Recess-Free Nonalloyed Ohmic Contacts on Graded AlGaN Heterojunction FETs

We report on a new approach to achieve nonalloyed ohmic contacts with record low resistance in GaN-based transistors. We use upward and reverse grading of Al composition in a polarization-graded field-effect transistor structure to eliminate abrupt heterojunction band offsets and create direct contact to the channel. Total contact resistance of 73 mQ · mm and interfacial resistance of 29 mQ · mm to the 2-D electron gas were obtained. The method adopted here could enable to ultralow-resistance contacts without ohmic recess.