UV Thermal Imaging of RF GaN Devices with GaN Resistor Validation

Shrinking features and growing device complexity in advanced microwave devices has increased the challenge of fully understanding device thermal behavior on the sub-micron scale. Predicting the device static and dynamic thermal behavior is essential for ensuring optimal tradeoffs between performance and device reliability. Thermal imaging based on the Thermoreflectance Principle can meet the challenges imposed by these compact, high power density RF devices by providing submicron spatial resolution and temporal resolution in the picosecond range. This technique overcomes the limitations of traditional thermal imaging techniques such as IR and Micro-Raman and some of the specific challenges in measuring GaN devices. In the past, Thermoreflectance Imaging has been shown to accurately estimate the temperature rise of metals using visible wavelength excitation sources. This paper presents a novel method to estimate the temperature directly on GaN surfaces using UV wavelengths. These UV thermoreflectance measurements were verified with measurement of an on-chip GaN mesa resistor. Temperature measurements on top of the field plate and inside the GaN channel were compared for a commercial GaN HEMT both on Si and SiC substrate. The advantages and disadvantages will be presented for the thermoreflectance technique for thermal imaging.