# A Thorough Evaluation of GaN HEMT Degradation under Realistic Power Amplifier Operation
## 2 Characterization Technique  
The proposed technique for evaluating the degradation effects on microwave transistors is based on the exploitation of low-frequency time-domain load-pull measurements. The term \"low frequency\" as defined herein refers to an operating frequency in the megahertz range, at which the microwave device behaves approximately as a \"resistive\" component, since the reactive effects associated with the parasitic elements and the intrinsic nonlinear capacitances produce negligible effects (Figure 1) [33]. This enables the access to electrical quantities that cannot be directly monitored under a microwave regime, such as the resistive gate current, i.e., the current flowing through the gate Schottky junction in a HEMT when excited under forward and/or reverse conduction. This quantity is usually hidden at microwave frequencies due to contributions of the reactive currents, which are orders of magnitude higher, leaving the average gate current as the only indicator related to the Schottky junction conduction regime. Nevertheless, this might lead to misleading information. As an illustrative example, Figure 2 shows two different gate current waveforms having the same average value but representing very different conditions. The blue waveform shows only the forward conduction of the Schottky junction (i.e., the positive current), whereas the red one also exhibits the reverse conduction, with a consequent increase of the forward current to keep the average value constant. Clearly, the latter is a more critical condition for the device, but it is impossible to infer only based on the average current. Operating at a low frequency also enables the direct access to the so-called current-generator plane of the transistor, i.e., the intrinsic active part of the device. Power amplifier design techniques are generally related to this reference plane, defining the waveforms and/or the load terminations that the transistor current generator must experience to implement a specific class of operation [34; 35]. The direct access to this plane enables the possibility of accurately controlling and monitoring the operating condition of the device by properly setting the input signals and the load conditions. The choice of an operating frequency in the megahertz range for the characterization of microwave devices also allows one to account for the so-called low-frequency dispersion phenomena, i.e., the effects of thermal and trapping phenomena, which influence the electrical characteristics of the transistor [36; 37; 38; 39; 40]. Thermal effects depend on the junction temperature, i.e., the temperature that the device channel reaches during operation because of the power dissipation. Trapping phenomena are related to the presence of electron traps into the layers of material composing the transistor and consisting of energy levels that the carriers can occupy, being subtracted from conduction. These phenomena depend on the electric fields applied to the device, which are strictly related to the operating condition, especially in GaN devices where fast-trapping is an unavoidable issue connected to the maximum field instantaneously applied to the transistor [41]. Thermal and trapping effects are intrinsically intertwined [39; 42; 43] and are characterized by time constants relatively  
Figure 1: Equivalent-circuit model of a microwave field-effect transistor operating at a low frequency. Figure 2: Examples of gate current waveforms having the same average value but representing very different conditions: forward and reverse conduction (red line), and only forward conduction (blue line). low when compared to the timing of a microwave signal [33; 36; 39; 40], as well as that of the low-frequency characterization. Therefore, the data gathered from the low-frequency measurements are consistent with the microwave behavior of the device, being the cut-off frequency of these effects below a few megahertz.