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 (n_s) 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 In_0.17Al_0.83N barrier thickness has a minimal impact on n_s and μ, and an AlGaN cap layer decreases both n_s and μ. Optimization of AlN and GaN interlayers can be used to maximize μ and set n_s in the range of 1-3 × 10¹³cm^-2. The authors use this approach to demonstrate N-polar HEMTs grown on freestanding GaN substrates with sheet resistance R_s = 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.