N-Polar GaN HEMT With Deep Wet Etched Recess Demonstrating Record G-Band Performance at 170 GHz

In this paper, we report on record large signal performance at G-band (170 GHz) from novel N-polar GaN high-electron-mobility transistors (HEMTs) with Schottky-barrier TiN/Ru gate with enhanced gate aspect ratio. Unlike traditional N-polar GaN Schottky-gate HEMTs, this work establishes the Schottky barrier directly on GaN channel, as opposed to on AlGaN cap, by wet removal of AlGaN cap using citric acid. The developed novel process combines the wet selective GaN cap etch and wet AlGaN cap etch for a fully-wet deep recess etching. As a result, a HEMT with a gate length (LG) of 67 nm demonstrated a peak DC transconductance of 924 mS/mm with ~0.8 A/mm saturation current. The device demonstrated a near enhancement-mode (E-mode) behavior with +75 mV threshold voltage. The same transistor showed a cut-off frequency (fT) of 145 GHz with maximum oscillation frequency (fMAX) of 330 GHz. Large signal characterization at 170 GHz revealed a record performance of 1.1 W/mm with an associated power-added efficiency (PAE) of 11.9%, and 1 W/mm with 13.3% PAE on sapphire substrate, which demonstrates the best performance among the previously reported >150 GHz GaN results.