Submicron local and time-dependent thermal resistance characterization of GaN HEMTs

This paper presents an optical thermal characterization of a microwave power device with submicron features using thermoreflectance imaging. Recent advanced gallium nitride (GaN) technology is the basis for a microwave power amplifier consisting of an array of high electron mobility transistors (HEMTs) for very high frequency operation. With a micron or narrower scale gate and surrounding submicron features, a significantly high density of heat is generated. It occurs especially in a 2D electron gas channel consisting of aluminum gallium nitride (AlGaN) layer beneath the gap between the gate and drain. Due to the relatively long span gate finger width (longitudinal length ~100 μm) with very large aspect ratio, some non-uniformity in thermal resistance along the line may occur. The objective region comprises multiple materials with submicron features having different reflective properties as a function of illumination wavelength. A previously developed hyperspectral full-band wavelength thermoreflectance imaging technique enabled an accurate characterization of the time-dependent temperature distribution to a pulse input along the gate finger. The localized and time-dependent thermal resistance helped further characterization of the thermal impact by utilizing a Field Plate (FP) on top of the gate line, which is known to improve the quality of the waveform from GaN HEMT power amplifiers. It showed only a minor increase of the local thermal resistance at the drain side in a submicron gap with the FP device. However, the method of hybrid analytic modeling also showed the potential extension of transient thermal design and analysis of GaN HEMT devices.