**f**), the black dashed lines represent the initial state of the DUT, whereas the red continuous lines represent the final state of the DUT.  
The data concerning the DC control measurements are presented in Figure 13. Again, we observed a reduction in the saturated drain current from 720 mA/mm to 690 mA/m (\(-4.2\%\), Figure 13b), whereas the device threshold voltage changes during the stress but after 60 h settles on approximately the same value measured before the stress.