Ultrathin fully strained GaN channel high electron mobility transistors on AlN-on-sapphire templates engineered by ferroelectric ScAlN epilayers

In this work, we demonstrate fully strained GaN channel high electron mobility transistors (HEMTs) with superlattice barriers on AlN-on-sapphire templates by plasma-assisted molecular beam epitaxy. Here, ferroelectric Sc0.18Al0.82N epilayers are introduced into the heterostructure to engineer the device performance. Instead of the conventionally thick AlN buffer, an ultrathin Sc0.18Al0.82N layer is positioned between the AlN template and the GaN channel to effectively suppress the leakage at the interface between the secondary epilayers and the AlN template, realizing fully strained epitaxial structures less than 60 nm. Besides, a Sc0.18Al0.82N layer serves as a part of the superlattice barrier with well-defined interfaces, which results in a stronger polarization effect and larger conduction band offset and thus a high carrier density of 3.12 × 1013 cm−2. The processed GaN HEMT engineered by Sc0.18Al0.82N epilayers exhibits direct-current transfer characteristics with an on/off current ratio of ∼106 and distinct counterclockwise ferroelectric hysteresis windows, with a widely tunable threshold voltage ranging from −6.3 to −2.7 V. The fabricated device with a 180-nm T-shaped gate and gate-drain space of 1.5 μm achieves a breakdown voltage of 80 V, a cutoff frequency of 32 GHz, and a maximum oscillation frequency of 70 GHz, respectively. This work provides a new approach for developing next-generation multifunctional transistors on an ultrawide bandgap AlN platform through synergistically integrating ferroelectric gate modulation with a superlattice barrier, offering a promising candidate for highly reliable microwave power electronics with tunable switching characteristics.