Transient thermal dynamics of GaN HEMTs

Although GaN high electron mobility transistors (HEMTs) are one of the most promising semiconductor technologies for high power and high frequency applications, high device temperatures often lead to degraded performance and reliability. Most reports on the thermal characterization of GaN HEMTs have focused on the steady state temperature rise in spite of the prevalence of time-dependent power dissipation in aerospace, defense, and communications applications. In this work, we utilize analytical solutions to the transient heat conduction equation to investigate the thermal time constants associated with self-heating in GaN HEMTs. In contrast to previous reports and commonly held notions in the GaN device industry, we demonstrate that one or two thermal time constants does not adequately describe transient self-heating. Due to aggressive heat spreading from the small heat source in GaN HEMTs, a wide range of thermal time constants up to ≈10 ms are important for devices on sapphire substrates. These theoretical arguments are supported with transient temperature measurements obtained by time-resolved micro-Raman spectroscopy for a GaN-on-sapphire ungated HEMT.