Electrothermal Evaluation of AlGaN/GaN Membrane High Electron Mobility Transistors by Transient Thermoreflectance

A novel wet etch process for fabrication of large-area AlGaN/GaN membranes is reported, along with an evaluation of membrane-high electron mobility transistor (HEMT) electrothermal performance up to 1.9 W/mm. Hall measurements showed negligible post-etch change in membrane-HEMT sheet resistance, Hall mobility and carrier concentration. Static (dc) current-voltage characteristics showed negligible change in on resistance (R-O(N)), although I-DS,I-MAX was significantly reduced due to increased self-heating in the absence of the Si substrate. Pulsed output characteristics were similarly affected as self-heating was expected to be still present at ms pulse widths. In the off state, the drain leakage current was measurably lower by about an order of magnitude. Pulsed-mode off-state step stress showed a dynamic on resistance improvement by about a factor of 2 when both sides of the membrane were passivated by SiN. A peak temperature of 148.5 degrees C was measured on the membrane HEMT using transient thermoreflectance imaging. These initial results indicate that substrate removal does not necessarily cause device degradation, and can be a promising step in improving HEMT reliability in future generations of power devices.