Experimental and theoretical studies of self-heating in III-N power high electron mobility transistors

III-Nitride (III-N) High Electron Mobility Transistors (HEMTs) are promising candidates for power electronics due to their high breakdown voltage and fast switching characteristics. However, their widespread deployment is hindered by reliability challenges linked to self-heating effects, especially in devices fabricated on thermally resistive substrates such as silicon and sapphire. In this work, we experimentally investigate self-heating in commercial GaN HEMTs through pulsed current–voltage (I–V) measurements and develop an extended Unified Charge Control Model (UCCM) that incorporates self-heating explicitly. We introduce a critical power parameter, P₀, which characterizes the onset of thermally induced degradation, and define self-heating figures of merit: SHFM_w (in Watts) and SHFM_wm (in Watts/mm, normalized by gate width). These metrics enable quantitative comparison of thermal performance across devices. Experimental results from three commercial HEMTs show SHFM_w values ranging from 21.2 W to 47.7 W, with normalized values converging around 0.4 W/mm. In contrast, previously reported single-finger-gate HEMT devices exhibit significantly higher SHFM_wm values, highlighting the critical influence of device geometry and substrate thermal conductivity. The model and metrics presented in this study offer a practical framework for evaluating and mitigating self-heating, with direct implications for device design and long-term reliability in GaN-based power electronics.