Charge control in N-polar InAlN high-electron-mobility transistors grown by plasma-assisted molecular beam epitaxy

N-polar InAlN-based high-electron-mobility transistors (HEMTs) have fundamental advantages relative to conventional Ga-polar AlGaN HEMTs for high frequency devices. An understanding of the epitaxial design space for controlling sheet carrier density (ns) and mobility (μ) is desirable to maximize power and frequency performance by improving breakdown voltage and reducing parasitic access resistance. In this work, the authors show that In0.17Al0.83N barrier thickness has a minimal impact on ns and μ, and an AlGaN cap layer decreases both ns and μ. Optimization of AlN and GaN interlayers can be used to maximize μ and set ns in the range of 1–3 × 1013 cm−2. The authors use this approach to demonstrate N-polar HEMTs grown on freestanding GaN substrates with sheet resistance Rs = 190 Ω/◻ and μ = 1400 cm2/V·s, leading to a maximum drain current density of 1.5 A/mm for HEMTs with a 5-μm source–drain spacing and Pt-based Schottky gates.