# A Thorough Evaluation of GaN HEMT Degradation under Realistic Power Amplifier Operation
## 1 Introduction  
The unique properties of Gallium Nitride (GaN) technology make it one of the enabling factors pushing forward the development of microwave electronics, ranging from mobile communication and radar systems to satellite communications [1, 2, 3, 4, 5, 6, 7]. The success of this technology lies in the physical properties of GaN [8, 9], such as its high electron mobility and the wide bandgap. These allow high-frequency operation, high power density, and the ability to sustain high electric fields, providing intrinsic robustness to the devices [10]. In this context, the analysis of the technology reliability plays a fundamental role to identify and understand the mechanisms of degradation of the electron devices that can lead to a limited performance or even to a premature failure, especially when the technology is pushed towards its limits to fulfill the challenging specifications of the demanding emerging applications. DC stress tests are a well-established technique adopted for investigating technology reliability [11, 12, 13, 14, 15]. In these tests, a high-voltage bias point is applied to the device, sometimes gradually, to activate specific degradation mechanisms, such as hot-electrons (i.e., carriers with a mean kinetic energy greater than the lattice thermal energy [16]), a typical issue in GaN devices [17, 18], especially under large-signal dynamic operation, when the transistor continuously switches between the on and the off state [19]. This type of experiment is easy to implement since it requires simple instrumentation and providesaccurate results. Nevertheless, if we consider the typical applications of microwave devices, DC operation is far from being a realistic condition, and RF stress tests are usually required [20; 21; 22; 23]. It has been demonstrated that the application of a realistic stress condition can lead to very different results with respect to DC stress tests because of the degradation mechanisms that can be triggered or accelerated by RF signals [24; 25]. Although RF tests provide a better understanding of the device reliability issues, the setups required for this characterization present some limitations. First, their complexity is higher than that of DC setups, which directly impacts the accuracy of the measurements [26; 27; 28]. Indeed, proper calibration procedures are required at RF [28; 29; 30], and they need to remain valid for the entire duration of the stress tests, which can reach hundreds or even thousands of hours. Second, the need of RF instrumentation increases the overall costs of the experiments, considering that such long-lasting measurements require dedicated setups, which become unavailable for other activities when stress tests are ongoing. Another limitation is related to the levels of power manageable at RF. In fact, if dealing with complete circuits, e.g., power amplifiers, could be relatively easy at high frequencies, the investigation of the technology's reliability issues needs to be performed at the transistor level, which can pose significant challenges when large peripheries need to be characterized, in terms of both power management and stability [27; 29]. In this paper, we propose a characterization technique for reliability investigation that tries to overcome the aforementioned limitations, whilst, nevertheless, providing accurate and consistent results directly comparable with RF stress results. It is based on a time-domain load-pull setup [31] operating at a few megahertz, that mixes the advantages of low-frequency measurements and the possibility of stressing the device under test setting realistic operating conditions, thereby enabling access to data not directly available at microwaves [32; 33].