# A Thorough Evaluation of GaN HEMT Degradation under Realistic Power Amplifier Operation
## 4 Measurement Description  
The aim of this experiment is to verify how different operating conditions act on the device degradation. Therefore, we selected two samples from the same 0.15 \(\upmu\)m gallium nitride HEMT technology with 600 \(\upmu\)m total periphery and characterized each of them with a different stress condition. A preliminary characterization of the technology, which is not reported here for the sake of brevity, has shown an extremely low level of process dispersion between samples, which was necessary to guarantee a fair comparison between the selected samples. The investigated stress conditions were selected in order to emphasize two possible degradation mechanisms. The first one puts the device under thermal stress, by exploiting a large-signal condition with a high level of power dissipation. To this aim, we selected the tuned-load class-AB operation [35; 44] under saturated output power. The second one, on the contrary, has a lower level of power dissipation but forces the device to sustain high electric fields. That is a class-E condition [45; 46], which is characterized by a load line that reaches very high drain voltage values [47; 48]. We want to point out that the effects of high temperature and electric fields can be (and usually are) investigated by DC measurements. Nevertheless, that condition is not realistic, whereas the low-frequency characterization  
Figure 4: Typical sequence for a stress experiment. provides data under nonlinear dynamic operation that could be directly related to the microwave performance of the device. Each stress condition was initially applied to a fresh device for a total time of 60 h, keeping the backside temperature at 30 \({}^{\circ}\)C. The total stress time was divided into eight shorter sessions of different durations, with DC control measurements in between. Then, the same measurement was repeated on each sample with the same timings but whilst increasing the substrate temperature up to 100 \({}^{\circ}\)C.  
We report a summary of the measurement sequence in Table 1, whereas the details of the two stress conditions applied to the devices are reported in Table 2. The latter are the results acquired at the very beginning of the measurement at 30 \({}^{\circ}\)C, corresponding to the waveforms reported in Figure 5. Figure 5c clearly represents one of the advantages of the low-frequency characterization, i.e., the acquisition of the resistive gate current waveform. In the class-AB condition under saturated power, the forward conduction of the gate junction is evident, with a maximum instantaneous value of about 9 mA/mm. Under class-E operation, there is no forward conduction, but the drain voltage reaches a maximum value that is 63% higher with respect to the class-AB operation. The load lines synthesizing the two classes of operation, i.e., tuned-load class AB and class E, were not selected with the aim of designing best-in-class amplifiers, but in order to evidence the theoretical differences existing between the two classes. In particular, the investigated load lines enforced a nonlinear dynamic regime for the device, showing a different drain-gate voltage peak, and the conduction of the Schottky junction, as well as efficiency and output power level. The variations in the RF performance, expected during a degradation experiment, could be partly related to some drift of the load condition during such a long-lasting characterization. To manage this issue, the adopted load terminations were carefully