Impact of a thin AlN buffer layer on the characteristics of GaN high electron mobility transistors on a silicon substrate

The epitaxial structure of a GaN high electron mobility transistor (HEMT) on a silicon substrate typically involves a thick GaN buffer together with a graded AlGaN or superlattice transition layer (more than 1 μm). In this Letter, a thin epitaxial AlN buffer layer (200 nm) is proposed to replace the conventional GaN buffer and III-nitride (III-N) transition layer in GaN device epilayers grown on Si. GaN HEMTs with a gate length of 190 nm were fabricated. Compared to conventional GaN HEMTs on Si with thick GaN buffer/III-N transition layers, the proposed device featuring a thin AlN buffer shows a negligible kink effect in the dc output characteristics. The current collapse characterized by pulsed dc measurement is also significantly suppressed. The breakdown voltage is improved from 70 to 106 V. Infrared surface temperature measurements reveal that the thermal resistance of the thin AlN buffer device was 14.9% lower than that of the thick GaN buffer device. Additionally, a higher ratio of maximum oscillation frequency (fmax) to cutoff frequency (fT) indicates an enhanced output impedance. These results highlight the potential of the proposed GaN HEMTs with a thin AlN buffer for high-performance RF applications.