High Performance CMOS-Compatible RF GaN-on-Silicon HEMTs With Low-Resistive and Highly-Conformal Ohmic Contacts

This article presents Au-free ohmic contacts with low contact resistance for AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates, featuring a hybrid-deposited Si/Ti/Al/Ti/TiN metal stack and incorporating deep ohmic trenches. The proposed ohmic contact design achieved a low contact resistance of $0.12~\Omega \cdot \text {mm}$ and a smooth surface with a root mean square (rms) roughness of 5.68 nm concurrently. A high-resolution transmission electron microscope (HRTEM) reveals an excellent ohmic metal-semiconductor interface, and a contact mechanism for the optimized topological structure was proposed. Attributing to the excellent ohmic contact performance, the fabricated hybrid-deposited Si/Ti/Al/Ti/TiN ohmic contacts (HDOC) HEMTs exhibited a low on-resistance ( ${R}_{\text {on}}$ ) of $1.58~\Omega \cdot \text {mm}$ , a low knee voltage of 1.7 V, and a peak transconductance of 355 ms/mm. Moreover, the proposed ohmic topological structure improves the BV- ${R}_{\text {ON} }$ trade-off relationship, which enables low ${R}_{\text {ON} }$ at high operation voltage. RF small-signal measurements delivered a cut-off frequency ( ${f}_{\text {T}}$ ) of 27.8 GHz and a maximum oscillation frequency ( ${f}_{\max }$ ) of 51.4 GHz, which indicates a significant ${f}_{\max }$ improvement of 54.3% compared to the reference HEMT. These results are expected to enhance the high-performance and cost-effective CMOS-compatible GaN-on-Si HEMTs applied for RF applications.