Experimental Characterization of the Thermal Time Constants of GaN HEMTs Via Micro-Raman Thermometry

Gallium nitride (GaN) high-electron mobility transistors (HEMTs) are a key technology for realizing next generation high-power RF amplifiers and high-efficiency power converters. However, elevated channel temperatures due to self-heating often severely limit their power handling capability. Although the steady-state thermal behavior of GaN HEMTs has been studied extensively, significantly fewer studies have considered their transient thermal response. In this paper, we report a methodology for measuring the transient temperature rise and thermal time constant spectrum of GaN HEMTs via time-resolved micro-Raman thermometry with a temporal resolution of 30 ns. We measured a broad spectrum of time constants from <inline-formula> <tex-math notation="LaTeX">$\approx 130$ </tex-math></inline-formula> ns to <inline-formula> <tex-math notation="LaTeX">$\approx 3.2$ </tex-math></inline-formula> ms that contribute to the temperature rise of an ungated GaN-on-SiC HEMT due to aggressive, multidimensional heat spreading in the die and die-attach. Our findings confirm previous theoretical analysis showing that one or two thermal time constants cannot adequately describe the transient temperature rise and that the temperature reaches steady-state at <inline-formula> <tex-math notation="LaTeX">$\approx {16}L^{{2}}/\pi ^{{2}}\alpha $ </tex-math></inline-formula>, where <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${\alpha }$ </tex-math></inline-formula> are the thickness and thermal diffusivity of the substrate. This paper provides a practical methodology for validating transient thermal models of GaN HEMTs and for obtaining experimental values of the thermal resistances and capacitances for compact electrothermal modeling.

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