Record high electron mobility and low sheet resistance on scaled-channel N-polar GaN/AlN heterostructures grown on on-axis N-polar GaN substrates by plasma-assisted molecular beam epitaxy

GaN-based high electron mobility transistors (HEMTs) have demonstrated high frequency power amplification with considerably larger output power densities than that available from amplifiers based on other material systems such as GaAs or InP. To further increase the operating frequency while maintaining the high output power in HEMTs, the gate-to-channel distance needs to be reduced significantly. This leads to a reduced two-dimensional electron gas (2DEG) density (ns) and mobility (μ) in Ga polar HEMT structures resulting in a larger sheet resistance. This work demonstrates that by proper design of the back-barrier in N-polar GaN-based scaled-channel HEMT structures, a high 2DEG density can be maintained while scaling the channel thickness. Scaled-channel GaN-based HEMT structures with an AlN/GaN (0.5 nm/1.5 nm) digital alloy as the back-barrier were grown on an on-axis N-polar GaN substrate via plasma-assisted molecular beam epitaxy. A record high electron mobility of 2050 cm2/vs was achieved on an N-polar HEMT structure with a 10 nm-thick channel, while maintaining 8 × 1012 cm−2 2DEG density. By modifying the barrier structure, we demonstrated a combination of 2DEG density and a mobility of 1.7 × 1013 cm−2 and 1420 cm2/V s, respectively, leading to a record low sheet resistance of ∼258 Ω/□ on 7 nm-thick channel N-polar HEMT structures.