Thermal Characterization and Simulation of GaN-on-SiC HEMT Transistors in Transient and Steady-State Regimes

The temperature of the channel significantly affects the performance of GaN HEMT transistors. In this study, we employed various methods, including three measurement techniques and Finite Element Method (FEM) simulations, to thermally characterize the $2 \times 150 \mu \mathrm{m}$ transistor transiently and in a steady-state regime. The transistor is a GaN HEMT from the UMS (United Monolithic Semiconductors) foundry. The transient analysis allowed the extraction of thermal time constants governing the temperature evolution within the transistor, while the steady-state study was used to measure the temperature as close as possible to the hotspot. Two electrical methods and one optical method are used. One of the electrical methods is based on Gate Resistance Thermometry (GRT) [1], [2], and the other relies on the On-State resistance (Ron) obtained from Id(Vgs, Vds) drain current characterization [2], [3]. The optical method uses the principle of reflectivity variation and is based on CCD-thermoreflectance. Measurement results were supported by nonlinear FEM simulations. The GRT electrical method [1] enabled the characterization of temperature distribution along the gates in the proximity of the hot spot, providing an average temperature over the gate width. The optical method, based on Thermoreflectance (TR) [4], [5], effectively measure the surface temperature of the component, particularly on the gates. These methods exhibited relative linearity in the variation of thermal resistance according to the applied dissipated power, with higher thermal resistance values compared to the conventional R
ON
extraction method [2]. It is remarkable that the values obtained from the optical method were consistent with those obtained using GTR electrical methods and FEM simulations. The transient study allowed the extraction of the thermal time constants according to which the temperature in the transistor evolves. Transistor measurements show that the transistor temperature varies according to five-time constants, and that the temperature varies in the same way in the channel as on the gates.