## 1 Introduction  
The demand for transistors operating at frequencies exceeding 300 GHz is witnessing a notable surge across an array of sectors, encompassing next-generation communication, medical imaging systems, Internet of Things (IoT) applications, radar technologies, and satellite communication systems. Notably, high-electron-mobility transistors (HEMTs) featuring InGaAs channels have garnered considerable attention due to their remarkable properties, such as high electron mobility (\(\mu_{n,\mathit{off}}\)) and virtual source injection velocity (\(v_{inj}\)) [1]. This renders them exceptionally suitable for driving forward the realm of terahertz communication and serving as proficient low-noise amplifiers. The unparalleled high-electron-mobility properties of these devices have led to pseudomorphic high-electron-mobility transistors (pHEMTs) on InP substrates, achieving remarkable milestones, boosting cut-off frequencies (\(f_{T}\)) and maximum oscillation frequencies (\(f_{max}\)) surpassing the 700 GHz threshold. This achievement of reaching among the highest values for field effect transistors owes its success to the inherently elevated high electron mobility characteristics [2, 3, 4]. While InGaAs HEMTs have undeniably showcased impressive performance benchmarks on InP substrates, these substrates have their own limitations. Their relatively diminutive size, often measuring 3 to 4 inches, carries certain economic disadvantages owing to the exorbitant production costs associated with such dimensions. Additionally, their inherent fragility compounds these limitations. Furthermore, the process technology aligned with these smaller-sized wafers remains incomplete, translating to lower yields. In a bid to surmount these challenges, researchers have diligently worked on the development of metamorphic HEMTs (MHEMT) on GaAs substrates. The rationale behind this lies in the manifold advantages presented by GaAs substrates, such as their cost effectiveness facilitated by large-volume manufacturing, augmented mechanical robustness, and the potential for scaling the substrate size to 6 inches. Various studies have been conducted on MHEMT, which offers numerous advantages in substrates. These numerous studies have resulted in MHEMTs exhibiting DC and RF performance equivalent to the pHEMTs achieved through various buffer structures [5; 6; 7]. Since both devices have shown high RF performance, they were expected to find applications in various fields requiring such capabilities. Among these, especially for commercial applications like low-noise and high-power millimeter-wave amplifiers, the reliability performance of pHEMTs and MHEMTs has been a significant concern. The degradation mechanisms in pHEMTs and MHEMT on InP and GaAs substrates have been studied, taking into account factors such as the diffusion of the gate metals [8; 9; 10], fluorine contamination [11], hydrogen absorption into Ti metallization [8; 12], corrosion associated with Al oxidation [10], gate sinking [13; 14], and ohmic contact degradation [13; 15]. These studies have mainly discussed the impact of changes in the process. Apart from this, research has predominantly centered on degradation mechanism analysis, including device lifetime and property degradation due to stress. Additionally, representative studies have been conducted on high-temperature operating-lifetime tests that cause a rapid decrease in transconductance, hot carrier-induced impact ionization at the gate, or bias stressing [8; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. During this research, factors related to lifetime and bias stress evaluation become crucial in assessing the reliability performance, especially when considering the operating environment of actual devices. However, it is essential to note that a significant portion of the research, excluding this, has primarily concentrated on reliability evaluation within various fabrication processes, without a specific focus on differences in device structure or substrate.