Very Low Ohmic Contact Resistance through an AlGaN Etch-Stop in Nitrogen-Polar GaN-Based High Electron Mobility Transistors

Abstract

Ultra-low ohmic contact resistance of 0.1 Ω mm has been obtained as a step towards a deep-recess structure using N-polar GaN-based high electron mobility transistors (HEMTs). An AlGaN etchstop layer was investigated to obtain smooth and reliable gate recess. However due to reverse polarization, AlGaN results in a polarization-induced Schottky barrier which prevents ohmic contact to the channel through the etchstop. In this work, we have proposed a novel methodology to contact the two-dimensional electron gas (2DEG) by etching through the GaN cap and the AlGaN etchstop to eliminate the barrier and angular-evaporation of metals to achieve side-alloying resulting in very low ohmic contact resistance of 0.1 Ω mm achieved to N-polar GaN 2DEG. This result is state-of-the-art for alloyed contacts achieved to GaN-based 2DEG.